My $0.02/ experience isn't specific to .277 bullets. I load for numerous rifles and have used many different bullets. I do all of my initial fire forming and barrel break-in with midway factory seconds at Jam. I've use the ones that seem like hornady and the ones that are clearly Sierra.
The first step I take when loading a new bullet is to seat everything either to jam or maximum magazine length over a middle of the road powder charge. Sometimes Jam is under mag length, especially with SMK bullets, sometimes with eld-m/x.
Next, i take all of my rounds, usually no fewer than 30, to the range. I bring a lee hand loader, seating die and my caliper with comparator. I measure and check that I have 2 rounds the same length. I shoot 2 well placed shots using all the proper fundamentals of marksmanship. If they don't touch, I push the bullet back 0.006" on 2 more rounds and repeat until bullet holes are touching. Then you go 0.006" further. If it opens up you go back 0.003" if it is touching you go back 0.006" if it's touching you seat somewhere in that 0.006" window. I fire another 3 rounds to verify seating depth, then, if everything is nice and tight, seat all remaining rounds at that depth and shoot.
I use this process after initial fireforming to get all of the brass evened out and get the new barrel broken in.
The point of this long winded spiel is to say, you need to run a seating depth test. There have only ever been 2 rifles I couldn't get down to where to where 3-5 shot groups were sub 1/2". The culprit in both cases were the hornady eld-m bullets. I changed to bergers on one and SMK on the other rifle and they both shoot small now. Sometimes it's bullet weight or construction that the rifle doesn't like. Sometimes it takes sending it to a different Smith to find out the throat is cooked, Lol.
