Robert357 - is it common to use different size bullets in the same rifle?
It is not common if you can find a proper sized bullet. Unfortunately the 7.62x54R does has a lot of variation in the rifles that shoot it and not many bullet manufacturers who produce bullets just for this round.
Especially on military surplus rifles you should slug the bore of your rifle with a lead fishweight or a cast lead bullet slightly larger than the bore size. My MN has about a .311 bore. Some that have been well shot have slightly larger and some Finnish reworks have .308 bores from what I understand.
(see the following site for a suggestion on how to slug your bore)
http://www.surplusrifle.com/reloading/slug/index.aspA .308 diameter (.30 caliber) bullet is very common. However for the Russian MN (and SKS) rifles finding a proper sized bullet is sometimes difficult.
Many Russian rifles (MN's & SKS's) have a nominal .310 bore. There are some jacketed bullets available in this diameter. There are principally in the 100 to 125 grain range (there are some 150 and 180 grain .310 bullets). Alot of the MN's were shot exensively and have larger bores prior to the re-arsenal work that was done on them.
I have used a micrometer to check on the diamter of pullet 7.62x54R bullets Warsaw-block milsurp bullets and found that the max diameter on many is .310 except for a small band near the base that is .311---go figure.
Some bullet manufactures produce jacketed bullets for the .303 Brit round. These are usually .311, or .3115. There are a lot of "well used" military surplus 303 Brit rifles out there and some bullet manufacturers sell .312 jacketed bullets to meet this need.
There are other rifles like the 7.7x58 Japanese Arisaka and the Argentine 7.65x53 and the Winchester 32-20 that require bullets of .311 to .312 diameter depending upon who you talk to. The 32 Auto (or ACP) pistol round also is about .311 to .312 diameter bullet, (but is generally found in jacketed bullet weights of 60 to 80 gr).
The difference between a jacketed bullet of .311 and .312 is such that you probably will not see any difference in accuracy from a given rifle. You might see a loss of accuracy is you tried to shoot .308 bullets in a .311 or .312 bore. Personally, I have run some experiments with my MN 1891/30 using identifcal powder loads and similar bullets in .308 and .311 diameters and found no noticeable difference in group size at 100 yards with iron sights. However I still try to buy bullets that are close to the proper size.
I can also interchanges with the same powder load in my MN1891/30 174 grain .312 bullets and 180 grain .311 bullets and see not differnce in group size at 100 yards with iron sights.
I have shot 32 ACP out of a .308 bore and it works. I just would not want to do it alot because of barrel wear and copper fouling.
Using either .310, .311, .3115 or .312 bullets in a 7.62x54 R is considered acceptable by most folks as long as the accuracy is OK and if the bore is a well used Russian MN, it probably is just fine.
Now cast bullets is another matter, but there you need to size the bullet to just slightly more than the max bore size.