Jake,
If those scuff marks haven't been made "all" the way around the bullet, and are only half way around or so, you're in trouble! That would mean it has an offset neck and/or leade (freebore). The leade is usually around .0005" over bullet diameter, which would be .3085" in your case. If the scuff marks are all the way around the bullet and not just the side we can see in the pic, the bullets are oversize. If you think the end of the chamber where the case mouth starts into the leade has a burr around it and is shaving copper, you can polish it up to make sure it isn't. This transition from the mouth to the leade is usually cut at a 45 degree angle, so it doesn't cause burrs anyway. Sierra bullets are sometimes oversize, my brother couldn't throat his 338/378wby because his 300gr SMK's were all .3388" in diameter. They had the reamer ground with a .3385" diameter leade, so they now had to polish the chambers leade up to .3388" just to handle that 500 lot of 300's he had. They only noticed the problem when attempting to throat the thing.
I can put those same marks on "one half" of a side in my 300 Ultra ammo if I chamber the rounds 180 degrees off from the position where they were originally fireformed in. This is because the neck in my chamber is eccentric, and it really sucks too!
If you were able to easily seat a bullet up to the lands to measure the OAL to land contact, I would assume the chamber is eccentric in either the neck or the leade area. First, I'd measure the RO (runout) on the case necks (fireformed ones) to see where you're at there. If they are .003-.006" eccentric, you could easily get scuff marks on one side of the bullet if they are chambered with the RO at 9 O'Clock when that case was originally fired with the RO in the 3 O'Clock position. The quick cure for this problem is to alway FL size the cases to reduce or elliminate RO, or mark and index your cases when firing them all the time. Mine happens to shoot much tighter groups by indexing and neck sizing so that's what I do... it's a pain in the *** though!
If you don't find marks on just one side, and they show up all the way around the bullet, I'd suggest calling Sierra and having a talk with them on getting some smaller bullets... FREE! Second, I'd call Mr. Romain and have him measure that reamer he uses in the leade area and segregate the bullets you have on hand so you end up with some to try that are .0001" under size, .0002" undersize and so on to find how much smaller the need to be to chamber easily. My guess is, they will work at .0000" or maybe even +.0001" larger than his reamer spec. A chamber caste will tell you exactly how big your leade diameter is too.
I can tell by the pic that you are not into the lands at all, they leave distinct grooves in the bullet... not scuffs. By the marks toward the tip of the bullet, you may be pressing the lands, but not into them.
Besides, you'd never get them that far into the throat if you drove the bolt closed with a hammer... it would push the bullet into the case LONG before that point, you'd also leave the bullet in the barrel and powder in the mag when withdrawn too. I'd say yours and Chris's OAL measurements are right on the money!
The bullet is either not being allowed to align with the throats axis ( chamber and/or Leade eccentric), or it's too big to fit into the leade itself.
The helix marks at the rear just indicate the bullet rotated in the leade a little but, much less than the case head did on the boltface or the bullet inside the neck when camming and closing the bolt.
I'd worry little about the carbon on the neck, but it does look like it's up the length of the neck farther on the "top" of the pic than that of the "bottom." This could indicate better sealing on the lower side and thus and an offset neck to that side... that's a big maybe though.
I wouldn't let the carbon build up there or headspace can get tight. Clean em off each time and when they get sooted the whole length, I'd anneal them. You obviously sized the neck down to hold this bullet here, so maybe the neck already is hard enough it sooted up badly and was just cleaned up by the bushing passing over the neck.
I doubt Ray cut an eccentric neck or throat, unless he used a seperate neck and throat reamer and had a loose fitting pilot...
I'll bet your bullets are just tooooooo big.
Mic the bullets and the reamer and you'll clear that one up quick.
Good luck, and I hope that's all it is too. Believe me, you don't want nothin to do with an eccentric chamber!! Factory chambers suck hind tit!