Gas Gun Neck Tension and sizer ?


Sergeant of the Hide
Jul 11, 2013
Coldwater, MI
Thinking about doing a semi-auto precision build (6.5CM or 224 Val). I’m set up for my bolt guns with Redding type S FL dies and use sinclair expander and turning mandrels for neck tension. Is that sufficient for gas guns or do I need to pick up something like a lee factory crimp die as well? Also is the use of a small base sizer die something still used for precision semi-auto work? My only previous semi-auto reloading experience was just 556 range ammo, so I’m wondering what all is needed pst what I have to get started.


Sergeant of the Hide
Feb 13, 2017
I have had .002 neck tension for loads but had them set up expecting bullet set back. For my 556 it was .001ish set back. WIth my 308 AR it was more of set back and caused problems. I ordered K&M custom expanders and that has been titties. You should not need a SB die.


Sophisticated Redneck
Dec 13, 2017
Not a fan of crimping as I have experienced more bullet movement with that method vs neck tension with a Redding comp S die. .002 typical and test the round without primer or powder with 10 chamberings to check for movement. Most I have seen is .001 change in COAL which is good eneough for me. I load for 556 and 6.5CM on AR platforms.
Jan 15, 2005
I've had great luck using a Lee crimp die but I've still gotten better results from other methods. I've found .003" tension to be the balance between enough to keep the bullet from moving most of the time, without having heavy tension. I also try to choose a powder that will end up being a compressed load and that helps too. The bullet can't be pushed back if there's a compression that's close to pushing the bullet back out of the case.

With .003" I still never chamber the same round twice from the mag, especially in loads that aren't compressed. I've found the second chambering to cause the bullet to be pushed back most of the time. If a round gets fed from the mag and not shot I simply insert it by hand and close the bolt the next time.

I've never used small base dies, never found a need. A standard FL of FL bushing sizer has always worked for me.