LR-308 COL change after chambering. Help!

[generalist]

Hi new poster.

I was hoping I could get some help. I am setting up my first "benchrest" rifle. Just put on a new 20"' 1:12 Kreiger barrel. Working up first set of oads with Redding's new 308 win NM die set for 308 gas guns. sizer, comp seater, and a taper crimp die.

Anyway. so I load up some 168MK's to max COL with magazine, which is not as close to lands as I am used to, but best we can do if we don't single load it.

I chambered a dummy rounds loaded like the rest but no primer or powder. Same new Nosler brass, bullet, etc.

So I put the dummy round in the mag, and let the bolt close on it's own force using the the bolt catch to simulate a cycling gun loading itself.

I withdrew the round and the COL grew about 0.005" to 0.01" due to the inertia of the bolt slamming then the round then stopping on its headspace. Still off the lands (so that is good) No surprise about the creep; but I figure that can't be good for match accuracy since every round the bullet will likely not "jump" during chambering the same each time.

So I try the redding taper crimp die that redding included to add some post loading neck tension thinking this will stop it. Even to the tightest setting, the bullet still jumps the same.

So I broke out the trusty Lee Factory Crimp die I had, cranked it down to max and even distorting the neck a little with a VERY tight crimp, it still jumped!

Anyone have any ideas? Should I be concerned about this? how to fix?

The sizer is just a FL sizer die, not a neck sizer. Carbide button, so no neck lube is used.

Thanks in advance.

 

[gcoulsey]

Re: LR-308 COL change after chambering. Help!

I am not familiar with gas guns, others will jump in here better informed to help you out, but I'm surprised your getting movement after all the crimping etc..
The brass isn't getting marked somehow on the head of the caseduring the cycling action, giving a false reading? Have you double checked with a factory round?

 

[memo43]

Re: LR-308 COL change after chambering. Help!

for a gas gun this is somewhat normal. think about it, u have a stationary object that is getting slamed foward, how much should it "jump" foward I dont know. It varies, that is why a boltgun is more accurate. every round is same OAL. unless you can walk the bolt foward, or get a looser buffer spring that will not hurt cycleing.
hope it helps

memo

 

[generalist]

Re: LR-308 COL change after chambering. Help!

Don't have factory rounds to check, but the case is not getting hung up on anything. How "tight" could I neck size this down to i.e., how many thous squeeze? Current loaded rounds with the new Nosler brass measures 0.3370" OD at the neck. How small could I squeeze this neck down to and still get good accuracy? The FL die is giving it a 0.002 squeeze right now, obviously that is not enough. Even if I remove the expander ball in the FL die, the neck will come out 0.3290" OD. That is a 0.008" squeeze and it STILL jumps about 0.007".

Anyone else tried this with a .308 gun and their loads to see?

 

[CTone03]

Re: LR-308 COL change after chambering. Help!

Perfectly normal in auto loaders.

http://www.snipershide.com/forum/ubbthre...376#Post1410376

Think about an inertia bullet puller. Same concept. As long as you have brass with decent neck tension, you'll be fine.

 

[Goose375]

Re: LR-308 COL change after chambering. Help!

yeah the super crimp you put on it should have not jumped..