Semi-Auto Neck Tension Issue

[JerkeeJoe]

I've been loading for my OBR for a while now with good results. However, yesterday I was messing with it and after measuring, then chambering and ejecting a mag worth of reloads, I discovered that the OAL of the loaded rounds is increasing by .004-.005 after chambering them.

So, I checked my neck tension and found that I am getting .0015-.002 neck tension when loading. Thinking this may not be sufficient, I went ahead and tried crimping some rounds, but even after progressing to what I think is a pretty aggressive crimp, I'm still getting increasing OAL after chambering.

I'm out of ideas, so I though I'd ask people more experienced than I what they think.

In case it is important, I am using twice fired RP brass sized in a RCBS full-length competition sizing die and loading Nosler custom competition 155s.

 

[JimGnitecki]

Re: Semi-Auto Neck Tension Issue

It is POSSIBLE that your problem is not the amount of interference fit bwteen the bullet and the case, but rather that you are jamming the bulelt into the lands when chambering, and then partially extracting it from the case when manually ejecting it.

This can cause an even bigger issue - higher than planned chmaber pressures - unless you safely worked yourself up to this load without pressure symptoms. You should not be jamming into the lands without having done the safe load workup, starting from minimum powder and working upward carefully.

Normally, on a semi-auto rifle, .002 to ..03" of itnerference fit is desirable. While .001" can be sufficient on many bolt rifles, it is not enough in general for a semi-auto. Going PAST .003" in interference though is futile, as the case will simply stretch more to accommodate the bullet, and thus "work" the brass more than is necessary.

I have found that if you use bullets, case neck wall thickness, and sizing bushing that SHOULD result in .003" interference, you actually "net" only about .001" or less interference AFTER the neck walls have stretched a bit as you seat the bullets. This does NOT mean though that you should only size for .001" interference. That would NOT produce the same result. The point is that the brass in the neck walls has to encounter a certain amount of resistance in order to net out the correct amount of "hold" on the bullet.

Hopefully, the above makes sense to you. If not, I can try again.

Jim G

 

[sititunga]

Re: Semi-Auto Neck Tension Issue

You might want to look at getting a Redding resizing die with drop in bushings. The last time I spoke to Redding about this I was told 0.02 - 0.03 should be good for a gas gun. You might want to give them a call, their technical support guys are pretty good.

 

[JerkeeJoe]

Re: Semi-Auto Neck Tension Issue

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: JimGnitecki</div><div class="ubbcode-body">It is POSSIBLE that your problem is not the amount of interference fit bwteen the bullet and the case, but rather that you are jamming the bulelt into the lands when chambering, and then partially extracting it from the case when manually ejecting it.

This can cause an even bigger issue - higher than planned chmaber pressures - unless you safely worked yourself up to this load without pressure symptoms. You should not be jamming into the lands without having done the safe load workup, starting from minimum powder and working upward carefully.

Normally, on a semi-auto rifle, .002 to ..03" of itnerference fit is desirable. While .001" can be sufficient on many bolt rifles, it is not enough in general for a semi-auto. Going PAST .003" in interference though is futile, as the case will simply stretch more to accommodate the bullet, and thus "work" the brass more than is necessary.

I have found that if you use bullets, case neck wall thickness, and sizing bushing that SHOULD result in .003" interference, you actually "net" only about .001" or less interference AFTER the neck walls have stretched a bit as you seat the bullets. This does NOT mean though that you should only size for .001" interference. That would NOT produce the same result. The point is that the brass in the neck walls has to encounter a certain amount of resistance in order to net out the correct amount of "hold" on the bullet.

Hopefully, the above makes sense to you. If not, I can try again.

Jim G </div></div>

First, thanks for your thoughts.

I thought about the bullet in the lands issue after I posted this, but I don't think that is the issue. While I don't remember the exact numbers now, when I first started loading the 155s I checked how far out I could seat the bullet before hitting the rifling, and then discovered that the maximum I could load for the magazines was significantly shorter that what the rifle would allow. I'm going to recheck this when I get home this evening just to be sure though.

Everything else made sense, but leaves me even more puzzled than before. I've run this same exact formula of loads through this gun before without issue, and with good accuracy, but didn't realize that the rounds were growing in the gun (or at least may be). I guess a viable solution to this may be not to mess with it as long as it is shooting well, but the perfectionist in me wants to fix it.

 

[Grump]

Re: Semi-Auto Neck Tension Issue

It's also possible that the CASE is getting kinda slam-sized at the shoulder as the extractor snaps over the rim.

That is a known issue with *shortening* headspace of loaded rounds. What gets short in one place *can* pop out as longer somewhere else.

Check all variables carefully. But most importantly, maybe you want to run a 5-round test at 500 yards *on paper* to see whether it changes POI. Make it a true A/B test. Maybe even mess with your head and make it blind--assistant gives you one round at a time and tells you which target to shoot at, based on which ammo it is (re-chambered vs. "virgin").

You might be chasing something of no net effect. .005 ain't a whole lot if you're not talking bullet or bore DIAMETER.

 

[MSTN]

Re: Semi-Auto Neck Tension Issue

0.002" neck tension is on the light side for a gas gun.

I've noticed that M118LR does the same 0.004" OAL increase after cycling a round through the Larue OBR chamber.