I would not worry about the difference in the size of those groups much. Strange thing about shooting groups, you seldom shoot the same spread group twice in a row, Has something to do with most us shooters! The difference between 1.380 and .908 just isn't that much! That's about .472, bit less than a 1/2". What would determine for me the value of the load would be the overall size of a number of groups. Somedays we just don't shoot as well as we do other days! Years ago I had a 7mm Rem Mag that I'd completely re-bedded and it shot great, averaged 5/8" groups. That does not mean every group was 5/8" but take a number of them and average them out and that's what I got. One of my most accurate shooting loads in it was with a 115gr bullet, not much of a big game bullet but boy did it shoot well. The way I decided what bullet to settle on is it got into a run off with the 160gr Speer and 154gr Hornady Spire Point! Speer won out by about 1/8" groups which didn't really amount to squat! But those were early years in reloading and at that time it ment a lot to me. Kind of ho-hum today!
Something you'll notice when shooting groups, 3 shot groups should be the smallest and it's one dumb animal that allows you more shots than that. But take that three shot group and shoot two more shot's often the group will open in size. Go to ten shots and normally will open more. I believe a lot of that is the shooter and the rest is a barrel getting dirty and hot. I've tried letting a rifle I was shooting on a to warm day sit about five minutes then touch the barrel and it's still hot! No idea how much that much heat actually effect's the bullet but probably some. Also I've super cleaned barrels thinking they would have to be more accurate and over the years I decided that I over cleaned them, the world famous fouling shot always seems to open the group some. Your out hunting and get a shot, the first shot you take will be the fouling shot!
if you shoot a number of three shot groups at a time, seem's they grow then too. maybe the heat, maybe the dirty barrel but some, has to be blamed on the shooter getting beat up with recoil! Seem's to wreack havoc with trigger control! So, go shoot several three or five shot groups over several days and figure out what the average size come out t
Your rifle is perfectly adequate, buy a steel plate, get off the bench and go shoot from field positions, that's what I do when I have a rifle/load that's not shooting to my satisfaction. It's pretty gratifying to walk out to 300 hang the plate and then over the next hour hammer the general center of the plate from an improvised position. That 1/2 inch variable is moot. If every round hit the 8 inch plate you got your Antelope or anything bigger.
I've seen other stuff you've written, you know your way around a rifle, I would tell you you're over thinking it. I would also ask if you're.01 to.02 off the lands with the Ballistic Tips.
