lapping lugs

shdybrady · December 21, 2015, 06:47 AM

The off season is approaching for us and this ia when I usually tear down my guns and tinker. This year I am starting to get alittle bit more ambitious with my projects. I have a marlin .17hmr and a model 700 .270 I am wanting to check the lugs for contact.

How do ya'll feel about the shell cut in half with a spring in the middle method? From what I have seen it keeps you from having to remove the barrel which caught my eyes because I dont have the necessary equiptment to do so. I was going to check the lugs with layout fluid, lap the surfaces with 600 grit compound until I get 75% contact and throughly clean out the action to remove the compound. Has anybody used this method?

shdybrady · December 21, 2015, 07:21 AM

I forgot to add i would be buying a go and no go headspace guage for this.

For those who might not be familiar with the method mentioned above here is a link.

http://www.sniperforums.com/forum/di...ping-tool.html

Roughedge · December 21, 2015, 07:32 AM

If you happen to get excess head-space do you know how to do a barrel set back?

shdybrady · December 21, 2015, 09:02 AM

Of course, If the guages indicate the headspace is out of spec. I put the tools down, contact a gunsmith and accept I am out of my league. But, I do want to at least try. I realize it would be easier to let a gunsmith do it from the start. But, to me knowledge is worth more than the price of a dollar

hooligan1 · December 21, 2015, 05:56 PM

What good is 75% contact? It's probably close to that now, most people who have their lugs lapped want full contact....usually a Smith does this before barreling.
Also I've heard some respected members here say it matters not to have lugs lapped....
And your hands and your arm will be wore slick before you lose headspace of any concern using 600 grit.

shdybrady · December 21, 2015, 06:02 PM

Reason I decided 600 grit is just to control how much I am taking off. I didn't want to be over ambitious with it and take too much off. I cant say definitely that I don't have 75%, because I haven't checked it. But I have heard a fair amount of people saying they had 10-30% percent contact on the model 700. Of course I do plan on checking before hand.

Has anyone tried the shell cut in half method?

Clark · December 21, 2015, 11:41 PM

When I started building Mausers in 2002 I cut a brass case in half, put a coil spring in it, and valve grinding compound on the lugs. I pushed and pulled up and down on the bolt handle until some of the di-chem came off the surfaces being lapped.

I did a couple rifles that way in 2002. I build 3 or 4 rifles per year for myself, and have not been lapping.

What I run into is gun culture accuracy improvement rituals that have improvements so small, I cannot detect or measure the improvement. So I give up. I make enough rifles that are sub 0.5 moa with no wind at the range.... so any more accuracy would be academic when hunting in wind.

James K · December 22, 2015, 12:57 AM

It sounds like you want to lap the lugs just so you can lap the lugs. How do the rifles shoot now, and what degree of improvement do you expect?

Jim

Gunplummer · December 22, 2015, 05:02 AM

I have to go with Clark and JamesK, I have also found lapping lugs a waste of time. The ultimate folly is guys that lap the lugs on a build and then send the receiver out to be re-heat treated. If lapping the lugs makes a noticable difference, I would have to say you have other problems.

shdybrady · December 22, 2015, 08:47 AM

Im not just wanting to lap lugs. I am going to first check to see ifbits needed. But before I send off 4 bolt actions, again if needed, I might as well see if its something that I can handle. I have it somewhat made up in my head I can do this. Now if its needed is yet to be seen. Im waiting for my layout fluid to get here.

Clark · December 22, 2015, 08:50 AM

Gunplumber,
I think we are on the same page.

I never fell for the re heat treat in Kuhnhausen's Mauser book, but I did buy the book Dec 12, 2000.
I did not like the few pages of the book and gave it to my brother.
He is the one that made the spring loaded case for lapping. [I think he did another one in 2015]
He trued a bolt face to get rid of primer leakage erosion. [mistake].
He made a mandrel and trued the large ring face. [I fell for that too]

That is THE worst gunsmithing book I ever bought, out of ~50.
And yet Kuhnhausen books on double action Colt revolvers are the best gunsmithing books I ever read.

shdybrady · December 22, 2015, 08:58 AM

What is one book that you recommend for general smithing?

F. Guffey · December 22, 2015, 09:35 AM

Quote:

What is one book that you recommend for general smithing?

The complete illustrated guide to precision rifle barrel fitting by John L. Hennant

F. Guffey

Gunplummer · December 22, 2015, 10:33 PM

I had an old book that was about barrel making, but the exact title escapes me. I would have to say some of the methods were somewhat crude, but attribute that to the time it was written. It was kind of a cross between a machinist and gunsmith manual. It was really a handy little book and I misplaced it somewhere along the line. Does it ring a bell with anyone here?

Clark · December 23, 2015, 01:50 AM

I can see I wrote a review of this book in 2006

http://www.amazon.com/Introduction-m.../dp/0064634264

6 of 7 people found the following review helpful
Can't live without that book
By Clark Magnuson on February 8, 2006
Format: Paperback
Sporterizing Mausers?
This book is a must have.

Other things of interest:
He makes sight dovetails in the barrel with a hack saw!
The guy was a real bare foot doctor.

The table of contents:
1 Proper Methods and the right tools....15
2 How to make your own tools and parts...26
3 Diagnosis and eliminating malfunctions .. 47
4 Selecting and working different steels.. 68
5 How to pick the action for your sporter..86
6 Basic ways of improving military actions ..110
7 How to do your own shotgun work..145
8 Altering and improving the handgun ..163
9 How to achieve the best trigger pull ..181
10 How achieve the most accuracy.. 195
11 How to mount your own scopes and sights.. 218
12 Getting better than new metal finishes.. 234
13 Your military rifle an it's sporting stock.. 249
14 How to handle a hundred disassembly problems ..274
15 Commonly used data and reference tables..297
index ..316

The things I have used from this book:
1) How to do a trigger job on a Mauser with Silver solder
2) Inside radius and distance between holes in Weaver scope mounts
3) Thread length, shoulder diameter, thread major diameter, TPI, and type of thread on barrels for different actions.

This is an old writing.
The newer Weaver mounts are not there.
The newer actions, like Rem700 are not there.
The newer cartridges, like .223 are not there.

Gunplummer · December 23, 2015, 05:12 AM

I should pick that up. It looks like an interesting read.

mete · December 23, 2015, 11:26 AM

In a Mauser M98 , the receiver is soft steel like 1018. To be serviceble it has to be case hardened. Many wartime M98s are poorly machined and should have lugs lapped so each lap takes half the forces.This i s important for accuracy also. If the lapping is too severe you have the risk of cutting through the case andthe receiver must be rehardened to replace it. This may improve other things also.
Do everything right and you'll have an accurate durable gun .

Slamfire · December 23, 2015, 11:44 AM

I am not a fan of lapping. If the receiver seat to lug contact is 75%, I would leave it alone. Lapping removes the surface hardness. Many, if not most, bolts have hardened lug surfaces. This surface hardness is only thousands of an inch thick, grind that off and you are down to a softer substrate. I quit having lugs lapped after my first stainless M70 target rifle. Enough material was removed that I had galling on the back of the lugs.

This bolt has never been lapped. It is on its third barrel and contact between receiver and lugs has increased over time. This is the only lapping necessary on a well made modern gun: by shooting it.

shdybrady · December 23, 2015, 04:22 PM

Alright, alright. I'm still going to check my lugs for contact but I actually plan on sending this action off to be blueprinted. But I am going to buy a stock first. that's proving to be a task being the gun combination that I have.

Jimro · December 23, 2015, 05:44 PM

There are things you do for competition rifles that are utterly wasted on a hunting rifle.

Don't lap the lugs on a factory rifle with a factory barrel. It's a waste of effort. When the rifle is rebarreled, that is the time to true up the action shoulder, lap the lugs, and true the bolt face to the receiver threads. Then the new barrel can be installed true to it all.

Lapping the lugs and truing the bolt face and shoulder don't do anything for accuracy of the rifle though. What it does, assuming the barrel is installed true, is that reloads are more accurate as the cartridge head is more true than a rifle that has a bolt face off bore with the barrel axis.

If your virgin brass handloads are .50 MOA or less, and your first reloads off once fired brass are .75 MOA, odds are your rifle could benefit from having the bolt face trued to the bore axis. If the barrel isn't true to the receiver tennon threads then it's wasted effort though, any gains in accuracy on reloads will be destroyed at the next rebarrel as it is unlikely to be off bore in the exact manner as the previous barrel.

In the benchrest game where .25 MOA is a huge group, little things like truing the action and barrel are worthwhile. For a hunting rifle it is a waste of time and effort.

Jimro

Gunplummer · December 23, 2015, 06:14 PM

There you have it. If you get a noticeable difference just from lapping the lugs, something else is wrong.

tobnpr · December 24, 2015, 06:20 PM

Jimro is spot on.
Waste of effort on a factory rifle.
You can lap the lugs 'till the cows come home- if the bolt when in battery isn't true to the central axis of the bore, if the chamber isn't cut concentric to the bore, all the things that make up a precision, trued barreled action- it's not going to matter a bit, and might make accuracy suffer.

It's part of the big picture, never a "standalone"...

Longshot4 · December 27, 2015, 08:54 PM

I'm with shadybrady 100%

HiBC · December 31, 2015, 11:22 PM

If I may suggest,consider the machining operations of making a bolt,and of cutting the locking shoulders inside the receiver.
Wouldn't both of the rear faces of the bolt be cut in the same pass?

Same with the milling inside the receiver.

I would expect,as cut,to see little if any variation side to side.

Other variables enter in later,but as machined I would expect variation to indicate well less than .001.
I say that because its important to understand the scale of "lapping"
It should be a few tenths worth of tool finish.
Under those conditions,neither headspace or surface hardness are an issue.
Lapping is not the method to take out .002 or .003.

It CAN happen that the receiver bore and locking lug recesses are not perpendicular.or a similar condition with the bolt.If the bolt is "tilted" one lug can be lifted.
Funny thing,if you pull back on the bolt handle to lap the lugs,you create this condition with the off center load.
Another source of trouble post machining is the polishing wheel.One lug can be overpolished,compromizing the machined part accuracy.

Check it if you must with some sharpie or cold blue on the bolt lugs.The load must be in the center of the bolt face.

If you see that a lug does not bear...well.I might not like that either.

But lapping a finished rifle is somewhat like putting on your sox after you tie your shoes.
There is an order to these things.
Spooning valve grinding compound into the receiver is not the answer.

I have my own way of hard fitting parts to bear.I only want to take the peaks of the high spots.I want to take nothing off the low spots.

Cold blue,shiny spots,and small strokes with a very fine hard stone,only cutting the shiny spots,will make the shiny spots get bigger.

Dixie Gunsmithing · January 1, 2016, 10:02 AM

The receiver is milled and broached in several steps or operations. However, the front, where the barrel thread and lugs are machined, are done in one or two operations, and the rear of the receiver is machined in another. That is why there can be a variance in concentricity and longitudinal alignment. This is why that blueprinting is done, using the bores center line, so the front and rear will be closer to matching it. Here, you use a centering mandrel, in the rear of the receiver, to machine the front. AGI has a good video about doing this, by Darrel Holland (see below).

The factory operations are all done in jigs and fixtures, mounted on the machines, and though they try to get them as close as possible, there are some variance over the several operations. The use of CNC machining has gotten some of them closer, but it depends on what the designer chose as far as tolerances, even on these.

Not-Holland [Ken Brooks] DVDs:

http://www.midwayusa.com/product/676...volume-set-dvd

Edit: That is Ken Brooks in those two videos, and not Darrel Holland. Brooks is an instructor at Lassen College Gunsmithing School.

December 21, 2015, 06:47 AM	#1
shdybrady Member Join Date: December 20, 2015 Posts: 32	lapping lugs The off season is approaching for us and this ia when I usually tear down my guns and tinker. This year I am starting to get alittle bit more ambitious with my projects. I have a marlin .17hmr and a model 700 .270 I am wanting to check the lugs for contact. How do ya'll feel about the shell cut in half with a spring in the middle method? From what I have seen it keeps you from having to remove the barrel which caught my eyes because I dont have the necessary equiptment to do so. I was going to check the lugs with layout fluid, lap the surfaces with 600 grit compound until I get 75% contact and throughly clean out the action to remove the compound. Has anybody used this method?

December 21, 2015, 07:32 AM	#3
Roughedge Senior Member Join Date: January 24, 2011 Location: Monroe,NC Posts: 669	If you happen to get excess head-space do you know how to do a barrel set back? __________________ The man that die's with the most stuff win's!

December 21, 2015, 05:56 PM	#5
hooligan1 Senior Member Join Date: July 18, 2010 Location: Independence Missouri Posts: 4,585	What good is 75% contact? It's probably close to that now, most people who have their lugs lapped want full contact....usually a Smith does this before barreling. Also I've heard some respected members here say it matters not to have lugs lapped.... And your hands and your arm will be wore slick before you lose headspace of any concern using 600 grit. __________________ Keep your Axe sharp and your powder dry.

December 21, 2015, 11:41 PM	#7
Clark Senior Member Join Date: December 4, 1999 Location: WA, the ever blue state Posts: 4,678	When I started building Mausers in 2002 I cut a brass case in half, put a coil spring in it, and valve grinding compound on the lugs. I pushed and pulled up and down on the bolt handle until some of the di-chem came off the surfaces being lapped. I did a couple rifles that way in 2002. I build 3 or 4 rifles per year for myself, and have not been lapping. What I run into is gun culture accuracy improvement rituals that have improvements so small, I cannot detect or measure the improvement. So I give up. I make enough rifles that are sub 0.5 moa with no wind at the range.... so any more accuracy would be academic when hunting in wind. __________________ The word 'forum" does not mean "not criticizing books." "Ad hominem fallacy" is not the same as point by point criticism of books. If you bought the book, and believe it all, it may FEEL like an ad hominem attack, but you might strive to accept other points of view may exist. Are we a nation of competing ideas, or a nation of forced conformity of thought?

December 22, 2015, 08:50 AM	#11
Clark Senior Member Join Date: December 4, 1999 Location: WA, the ever blue state Posts: 4,678	Gunplumber, I think we are on the same page. I never fell for the re heat treat in Kuhnhausen's Mauser book, but I did buy the book Dec 12, 2000. I did not like the few pages of the book and gave it to my brother. He is the one that made the spring loaded case for lapping. [I think he did another one in 2015] He trued a bolt face to get rid of primer leakage erosion. [mistake]. He made a mandrel and trued the large ring face. [I fell for that too] That is THE worst gunsmithing book I ever bought, out of ~50. And yet Kuhnhausen books on double action Colt revolvers are the best gunsmithing books I ever read. __________________ The word 'forum" does not mean "not criticizing books." "Ad hominem fallacy" is not the same as point by point criticism of books. If you bought the book, and believe it all, it may FEEL like an ad hominem attack, but you might strive to accept other points of view may exist. Are we a nation of competing ideas, or a nation of forced conformity of thought?

December 21, 2015, 07:21 AM	#2
shdybrady Member Join Date: December 20, 2015 Posts: 32	I forgot to add i would be buying a go and no go headspace guage for this. For those who might not be familiar with the method mentioned above here is a link. http://www.sniperforums.com/forum/di...ping-tool.html

December 21, 2015, 09:02 AM	#4
shdybrady Member Join Date: December 20, 2015 Posts: 32	Of course, If the guages indicate the headspace is out of spec. I put the tools down, contact a gunsmith and accept I am out of my league. But, I do want to at least try. I realize it would be easier to let a gunsmith do it from the start. But, to me knowledge is worth more than the price of a dollar

December 21, 2015, 06:02 PM	#6
shdybrady Member Join Date: December 20, 2015 Posts: 32	Reason I decided 600 grit is just to control how much I am taking off. I didn't want to be over ambitious with it and take too much off. I cant say definitely that I don't have 75%, because I haven't checked it. But I have heard a fair amount of people saying they had 10-30% percent contact on the model 700. Of course I do plan on checking before hand. Has anyone tried the shell cut in half method?

December 22, 2015, 12:57 AM	#8
James K Member In Memoriam Join Date: March 17, 1999 Posts: 24,383	It sounds like you want to lap the lugs just so you can lap the lugs. How do the rifles shoot now, and what degree of improvement do you expect? Jim

December 22, 2015, 05:02 AM	#9
Gunplummer Senior Member Join Date: March 11, 2010 Location: South East Pa. Posts: 3,364	I have to go with Clark and JamesK, I have also found lapping lugs a waste of time. The ultimate folly is guys that lap the lugs on a build and then send the receiver out to be re-heat treated. If lapping the lugs makes a noticable difference, I would have to say you have other problems.

December 22, 2015, 08:47 AM	#10
shdybrady Member Join Date: December 20, 2015 Posts: 32	Im not just wanting to lap lugs. I am going to first check to see ifbits needed. But before I send off 4 bolt actions, again if needed, I might as well see if its something that I can handle. I have it somewhat made up in my head I can do this. Now if its needed is yet to be seen. Im waiting for my layout fluid to get here.

December 22, 2015, 08:58 AM	#12
shdybrady Member Join Date: December 20, 2015 Posts: 32	What is one book that you recommend for general smithing?

December 22, 2015, 10:33 PM	#14
Gunplummer Senior Member Join Date: March 11, 2010 Location: South East Pa. Posts: 3,364	I had an old book that was about barrel making, but the exact title escapes me. I would have to say some of the methods were somewhat crude, but attribute that to the time it was written. It was kind of a cross between a machinist and gunsmith manual. It was really a handy little book and I misplaced it somewhere along the line. Does it ring a bell with anyone here?

December 23, 2015, 01:50 AM	#15
Clark Senior Member Join Date: December 4, 1999 Location: WA, the ever blue state Posts: 4,678	I can see I wrote a review of this book in 2006 http://www.amazon.com/Introduction-m.../dp/0064634264 6 of 7 people found the following review helpful Can't live without that book By Clark Magnuson on February 8, 2006 Format: Paperback Sporterizing Mausers? This book is a must have. Other things of interest: He makes sight dovetails in the barrel with a hack saw! The guy was a real bare foot doctor. The table of contents: 1 Proper Methods and the right tools....15 2 How to make your own tools and parts...26 3 Diagnosis and eliminating malfunctions .. 47 4 Selecting and working different steels.. 68 5 How to pick the action for your sporter..86 6 Basic ways of improving military actions ..110 7 How to do your own shotgun work..145 8 Altering and improving the handgun ..163 9 How to achieve the best trigger pull ..181 10 How achieve the most accuracy.. 195 11 How to mount your own scopes and sights.. 218 12 Getting better than new metal finishes.. 234 13 Your military rifle an it's sporting stock.. 249 14 How to handle a hundred disassembly problems ..274 15 Commonly used data and reference tables..297 index ..316 The things I have used from this book: 1) How to do a trigger job on a Mauser with Silver solder 2) Inside radius and distance between holes in Weaver scope mounts 3) Thread length, shoulder diameter, thread major diameter, TPI, and type of thread on barrels for different actions. This is an old writing. The newer Weaver mounts are not there. The newer actions, like Rem700 are not there. The newer cartridges, like .223 are not there. __________________ The word 'forum" does not mean "not criticizing books." "Ad hominem fallacy" is not the same as point by point criticism of books. If you bought the book, and believe it all, it may FEEL like an ad hominem attack, but you might strive to accept other points of view may exist. Are we a nation of competing ideas, or a nation of forced conformity of thought?

December 23, 2015, 05:12 AM	#16
Gunplummer Senior Member Join Date: March 11, 2010 Location: South East Pa. Posts: 3,364	I should pick that up. It looks like an interesting read.

December 23, 2015, 11:26 AM	#17
mete Senior Member Join Date: June 14, 2004 Location: NY State Posts: 6,575	In a Mauser M98 , the receiver is soft steel like 1018. To be serviceble it has to be case hardened. Many wartime M98s are poorly machined and should have lugs lapped so each lap takes half the forces.This i s important for accuracy also. If the lapping is too severe you have the risk of cutting through the case andthe receiver must be rehardened to replace it. This may improve other things also. Do everything right and you'll have an accurate durable gun . __________________ And Watson , bring your revolver !

December 23, 2015, 11:44 AM	#18
Slamfire Senior Member Join Date: May 27, 2007 Posts: 5,261	I am not a fan of lapping. If the receiver seat to lug contact is 75%, I would leave it alone. Lapping removes the surface hardness. Many, if not most, bolts have hardened lug surfaces. This surface hardness is only thousands of an inch thick, grind that off and you are down to a softer substrate. I quit having lugs lapped after my first stainless M70 target rifle. Enough material was removed that I had galling on the back of the lugs. This bolt has never been lapped. It is on its third barrel and contact between receiver and lugs has increased over time. This is the only lapping necessary on a well made modern gun: by shooting it. __________________ If I'm not shooting, I'm reloading.

December 23, 2015, 04:22 PM	#19
shdybrady Member Join Date: December 20, 2015 Posts: 32	Alright, alright. I'm still going to check my lugs for contact but I actually plan on sending this action off to be blueprinted. But I am going to buy a stock first. that's proving to be a task being the gun combination that I have.

December 23, 2015, 05:44 PM	#20
Jimro Senior Member Join Date: October 18, 2006 Posts: 7,097	There are things you do for competition rifles that are utterly wasted on a hunting rifle. Don't lap the lugs on a factory rifle with a factory barrel. It's a waste of effort. When the rifle is rebarreled, that is the time to true up the action shoulder, lap the lugs, and true the bolt face to the receiver threads. Then the new barrel can be installed true to it all. Lapping the lugs and truing the bolt face and shoulder don't do anything for accuracy of the rifle though. What it does, assuming the barrel is installed true, is that reloads are more accurate as the cartridge head is more true than a rifle that has a bolt face off bore with the barrel axis. If your virgin brass handloads are .50 MOA or less, and your first reloads off once fired brass are .75 MOA, odds are your rifle could benefit from having the bolt face trued to the bore axis. If the barrel isn't true to the receiver tennon threads then it's wasted effort though, any gains in accuracy on reloads will be destroyed at the next rebarrel as it is unlikely to be off bore in the exact manner as the previous barrel. In the benchrest game where .25 MOA is a huge group, little things like truing the action and barrel are worthwhile. For a hunting rifle it is a waste of time and effort. Jimro __________________ Machine guns are awesome until you have to carry one.

December 23, 2015, 06:14 PM	#21
Gunplummer Senior Member Join Date: March 11, 2010 Location: South East Pa. Posts: 3,364	There you have it. If you get a noticeable difference just from lapping the lugs, something else is wrong.

December 24, 2015, 06:20 PM	#22
tobnpr Senior Member Join Date: August 1, 2010 Location: Tampa Bay Posts: 4,556	Jimro is spot on. Waste of effort on a factory rifle. You can lap the lugs 'till the cows come home- if the bolt when in battery isn't true to the central axis of the bore, if the chamber isn't cut concentric to the bore, all the things that make up a precision, trued barreled action- it's not going to matter a bit, and might make accuracy suffer. It's part of the big picture, never a "standalone"... __________________ Remington 700/Savage Rebarreling /Action Blueprinting 07 FFL /Mosin-Nagant Custom Shop/Bent Bolts Genuine Cerakote Applicator www.biggorillagunworks.com

December 27, 2015, 08:54 PM	#23
Longshot4 Senior Member Join Date: February 22, 2014 Posts: 868	I'm with shadybrady 100%

December 31, 2015, 11:22 PM	#24
HiBC Senior Member Join Date: November 13, 2006 Posts: 8,286	If I may suggest,consider the machining operations of making a bolt,and of cutting the locking shoulders inside the receiver. Wouldn't both of the rear faces of the bolt be cut in the same pass? Same with the milling inside the receiver. I would expect,as cut,to see little if any variation side to side. Other variables enter in later,but as machined I would expect variation to indicate well less than .001. I say that because its important to understand the scale of "lapping" It should be a few tenths worth of tool finish. Under those conditions,neither headspace or surface hardness are an issue. Lapping is not the method to take out .002 or .003. It CAN happen that the receiver bore and locking lug recesses are not perpendicular.or a similar condition with the bolt.If the bolt is "tilted" one lug can be lifted. Funny thing,if you pull back on the bolt handle to lap the lugs,you create this condition with the off center load. Another source of trouble post machining is the polishing wheel.One lug can be overpolished,compromizing the machined part accuracy. Check it if you must with some sharpie or cold blue on the bolt lugs.The load must be in the center of the bolt face. If you see that a lug does not bear...well.I might not like that either. But lapping a finished rifle is somewhat like putting on your sox after you tie your shoes. There is an order to these things. Spooning valve grinding compound into the receiver is not the answer. I have my own way of hard fitting parts to bear.I only want to take the peaks of the high spots.I want to take nothing off the low spots. Cold blue,shiny spots,and small strokes with a very fine hard stone,only cutting the shiny spots,will make the shiny spots get bigger.

January 1, 2016, 10:02 AM	#25
Dixie Gunsmithing Moderator Emeritus Join Date: April 27, 2013 Location: Ohio Posts: 1,923	The receiver is milled and broached in several steps or operations. However, the front, where the barrel thread and lugs are machined, are done in one or two operations, and the rear of the receiver is machined in another. That is why there can be a variance in concentricity and longitudinal alignment. This is why that blueprinting is done, using the bores center line, so the front and rear will be closer to matching it. Here, you use a centering mandrel, in the rear of the receiver, to machine the front. AGI has a good video about doing this, by Darrel Holland (see below). The factory operations are all done in jigs and fixtures, mounted on the machines, and though they try to get them as close as possible, there are some variance over the several operations. The use of CNC machining has gotten some of them closer, but it depends on what the designer chose as far as tolerances, even on these. Not-Holland [Ken Brooks] DVDs: http://www.midwayusa.com/product/676...volume-set-dvd Edit: That is Ken Brooks in those two videos, and not Darrel Holland. Brooks is an instructor at Lassen College Gunsmithing School.