Neck Tension for 308 Semi-Auto?

[ashiha]

What options do I have for preventing the projectile from being slightly pulled during chambering in a semi-auto?

What's happening:
The bullet pulls ~0.010" when being chambered from magazine in my Aero AR10.

LC once-fired brass annealed with AMP.
Resized using Forster Bushing Full Length w/ 332 bushing. 0.002" shoulder bump
Final neck tension is ~0.005"
Normally I use a 0.306" expander, but I abstained for this test as it would be even less neck tension
I can't test any tighter without buying stuff.

Bullet is a 155 TMK measuring 0.3075" but I've also tested a 175 SMK.

Is going tighter reasonable? Do I HAVE to crimp? I've never had good results with accuracy after crimping anything. I don't have one in this caliber yet to test.

 

[Walt Krafft]

Are you cleaning your necks? If not try with a bore brush. What is your loaded round neck diameter?

 

[SeabeeKen]

Sounds like time to crimp

 

[MisterHelix]

The Lee Factory Crimp Die is really cheap.

 

[DiffEQ]

The Lee Factory Crimp Die is really cheap.

That's what I use for every one of my seven different semi-auto AR cartridges.

 

[ashiha]

Are you cleaning your necks? If not try with a bore brush. What is your loaded round neck diameter?

Lately I've been using a nylon brush, but these in particular were cleaned with stainless media. So they're pristine. Loaded OD is 0.337"

 

[robert8904]

I crimp if the bullet has a groove. Otherwise, I have minimal to no bullet pull by not annealing, setting .003 interference with a collet die, and getting the brass squeaky clean in a sonic cleaner.

I feel un-annealed brass has more spring back and so grips the bullet somewhat tighter. Much more than .003" interference fit is just farther expanding the case necks without increasing grip on the bullet.

I'll follow this thread since the OP's question is interesting and I'm sure open to any better solution for this problem.

 

[6Creed73]

Another vote for the Lee Factory Crimp die. I use it for all my AR loads and never have an issue with bullet movement or accuracy.

 

[243winxb]

Final neck tension is ~0.005"

Gun needs different lighter spring? Carbine & Rifle different springs? Adjustable gas block?

https://www.sierrabullets.com/reload-basics/reloading-for-semi-autos-and-service-rifles/ At Sierra link- See Neck Tension

 

[ashiha]

Gun needs different lighter spring?

https://www.sierrabullets.com/reload-basics/reloading-for-semi-autos-and-service-rifles/ At Sierra link- See Neck Tension

Cool data! I don't have an arbor press yet to measure something similar. Unsure how he even did it for the pulling.

I can't really mess with the spring. It was difficult enough getting the gas/buffer system balanced to work suppressed and unsuppressed.

I appreciate the feedback everyone. I'll get a crimp die and do some more testing.

 

[243winxb]

measure

Go by outside neck diameter before & after bullet seating. The 5.56 needed to grow by .002" for 35 pounds bullet pull.

Your Final neck tension ~0.005" should be enough? Over annealed brass?

 

[Zero333]

For my M1A... I'm good with 0.0015" - 0.002"... then I use the Lee Factory Crimp die after seating the bullets. Never had any issues. And the M1A beats the crap out of the brass when feeding and ejecting.

Some 308win seating dies (like the green box RCBS dies) allow you to taper crimp the end of the case mouth at the same time as seating a bullet.
I don't recommend seating and crimping at the same time.
Seat all the bullet first, then set the die to crimp if not going the Lee Factory Crimp die route. Just make sure you back out the seating stem before setting up the taper crimp on the seating die.

Before I got the Lee Factory crimp die I never used to crimp for the M1A and the neck tension I mentioned did the job without any problems. I've checked several times if bullet set back was an issue without the crimp.