Are My Slugging Measurements Correct? (30-30 Winchester)

UtopiaTexasG19 · August 31, 2011, 12:46 PM

I just slugged my 336 in 30-30 Winchester with a very soft BHN 5 cast bullet that had been sized to .310 and had a devil of a time getting it through the well oiled barrel. No matter which area I measure with my micrometer I get a reading of .307 Is this possible? I know the micrometer works properly as I measured the slug after running it through the sizing die and got the correct measurement. Thanks...

PawPaw · August 31, 2011, 01:02 PM

If my memory serves, and this article seems to help me remember, the current Microgroove barrel has 12 grooves, each 0.0028 deep. Some reports indicate that the nominal bore diameter has run from 0.300 to 0.305, with bores at 0.301-.302 being the norm. If that's the case, then your Marlin just might have a 0.307 groove diameter.

**Unclenick** · August 31, 2011, 01:20 PM

Yes, that's within range. However, if the slug was hard to push through, by which I mean hard all the way down the barrel and that is true in both directions (remove bolt to push from breech side) then is sounds like your slug is a lot harder than you think. I've had that experience when I tried pushing a BHN 14-16 slug through. You can do it, but they're so springy they push outward against the bore and create enough friction to drag the whole way. If you put them back through a second time, they are just as hard going.

A pure lead slug (BHN 4.5) to a swaged alloy of about BHN 6, will not spring outward and will be tight only where the bore is tight and be loose where it isn't. That's what you want. Your actual bore could be smaller than you are reading, due to spring-out, or larger if the alloy didn't bend enough to fill the grooves well. I use Hornady swaged lead balls which are pure lead. For .30 caliber I tale a .36 caliber ball and roll it between plates until it is narrow enough to tap in with a plastic mallet or a wood dowel and brass hammer. The tapping will cause it to fill out the grooves. Put oil on it and run an oily patch down the bore before you start.

Also, you said your micrometer was right. Did you actually mean a micrometer or a caliper? Sometimes folks get the terms mixed. What you want for slugging is an actual OD thimble micrometer, like this one. It will let you read tenths of a thousandth. (Harbor Freight has one for a little less, but their web site is down for maintenance as I compose this, so I can't give you a link.)

The old military spec for .30 cal barrels is a groove diameter of 0.3065"-0.3095". I suppose you may encounter anything in that range. A narrow bore is better than a wide one for cast bullet accuracy.

noylj · August 31, 2011, 02:41 PM

.307-.308" is what my .30-30s have run.

August 31, 2011, 12:46 PM	#1
UtopiaTexasG19 Senior Member Join Date: July 3, 2011 Location: S.E. Texas Gulf Coast Posts: 743	Are My Slugging Measurements Correct? (30-30 Winchester) I just slugged my 336 in 30-30 Winchester with a very soft BHN 5 cast bullet that had been sized to .310 and had a devil of a time getting it through the well oiled barrel. No matter which area I measure with my micrometer I get a reading of .307 Is this possible? I know the micrometer works properly as I measured the slug after running it through the sizing die and got the correct measurement. Thanks...

August 31, 2011, 01:02 PM	#2
PawPaw Senior Member Join Date: December 24, 2010 Location: Central Louisiana Posts: 3,137	If my memory serves, and this article seems to help me remember, the current Microgroove barrel has 12 grooves, each 0.0028 deep. Some reports indicate that the nominal bore diameter has run from 0.300 to 0.305, with bores at 0.301-.302 being the norm. If that's the case, then your Marlin just might have a 0.307 groove diameter. __________________ Dennis Dezendorf http://pawpawshouse.blogspot.com

August 31, 2011, 01:20 PM	#3
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,063	Yes, that's within range. However, if the slug was hard to push through, by which I mean hard all the way down the barrel and that is true in both directions (remove bolt to push from breech side) then is sounds like your slug is a lot harder than you think. I've had that experience when I tried pushing a BHN 14-16 slug through. You can do it, but they're so springy they push outward against the bore and create enough friction to drag the whole way. If you put them back through a second time, they are just as hard going. A pure lead slug (BHN 4.5) to a swaged alloy of about BHN 6, will not spring outward and will be tight only where the bore is tight and be loose where it isn't. That's what you want. Your actual bore could be smaller than you are reading, due to spring-out, or larger if the alloy didn't bend enough to fill the grooves well. I use Hornady swaged lead balls which are pure lead. For .30 caliber I tale a .36 caliber ball and roll it between plates until it is narrow enough to tap in with a plastic mallet or a wood dowel and brass hammer. The tapping will cause it to fill out the grooves. Put oil on it and run an oily patch down the bore before you start. Also, you said your micrometer was right. Did you actually mean a micrometer or a caliper? Sometimes folks get the terms mixed. What you want for slugging is an actual OD thimble micrometer, like this one. It will let you read tenths of a thousandth. (Harbor Freight has one for a little less, but their web site is down for maintenance as I compose this, so I can't give you a link.) The old military spec for .30 cal barrels is a groove diameter of 0.3065"-0.3095". I suppose you may encounter anything in that range. A narrow bore is better than a wide one for cast bullet accuracy. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

August 31, 2011, 02:41 PM	#4
noylj Senior Member Join Date: October 21, 2007 Location: Between CA and NM Posts: 858	.307-.308" is what my .30-30s have run.