Recently I picked up a Dillon 1050 with Ammobot, and I'm trying to load large quantities of reasonably accurate 5.56 ammo with it. For several range trips, I was frustrated to discover that I couldn't get my reloads to reliably shoot better than 2.5-4 MOA from my LMT MRP. Now I realize that this isn't a match rifle, but I do think it should be able to do 1.5 MOA on average.
My first thought was that my brass was the culprit; I had been using a bunch of old mixed brass with varying headstamps and number of times loaded. I eventually bought some virgin brass (RUAG brand if it matters) and found that it was extremely consistent in weight, length, and headspace. The case mouths were even nicely chamfered. I reloaded it right out of the bag without doing any kind of processing, except for slightly flaring the case mouth with a Lyman M die for ease of bullet seating. These rounds were loaded on the 1050 autodrive with the same die setup as my previous loads. After tinkering with a few bullets and powders, I noticed a significant accuracy improvement, and some loads would shoot 1.25-1.5 MOA.
Thinking that having new, consistent brass solved my problems, I gathered up the once fired RUAG brass and processed them using my usual setup which is as follows:
1. Decap
2. Dillon RT1200 trim and size
3. Neck expand to .223" with a Sinclair carbide expander stem
4. Corn cob tumble to remove lube
I then loaded this brass again with the same loads that performed well during the previous range trip. Much to my surprise, I was not able to replicate the same group sizes. In fact, this ammo only seemed marginally more accurate than the mixed brass bulk loads that I had started out with. I would say 2.5 MOA was about average.
This past weekend, I went out again and shot more groups using ammo loaded in once and twice fired RUAG brass. The accuracy was crappy. But then I brought out a few rounds that had been loaded in virgin brass, and accuracy immediately improved. After seeing numerous ~2.5 MOA groups all morning, I immediately shot four groups in a row that ranged from 1.15 to 1.5 MOA.
So this leads me to believe that there's something very wrong with my brass processing procedure. I checked many samples of my processed brass and didn't notice any that were improperly headspaced or had noticeably severe neck runout, at least based on a rudimentary check spinning the rounds in between the slots of my mini mill's table with a test indicator. What else could the problem be?