TOP PREDATOR.....keep bringing the info....your setup is very close to mine but no way could i print 3" groups at 400yds....I cant even print that at 100....
105GR, i just took alook back at page 1....i'd think that IMR4350 is way too slow for your barrel length, you may have to go with IMR3031, IMR4198, or something around that same burn rate. i have a .308 i cut from 22" to 17", couldn't get much more than 2400 out of it using 4064 so switched to 4198 and now getting 2800+ using 110 vmax.
this may be a silly point, but let's go through a little check list:
brass being trimmed to proper length? (i'm at 2.040, splitting the min and max) make sure to take the burr off with chamfer tool
are your dies properly set?
are you attempting to crimp these things?
are the bullets straight in the case?
you said your single feeding, that takes out the bullet creeping in the mag due to recoil, but make sure your feeding them correctly and not jamming them in there.
factory barrel or replaced one? get your headspacing checked
start from scratch keeping things simple - scrub the shit out of the chamber, the lugs races, and chamber face take out that variable just in case there is gunk in there. take off the can, inspect for damage / clean your crown. a 1976 rem700, perhaps the throat and barrel is just used up. .243 are known to be used up around 2,000 - i can't speak to this as i don't have that many rounds down the pipe yet.
buy a box of factory 100gr hornady or even remington ammo just to see if your having the same problem. if not, it's not the rifle, but something wrong with your handloads. plus you'll have 20 more pieces of once fired brass.
if there is no problems there, go back to basics...scrap neck sizing for the box you just shot and just FL them AFTER taking the die apart and cleaning it. re-read the directions on the die again to properly set them. tumble clean, trim to 2.040 if they get stretched that far making sure your not somehow deforming the neck with your trimmer, chamfer, check your primers to make sure they are fully seated. look at the case, compare it to a once fired or even a pulled apart not fired case. make sure there are no buldges in the neck, and especially where the shoulder meets the body of the case. < that'll definately cause some funky chambering. seat the bullet to 2.780. don't try to crimp 'em. take your time single feeding to make sure your not jamming the bullets further in the case or deforming the tip somehow.
if your still having feeding issues after that, i have no idea. if the feeding issues are now gone, stick to what works. if the groups are still funky, (but no feeding issues change powder - i think 4350 is too slow for that barrel length, there's no use F'n with it anymore your just eating up other components and adding to the frustration.
then mess around with neck sizing later after you have a successful foundation to go back to.
that's all i got.
i think ACC (the OP) has a whole bunch of info to go with too on the original topic through all these posts, just may not be stabilizing in a 1:10. i originally shot a box of hornady 100 BTSP through the 1:9.25 to "settle in" the barrel, clover leafed 5+ rounds at 100yds. if they are working for you, though it's worth a try, there's no need to try to force a "guccier" bullet if the SP are working for you.