Thumper,
Based on your barrel dimensions, the .3115" diameter bullet is being sized down to about .309" average (Is there a possibiliy that there is leading forward of the chamber which is making the barrel smaller there?) at the start. As the barrel gets a bit larger toward the muzzle, gas will get by the now undersized bullet which will cause it to be much less accurate and explains the scuffing at the target (partial keyhole).
Since the bullet is .3115" diameter and the muzzle is not sizing it down, there is a possibility that the dimension at the muzzle could actually be a bit larger than that.
If the rifle was cleaned quite a bit over the years, a bit of cleaning rod wear at and toward the muzzle is possible which could make it a bit larger there.
Ideally, it is much better that the smallest diameter be at the muzzle.
In lower velocity cartridges, shallow rifiling works just fine.
A couple of fixes.....
1.) Use soft bullets (lead or no harder than 50/1 lead /tin / commercial bullets are much harder) that will be more "plastic" which will conform much better to the various dimensions in the barrel.
2.) Use PSB (Poly Shot Buffer) which has a consistancy similar to corn meal but is plastic. It will keep the gas behind the bullet, thus enhancing accuracy in less than perfect barrels. To use, allow enough room between the powder and the bullet so when compressed, there is a minimum of 1/8" of space for the PSB. Compress the PSB at least 1/3, meaning that at 1/8" space between the powder and bullet, put at least 3/16" of PSB over the powder.
Do not use PSB over smokeless powders quicker than 2400. B.p. ok.
In straight walled cases, a .06" thich LDPE wads can also keep the gas behind the bullet and improve accuracy.
3.) Lap the barrel so that the dimension is close to the same for the entire length. That can be done by "fire lapping" which means that bullets with fine lapping compound are fired through the barrel at very low velocity.
I would be happy to send you some PSB and other bullets to try......
Do you plan on using smokeless as well?
w44wcf