Crimp, neck tension, and velocity

[azredhawk44]

I loaded up 200 rounds of the following last week:

Federal .308 Winchester brass, trimmed to 2.005"
CCI #34 primer
Hornady A-MAX 168gr bullet
IMR 4895 42.5gr powder
2.790" OAL, no crimp

I confirmed that it shot acceptably. I shot a 2MOA group from prone with 10 rounds, at 200 yards. I was happy.

I took the ammo to a match on Saturday. Prone, 60 rounds, at 500 yards. I had a LOT of vertical stringing. Bad enough that it was even off-paper. One round came so slowly out of the barrel, that it fell short of the backers by about 8 feet and slammed into the target pit wall so that I could hear it with hearing protection from 500 yards away. Very embarrassed by that, I erred to shoot high rather than low for the remainder of the match.

Later, I went home and decided that it was possible that the rounds weren't getting a consistent burn in the case to build matching pressures on exit. So, I ran the remaining rounds through my Lee Factory Crimp Die. Prior to crimping, the cartridges measured 0.338 at the neck. The neck walls were 0.015 thick. I crimped them to 0.332, which might have been a bit too much, but I wanted to make sure the bullets got a good shove from the burning powder.

I went to another match, exact same format, today. All the vertical variation went away (except for the stuff that was my own fault).

I'm concerned that over-crimping may be damaging the bullet, though.

I'd like to find a better solution to this than just crimping the heck out of the bullet.

I've heard that neck tension is the real goal, not just a tight crimp.

I'm using an RCBS X-die sizer die. How do I increase neck tension on my resized cases?

[Kawabuggy]

To increase neck tension simply reduce the O/D of the button that gets pulled back through the case mouth as the case comes out of the die. This applies ONLY to F/L dies that have the expander button inside them. If using a busing die, simply go with a smaller I/D on the bushing so that the case mouth is closed up further. Understand that as the bullet is seated, that it will stretch the case mouth open again to conform to the O/D of the bullet. I've been told that sizing the case mouth significantly smaller than the O/D of the bullet is a bad thing because then when you press the bullet into the case, the greater resistance can cause the case neck to get pushed off center. Just what I've heard-never experienced this personally. Makes sense though.

Also be aware that annealing brass will greatly soften the case neck, and actually reduce neck tension. Work hardened brass-such as brass that has been re-sized many times without annealing will provide much more neck tension on finished rounds. The problem with work hardened brass is that it will spring-back when pulled from the die and won't be sized to the actual dimensions of the die. Annealed brass will actually hold the dimensions of the die as it is pulled out. All of this is to say that finding conformity will mean that you treat every case exactly the same, each time you re-size it, which is difficult, if not impossible to achieve. Further, if you have case necks that are not uniformly thick (variances from side to side) this will introduce it's own set of problems. I see you mention neck wall thickness, are you turning your necks???? If not, it might be time to start, to reduce one more variable.

I don't shoot competition, but I do take re-loading, and accuracy, very seriously. I actually neck-turn my hunting rounds, as well as full case prep to include annealing, primer pocket uniforming, flash hole deburring. Only then can I reason that I have done all that I can do to my brass to insure that any problems that crop up are either shooter, or gun. I do use the LFCD (Lee Factory Crimp Die) on all loaded rounds.

One other thing you need to think about, if you do use the LFCD, you need to realize that you are potentially changing the bearing surface of the bullet. If you want to do an experiment, measure the bearing surface of a bullet. Then press that bullet into a case with no primer, or powder. Now use the LFCD and put a heavy crimp on it. Now pull that bullet out of that case, and look at it. You will see that the LFCD has reduced the O/D of the bullet! It will actually cave the brass neck down into the bullet and reduce the bullet O/D. where it makes contact. With that in mind, if you don't crimp exactly the same way every time, or one bullet is seated deeper, or more shallow, than the next one, you won't have the same bearing surface area from shot to shot. This again adds another variable that is extremely hard to stay in control over.

I've heard it said that vertical stringing is a sign of varying powder charges. How are you measuring your powder?

[Brian Pfleuger]

Have you considered a Lee Collet neck sizing die?

For one thing, I'm told that they produce just about the most concentric neck of any die. The instructions say to run the brass twice, turning the case 180dg in the shell holder before the second time

Second, the neck tension is "adjustable" because the die can be set lower of higher in the press which puts more or less tension on the neck at full stroke.

Also, it's an absolute PITA, but you may want to look into annealing.

[azredhawk44]

Peetza, I have to FL size since I'm shooting an M14. Can't use a neck-only sizing die, unless I guess I were to add that as a second sizing step, after the FL size process.

Kawa: I'm measuring powder by volume, in an RCBS uniflow. Not trickle-charging for precision weighted charges.

I suspected I was probably indenting the bullet's bearing surface with a heavy crimp. But, it definitely increased accuracy.

My X-die is bushingless, so fussing with the OD is not possible without changing dies. Perhaps I will try to shrink the expander button a bit, or see if replacement expander buttons are available.

If I were to sand or otherwise shrink the expander button, and assuming it is .308 diameter right now, what should I shrink it to? .3075? .307?

[Brian Pfleuger]

Peetza, I have to FL size since I'm shooting an M14. Can't use a neck-only sizing die, unless I guess I were to add that as a second sizing step, after the FL size process.

Ah, ok, well, you could always go with a Collet neck die and a Redding body die.

[zeke]

Take out the expanding plug and de prime with universal deprimer in seperate step, or kawabuggy's tip, or use an undersize expanding plug from smaller caliber. This also helps keeping your brass from stretching.

Use a Redding competition seating die and boatail bullet. This keeps the bullet straight, when seating.

Just use the LFC to keep the case mouth even with rest of case neck.

Load up a dummy round to check for the round maintaing COL, either shorter or longer. Both are possible with semi's.

[alloy]

I took .002 off my expander to increase neck tension.
But then my trimmer pilot wouldn't fit so that needed it too.

No more crimping bullets with no cannelure for fear of light neck tension in a semi.

[alloy]

I figure you have read this already, it's an excerpt from the Sierra manual, but it does go into increasing neck tension, among other common issues in loading for service rifles.

http://www.exteriorballistics.com/re...sgunreload.cfm

[Kawabuggy]

The article in that link emphatically states that crimping hurts accuracy. I have found the opposite to be true-as has the OP of this post. Makes you wonder what else in the article is erroneous, or based on the writers personal opinion.

I know that people from Sierra wrote it, but how often do things get published and then later are proven wrong? Quite often.

Lot's of good information in the article otherwise.

[azredhawk44]

I have found the opposite to be true-as has the OP of this post.

To be fair, what I have found so far is:

1 - Uncrimped ammo with loose neck tension shoots poorly.
2 - Ammo in case #1 is improved by applying a strong crimp, even if the bullet doesn't have a cannelure.

I have yet to replace step 2 with what seems to be the preferred methodology of overall tighter neck tension rather than a mere crimp.

[alloy]

http://www.zediker.com/downloads/14_loading.pdf

Another good one you have most likely read, but it does mention the ongoing ideas about crimping or no, this time between Mr Redding and Mr Lee.

[azredhawk44]

Realizing...

My hand lathe case trimmer uses a pilot pin that enters into the case mouth to hold the case still while cutting.

I wonder if that pilot is > .307. Never had a big problem with the X-die prior to this, but I've been shirking my responsibility to trim cases, and had more lax standards for accuracy and consistency, before.

I bet the X-die sized 'em fine, and the trimming operation flared the mouth a bit. I will check the diameter of the expander button and the case trimmer pilot, see if one is bigger than the other.

Or, it could be that the trimming operation naturally causes neck tension to open up. I may need to re-size again after trimming.

[alloy]

I'm not expert enough assume your rifle likes XYZ better than ABC, and I surely don't know where your rifling starts....but is your stated overall length a bit short?

Most 168gr match type ammo I have measured has been closer to 2.815, with alot of folks pushing 2.825 and I have seen drawings of M118LR(173 grain from memory) listing length as 2.83 +/-.003, short enough to fit the mag, but as close to the lands as possible.

My particular rifle won't get anywhere near the lands and still fit the magazine. Just wondering if that might tighten group a bit to allow you to experiment with the crimp/tension thing.

Probobly irrelevant, just wondering...for my own notes.

My pilot didn't fit but that was Hornady, again...irrelevant I guess. Especially with LC brass. Back to the lathe and emery cloth to fix the pilot. Ended up with two decapper pins and two pilots. One set for LC and one for commercial brass. Just another trimming pain for the list.

[Tuckerp229]

The article in that link emphatically states that crimping hurts accuracy. I have found the opposite to be true-as has the OP of this post. Makes you wonder what else in the article is erroneous, or based on the writers personal opinion.

I know that people from Sierra wrote it, but how often do things get published and then later are proven wrong? Quite often.

Lot's of good information in the article otherwise.

To be fair, what I have found so far is:

1 - Uncrimped ammo with loose neck tension shoots poorly.
2 - Ammo in case #1 is improved by applying a strong crimp, even if the bullet doesn't have a cannelure.

I have yet to replace step 2 with what seems to be the preferred methodology of overall tighter neck tension rather than a mere crimp.
June 30, 2010 10:10 AM

I read several posts similar to these here and on other forums. Crimping seems to be the "Holy Grail". Personally I agree the pro-crimpers but truth is some of the difference of opinion is purpose oriented. I hunt and I hunt with multiple rounds in the mag or tube. Competitive shooters probable load one bullet carefully...slowly ensuring they cause no bullet movement. These two groups will tend to have different experiences.
Having stated the above, all of my personal testing has shown that some amount of crimp is better than no crimp. I have also learned to really trust the Lee FCD. However what bothered me was that nagging variable of how much crimp?. "Good crimp", "light crimp", "heavy crimp" are "non-numbers" to me. There was no way to quantify such terms and then to make the quantification repeatable. In short crimping seemed to me to be such "SWAG" after the meticulous preparations leading up to the crimp. I discovered through some of my own research that the results of using the Lee FCD dies can vary dramatically from a "light crimp"-nearly open collets, to a "heavy crimp"-closed collets. I hypothesized that this range from open to closed is important. I devised a way to quantify this pressure incrementally and repeatably and then tested the results downrange. I discovered that each step of the way from open to closed collets affected Point-Of-Impact both in terms of accuracy and precision. With everything else being carefully controlled I changed only the crimp pressure. I watched my groups "move" around the face of the target and change in average spread...all because of to one thing....difference in crimp pressure. I was able to discover at least one "sweet spot" for each load I developed that produced accurate and precise groups. As a side benefit, case length variance was also dealt with since the pressure reading is not dependent on the press ram position. I fact I don't run the ram to the bottom of the swing.
I now finish my "pet loads" with this testing of crimp pressure to optimize bullet performance downrange. My 30-30 Marlin with plain old Rem. 150 grain softpoints and a 4 power scope went from "pie plate" groups at 100 yards to 1-1.25 inches. This from a brush gun.

[Tuckerp229]

http://www.accuratereloading.com/crimping.html

I don't know this guy from Adam however he made and interesting comparison about crimping and non-crimping.