.308 bullet moving when AR10 upper bolt closes

[swift]

Most of my reloading has been for bolt rifles and an AR15. I always load a couple of empty, primerless cases with a bullet and feed the powderless/primerless cartrige from the magazine.

I'm loading some brass I collected after shooting a couple of boxes of federal gold match. I trimmed the case length to 2.005" - 2.007" and put in a Sierra 168 gr MatchKing HPBT. The OAL is 2.774". When the dummy round is loaded from the magazine, the OAL changes to 2.780".
I'm using a rockchucker single stage press with RCBS .308 X-dies.

I didn't have this problem with the AR15 loads, but the AR10 upper's bolt is much heavier. What should I do to fix this problem?

[Solidsnake87]

Sounds like not enough neck tension to me...........Perhaps go tighter on the neck tension.

[polygunner]

I don't shoot an AR-10, but I do shoot a M1-A, a lot.
I have found all of the handloads I've tried grow upon chambering by .001".
No matter how much crimp I use.
I don't crimp the precision stuff but I do crimp plinking loads. They all grow by .001" due to inertia.

I've never seen them grow by .006" as you have but for me .001" is common.

Maybe the bolt on an AR-10 moves at a faster rate so there's more inertia or it could be a neck tension issue.

Try crimping a few to see if the movement is reduced.

I think the reason you didn't experience this problem with the AR-15 is that the projectile is so light, way less inertia.

Good luck.
pg

[swift]

Thanks for the advice. Is there a way to adjust the neck tension on a case that is already primed?

I would like to load for precision shooting, so I'm thinking that crimping is not an ideal solution.

[polygunner]

If you have a sizing die that you can remove the decapper from the mandrel or expander ball.

Measure the inside if the case mouth if it's at least .002" smaller than the projectile you should be OK.

Make sure there is no lube inside the case neck also.

pg

[swift]

The neck has an inner diameter that is 0.003" - 0.004" smaller than the projectile's outer diameter. I'll check for case lube inside the neck.

Thanks!

[Bug Splat]

You can get a lee factory crimp die for like $7. They work just fine. IMO that amount of movement is not that big of a deal. Unless you are going for some serious accuracy (1/4 MOA) you should not worry about it.

[swift]

still doing something wrong

I tried a few different cases that had no lube on them, and the bullet still moved. I tried adjusting the neck tension to make it tighter, the bullet still moved 0.005". All the loaded empty and primerless cartridges did fit into the Dillon .308 cartridge gauge and pass this test. I did not try a different bullet. Next time I go to the range, I'll measure commercial ammo before and after chambering from the magazine to see if its bullet moves. Maybe the movement of the bullet is not significant, but it does concern me.

I'll be ordering the crimp die. I'm also wondering if I should get a different set of dies. I'm currently using X-dies from RCBS which, after an initial trim to 0.020" shorter than the max case length, are supposed to remove the need for trimming in the future. What do you suggest?

[polygunner]

Good idea to try commercial ammo.
If it moves then it is what it is I guess.

This is where consistency is vital. If the neck tension is the same on all your reloads then the bullet movement will be the same and you can adjust for it.

Try a light crimp but in my experience the bullet will still move.

Maybe someone with a better answer will chime in and give you some help.

A bullet with a cannalure and a moderate crimp would solve your problem but that's not very consistent with match quality ammo.

Good luck
pg

[popeye4]

The neck has an inner diameter that is 0.003" - 0.004" smaller than the projectile's outer diameter. I'll check for case lube inside the neck.

Thanks!

I'll re-read Zediker's book on this, but that doesn't seem like a ton of neck tension, nor does it seem light. I do recall that he said it didn't make much sense to have more than 0.004" as the brass then just goes "plastic" and you don't get more neck tension by going tighter.

I'm just reading his new book on AR rifles, haven't gotten to the big chassis part yet, but I understand the big rifles may be a bit finicky about spring power. Perhaps you need to slow the bolt down a bit......

[swift]

solution

Thanks for your advice. I couldn't solve the problem, so I bought a box of 168 gr MKs and used a commercial cartridge to set up my bullet seating die. It turns out that my bullets were seated a little too deep. Once I adjusted the seating die, my test rounds (cartridge + bullet, no primer, no powder) had their OAL change by 0.002" when I chambered a dummy round. I loaded a couple dozen cartridges and they worked fine. Now I just need to work on the load to improve my groups.

I must have determined OAL differently from what was in the Lyman manual. (I measured bullet tip to cartridge base.)