Why do people say they need .002 or .003 to avoid pushback in semi-auto guns?
I'm working on making sure I have consistent neck tension on a bunch of 20 Vartarg rounds I'll be loading during the snowy winter season. Recently I realized I had some learning to do. I saw this thread, and there's some interesting discussion here.
The question was asked about neck tension for semi-auto guns, and I recalled a big lesson I learned with my AR loading.
I was "testing" seating depth on a 20 Practical AR rifle load. It seemed that regardless of the seating depth I tested, the results were all the same. Seemed odd. I asked for help here, and someone suggested I verify the bullet was staying in place, and not sliding out into the lands when the bolt cycled.
I put in a round I had loaded. letting the AR bolt slam forward as it normally does, and the bullet was jammed into the lands. When I tried to extract the round, powder went all over, and had to get the bullet out with a cleaning rod, oops! I should NOT have used a live round for the test. Brain fart.
I had used a 226 bushing for all the brass when testing seating depth. I then tried the 225 bushing, and it seemed to stay in place, but I wasn't convinced it was solved. To be sure, I settled on using 224 bushing and the bullets stay in place, didn't move forward at all when chambered.
So, regardless of the opinion expressed by some above, the neck tension changed when I used a smaller bushing.
Another factor I think needs to be included is work hardening to the brass, or if it was annealed. Work hardened brass has more spring-back, and that's a reason some anneal after every firing. Others might never anneal, and consistency from one loading to the next is probably the most important factor either way.
And lastly, the thickness of the neck would also vary how much "hoop tension" the neck can provide.
Since most of my 20 Vartarg brass is newly formed, I wanted to focus on final brass prep, and having it all uniform, and the ID of the necks, and neck tension is what I'm focusing on now, before I load a quite a few hundred.
I purchased some neck tension mandrels for .002 and .003 tension from K&M. I just got them, so still getting info together for my plan of attack. Looks like I'll try using a 225 bushing to
size down slightly, then expand with the .002 mandrel, to create a consistent inside diameter with 2 thousands of tension. I was using the expander ball, but I think this in then out sizing will give me the most consistency.
Also, going to lube the necks some way before loading, probably with Sinclair graphite dry lube, so I don't have the metal-to-metal contact of brass to bullet, since this is newly formed brass.
Stuff to think about, going down the rabbit hole deeper.
