Fit is king .
The more variables you remove at the bench the fewer you will have on the range.
I had what should have been 1 lot of LC 43 . After a lot of shooting with 2-3 of 5 kissing and the rest of 5 18" away I emptied 128 rounds with breathing, trigger , hold etc . I had 4 very clearly defined 2.5" groups and a few fliers . I discovered that the rifle put bullets in a different place for 2.2 gr of case weight and that I had cases that ran from 178-207 gr and varied about 10gr of water weight . When those variations were eliminated I had 5 touching in .980 outside groups .
Change one thing at a time .
I changed primers in an SKS and groups dropped from 8.5×11 at 50 to 3×5 at 100 .
If it chambers freely it's not too big .
I've had a 7.62 and 7.7 that needed .318 to fill the throat and .316 for the grooves . Also a 6.5 that needs .272 .
There are situations where filling the throat and swaging a bullet into the grooves won't work . A .301 would slip fit and chamber but the last 22" of barrel were .277×.284 .
With jacketed I have the example of jump to the lands doesn't matter , obviously it can but when you have an 06' that that groups the same with the bullet .08 in the neck as .350 , and will take a 4" OAL you get to use the argument. However I'm only now loading to test the theory in a 25-06' and 7×57 .
Alloy doesn't seem to be in the equation until you break 32kpsi . Obviously dead soft even PC with a check isn't going to be really happy in full boat 9 , 357 , and 40 but it works in 30/30 and 45-70 . Linotype will probably foul a 45acp , 38 , and 45-70 , so apply some sense to this .
Some bullet designs in some applications want/need/are best served by a huge kick in the base the next time they want a long flat push ......of course both happen in the same .007-9 seconds after the sear breaks so it's a perspective thing .
Have faith in the knowledge here .
My resume' includes learning to paper patch because gas checks were expensive and I only had to buy a sizer once .
15 yr and I still wander around in the weeds with some of these guys shaking their heads in both often disbelief and very rarely awe . My triumphs include matching jacketed speeds and groups in 222 ,30-30 ,32 Rem, 358 Win , 6.8 SPC ,7.62x39 , and 45-70 . Also every pistol from 380-45 Colts with cast , stops at 38 Short 357 mag and Ruger level Colts leaving little to nothing to collect if I wanted a 44 .
These guys and more like them got me here .