Cartridge growth in semi-autos

[CTone03]

Hi folks!

I have a question for those who reload for semi-autos: why is it that when I measure a finished cartridge, be it a handload or factory, and then load it by dropping the bolt on a magazine, that the round grows in length?

This happens with my handloads; and I just tried some FGMM 168s and 175s and they do it too. The 168s measured 2.224" to the ogive, and the 175s measured 2.223" to the ogive. Then I popped them in a mag, dropped the bolt, ejected the round in my hand, and then measured to the ogive again and they were 2.227" & 2.231" respectively. The rifle is a DPMS LR-308 with a 20" Titan barrel.

It took me awhile to diagnose an earlier velocity problem with handloads, and the culprit was the magazine feed lips presenting the round too low which cause feeding rounds to hit either the top of the magazine or the bottom of the feed ramps. The bullet was setting back into the case. I got that problem straightened out.

As far as the rounds now growing, the only thing I can think of is the inertia of the round being chambered causes the bullet to pull forward. Kinda like how a bullet puller works. Am I on the right track? I crimp my handloads with a Lee factory crimp die to help keep the length right when the rounds are banging around in the mag, but what do I do about factory ammo?

 

[The Mechanic]

Re: Cartridge growth in semi-autos

you guessed it. more neck tension or canalure

 

[The Mechanic]

Re: Cartridge growth in semi-autos

One more thing are you sure they are growing with factory? Are you sure you are measuring correctly? on the ogive?

 

[CTone03]

Re: Cartridge growth in semi-autos

Yes. I have tried this with factory Remington Cor-Lokt ammo as well, which has a cannelure and is crimped. It freaked me out the first time I noticed it. I was checking the Remingtons to see if I had the bullet-slamming-into-the-feed-ramps issue solved. I tried my uncrimped handloads and they did the same thing.

I just tried a handload with a good crimp not 60 seconds ago, and it went from 2.219" at the ogive to 2.222". I don't think that .003" is a deal breaker, but it could definitely be worse with uncrimped factory.

 

[CTone03]

Re: Cartridge growth in semi-autos

BTW, the crimp that I'm using on my handloads is pretty substantial. The bullet is a 155gr Nosler Custom Competition which does not have a cannelure. I use that Lee factory crimp die which says that you don't need one. I would rather not crimp at all, but I got worried that the bullet was getting even a little bit of setback from hitting the feedramps. Regardless, they shoot pretty well, but I don't want my rifle to grenade one day because the bullet seating got out of hand.

 

[Victor N TN]

Re: Cartridge growth in semi-autos

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: The Mechanic</div><div class="ubbcode-body">you guessed it. more neck tension or canalure </div></div>

You can check this with a marker by marking around the case mouth on the bullet and cycle it through. Check it and see if you can see a clean spot where the bullet moved forward.

Good luck with it. I've never had this problem myself.

 

[BOLTRIPPER]

Re: Cartridge growth in semi-autos

i was on a range here in Texas one day some time back , and had just miss witnessing a new JP getting the fuck blown apart, ....it pays to crimp your re-loads.

the owner of the JP is also a Hide member who shall remain nameless

 

[CTone03]

Re: Cartridge growth in semi-autos

Splody guns don't sound fun at all.

I tried the sharpy trick just now on a 168 gr FGMM, and one of my reloads. There is no difference by eye, telling me that this is not really a big deal. My reload grew again by .002", but I'm not concerned with it now. I am noticing that the bullets are gouged all to hell though. I know it's a rough ride in an AR, but damn! I don't know what's causing it, but I'm gonna find out.

Thanks for all your help!

 

[myerfire]

Re: Cartridge growth in semi-autos

As I read all this, my first thought was BS, but curious enough to put a few measured rounds in a magazine, pop it into my AR-15 and low and behold, the chambered rounds varied in growth by .002 to .004. Now I only did 3 rounds, but they all gained in length. Oh well. It hasn't been a problem and I'm not going to worry about it, I'll just make sure my neck tension doesn't decrease.
myerfire

 

[Clay_James]

Re: Cartridge growth in semi-autos

Same thing happens to me. It's never caused any problems so I don't worry about it.

 

[The Mechanic]

Re: Cartridge growth in semi-autos

I will venture a guess. Based on the projo not moving at all, I think that there is the possibility that in the process of chambering a hit at or near the neck or the possibility that the round is being sized slightly as it slams home in the chamber. Here is some direction for you. First try to slow chamber the round and use the forward assist with some sharpie marking along its length. remove and check for stretch. check for change in dimension at the base and at the shoulder. If nothing than try a bolt release to have a "normal" chambering. Check the dimensions again at the base and below the shoulder. My thought is either the cartridge is being slowed at the base on the chamber wall and the inertia of bullet is pulling the brass a little longer or the chamber is a little small and is "sizing" the brass ever so slightly and that will make the brass grow also if there is no contact of the shoulder.

 

[Grump]

Re: Cartridge growth in semi-autos

Methinks that you will find that The Mechanic has pegged this one.

Back in the days of the M1 Garand, it was well-known that the feed cycle "sized" the loaded round just a bit. Most of it was the extractor snapping over the rim.

Check the cartridge headspace on your Wilson Gage before and after and see what you find.

THEN look at a .008 (that's one of your larger reported dimensional drifts) gap in your calipers and think about whether this is something to worry about or just a factoid.

 

[Country]

Re: Cartridge growth in semi-autos

I was thinking the same thing as Mechanic . The sudden stop of the case in the chamber causes the shoulder to pull out slightly to the chamber wall by the enertia built up in the bullet weight.
If you sized a tight case with only a body die intil it just fitted into the chamber with no room at the shoulder I don't think this would happen. Naturally you can't run autos that way but it may prove or dissprove the issue.

 

[The Mechanic]

Re: Cartridge growth in semi-autos

Any update on this???