Interference Fit Question

flyguy958 · December 13, 2009, 10:36 PM

How much interference fit is optimum for bullet fit in a bolt action 243 Win and 30-06? Searching seems to find most do not crimp for these calibers in a bolt gun. I am using Lee collet neck size dies and have read that I need to reduce the size of the pilot on the die. Just wondering what the general consensus is on proper fit.

wncchester · December 14, 2009, 04:33 PM

"Just wondering what the general consensus is on proper fit. "

There really isn't one. And, saying "interference fit" is using the proper term, it's actually not "neck tension" as most refer to it, at all.

Most standard FL expander plugs will normally leave mouths 2 or 3 thou undersize while cases from most Collet die will run from 1 or 2 thou under, both depending largely on how hard the brass itself is. Harder brass tends to spring back more and that means a slighter tighter fit with standard dies. But that effect is reversed with the Lee collet, cases from it springs back larger each use so annealing from time to time is in order.

Many folks talk of wanting a lot of bullet 'tension', 3 to 5 thou, as if it's some kind of anvantage. It isn't, certainly not more than maybe 3 thou anyway because the brass is simply stretched passed its elastic limits at that difference. So, going smaller than 3 is a mistake, IMHO.

I also think it can be proven that the higher pressure requiired to seat a bullet in a case that's significantly smaller than it need be WILL produce undesirable tilt (run-out) when the bullet enters the mouth.

Seems Lee has it right when they strive for only 1 thou of interference fit in an already straight neck! The good neck sizer plus a proper diameter neck tends to make for much straighter ammo and that makes for smaller groups!

gearheadpyro · December 14, 2009, 08:39 PM

+1

Their really isn't any consensus other than what works best for you. I don't reload for those calibers, but in my .308 I run about .004" interferance. I anneal my brass though so it tends to be softer and resize easier.

I do crimp my .308's using the Lee Factory Crimp Die and have had fantastic results with it, you may not. At $10 though I feel it's worth trying.

**Unclenick** · December 14, 2009, 10:48 PM

Wncchester hit it. There is a limit to how much additional tension you get from making the neck smaller. Beyond a point the bullet just acts like its own expander, making the neck wider, but not increasing tension appreciably, for the reason he gave. I also agree 0.001" to 0.002" is a good interference fit number. Like all things related to reloading, consistency is the main ingredient, often more so than the absolute values. You may find you need to anneal more often when producing only a slight interference fit, as the collet die does, so the interference remains constant and is not diminished by work hardening. Some find they need to take a little off those collet die mandrels for their brass, but I've not had to do so yet. I think that is about annealing rather than needing more tension.

If you want to check the interference fit you are getting, just seat a bullet's bearing surface half way into the neck and measure the neck O.D. difference at the lower and upper halves. That is, where the brass is stretched over the bullet and where it is not. If you don't own a blade micrometer, a dial caliper is easiest for this task as you can see fractional thousands on the dial, where a digital that is rounding up may not show the difference at all.

On another board we have a former Aberdeen Proving Ground employee who did testing and failure investigations there. He points out that aging the ammo tends to increase tension, but not consistently. You may need, say, 60 lbs to seat a bullet, but he's seen pull up to 600 pounds just from it getting a few years old. Still shot fine to military grouping requirements, which are not tight by target shooting standards. But as a target shooter, that cold weld inconsistency is not helpful, so you do better not to load too far ahead of need for accuracy ammo.

flyguy958 · December 15, 2009, 12:06 PM

Thanks for the replies, and I'll try to remember "neck tension".

I did not consider aging to be a factor, but I see where it could have some definite affects. Seems all the little things add up.

Seeing that the temper of the brass has a good bit to do with how the sizing die preforms it also stands to reason the same neck tension with different brass hardness will result in a totally different "grip" on the bullet.

December 13, 2009, 10:36 PM	#1
flyguy958 Senior Member Join Date: June 27, 2009 Location: SC Posts: 200	Interference Fit Question How much interference fit is optimum for bullet fit in a bolt action 243 Win and 30-06? Searching seems to find most do not crimp for these calibers in a bolt gun. I am using Lee collet neck size dies and have read that I need to reduce the size of the pilot on the die. Just wondering what the general consensus is on proper fit.

December 14, 2009, 08:39 PM	#3
gearheadpyro Senior Member Join Date: September 3, 2009 Location: Charleston, SC Posts: 169	+1 Their really isn't any consensus other than what works best for you. I don't reload for those calibers, but in my .308 I run about .004" interferance. I anneal my brass though so it tends to be softer and resize easier. I do crimp my .308's using the Lee Factory Crimp Die and have had fantastic results with it, you may not. At $10 though I feel it's worth trying. __________________ www.southernmarksman.com Owner of & Chief Instructor for The Southern Marksman, LLC NRA Life Member, NRA CRSO, Multi-Discipline NRA Certified Instructor

December 14, 2009, 10:48 PM	#4
Unclenick Staff Join Date: March 4, 2005 Location: Ohio Posts: 21,057	Wncchester hit it. There is a limit to how much additional tension you get from making the neck smaller. Beyond a point the bullet just acts like its own expander, making the neck wider, but not increasing tension appreciably, for the reason he gave. I also agree 0.001" to 0.002" is a good interference fit number. Like all things related to reloading, consistency is the main ingredient, often more so than the absolute values. You may find you need to anneal more often when producing only a slight interference fit, as the collet die does, so the interference remains constant and is not diminished by work hardening. Some find they need to take a little off those collet die mandrels for their brass, but I've not had to do so yet. I think that is about annealing rather than needing more tension. If you want to check the interference fit you are getting, just seat a bullet's bearing surface half way into the neck and measure the neck O.D. difference at the lower and upper halves. That is, where the brass is stretched over the bullet and where it is not. If you don't own a blade micrometer, a dial caliper is easiest for this task as you can see fractional thousands on the dial, where a digital that is rounding up may not show the difference at all. On another board we have a former Aberdeen Proving Ground employee who did testing and failure investigations there. He points out that aging the ammo tends to increase tension, but not consistently. You may need, say, 60 lbs to seat a bullet, but he's seen pull up to 600 pounds just from it getting a few years old. Still shot fine to military grouping requirements, which are not tight by target shooting standards. But as a target shooter, that cold weld inconsistency is not helpful, so you do better not to load too far ahead of need for accuracy ammo. __________________ Gunsite Orange Hat Family Member CMP Certified GSM Master Instructor NRA Certified Rifle Instructor NRA Benefactor Member and Golden Eagle

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December 14, 2009, 04:33 PM	#2
wncchester Senior Member Join Date: December 1, 2002 Posts: 2,832	"Just wondering what the general consensus is on proper fit. " There really isn't one. And, saying "interference fit" is using the proper term, it's actually not "neck tension" as most refer to it, at all. Most standard FL expander plugs will normally leave mouths 2 or 3 thou undersize while cases from most Collet die will run from 1 or 2 thou under, both depending largely on how hard the brass itself is. Harder brass tends to spring back more and that means a slighter tighter fit with standard dies. But that effect is reversed with the Lee collet, cases from it springs back larger each use so annealing from time to time is in order. Many folks talk of wanting a lot of bullet 'tension', 3 to 5 thou, as if it's some kind of anvantage. It isn't, certainly not more than maybe 3 thou anyway because the brass is simply stretched passed its elastic limits at that difference. So, going smaller than 3 is a mistake, IMHO. I also think it can be proven that the higher pressure requiired to seat a bullet in a case that's significantly smaller than it need be WILL produce undesirable tilt (run-out) when the bullet enters the mouth. Seems Lee has it right when they strive for only 1 thou of interference fit in an already straight neck! The good neck sizer plus a proper diameter neck tends to make for much straighter ammo and that makes for smaller groups!

December 15, 2009, 12:06 PM	#5
flyguy958 Senior Member Join Date: June 27, 2009 Location: SC Posts: 200	Thanks for the replies, and I'll try to remember "neck tension". I did not consider aging to be a factor, but I see where it could have some definite affects. Seems all the little things add up. Seeing that the temper of the brass has a good bit to do with how the sizing die preforms it also stands to reason the same neck tension with different brass hardness will result in a totally different "grip" on the bullet.