Neck tension is a huge factor wthen trying to reduce Standard Deviation, either resistant or smooth straight accross the load. Too much is an issue for pressure problems, but I think a person would stop and rethink the reloading procedures. My 300 win mag is tighter than my 308, but I'm sure it's becuase of the dies. I only full length resize every 4 loads. My Thompson 300 win mag is really fussy about that. If I don't FL resize by the 4 th reload. They will enter the chamber just fine, but need a ram rod to remove them. I always cut them back to length on every load and debur. Thompson Ventures have a tight chamber. They also varry greatly from gun to gun. Out of 3 Venture's, all 300 win mags in our family, my nephew and i can reload for each other, but my bother's Venture could not be reloaded for at all. A call to the factory accomplished nothing, so the gun went down the road, hopefully to a hunter who doesn't try to reload. Well, I'm getting closer to the sweet load for my FCP-K 308. 44.5 gr. of Varget, 155 Sierra Match produces nice groups @ 2792 in 25 degree weather. I may back it off a little to see if they tighten up. Only 100 rds through the tube at this point, following all the break-in rules. It's getting there. They cancelled our shoot for tomorrow because of the weather, "WET". My philosophy came out of the Army, "If it ain't raining, it ain't traing"
. The heavy guns $ don't like it when it's wet
CFT, get that CM shooting out to 500 yds and see what kind of groups you get.