At range yesterday, seating depth test. 4 rd groups, .030 length increase between groups. At lands length is thought to be known. Last group is .010 off lands. Started at shortest length, working my way up one group at a time, have twelve groups loaded. Ok, so I get to the tenth group- first shot velocity is 100fps faster than the previous shot. Next shot is 20fps less, shot 3 is 7fps slower, and number 4 is 50 feet slower yet. No hard lift or hard close on bolt. First cartridge in group 11- I couldnt get it to chamber. Thought maybe I got math wrong on to-lands-length, just packed and went home. Then got to thinking, this makes no sense. Built a dummy rd shorter than last shot fired, it would not chamber. Sharpied up the bullet, and just above the case neck, I see marks, looks like lands, but way way shorter than the ogive.m must be the leade, right ? So now, I clean the rifle up, new dummy round at .008 longer that longest test group, and bolt closes. Obviously chamber was so powder fouled, the rounds would not go home. Or at least, that is what I hope. Is US869 a dirtier powder than others ? Any one else with a similar experience? Really scratching my head over this. Rifle was clean when I started, and only 40 rds thru it ?