YES, you can find different sizes, even in the same brand.. I did copied a design and used a comparator, they matched o.k.
remember with pressure, diameter can be lower than specified but never higher.
on certain 7.62 ammo I have found bullets measuring 0.3062 all the way to some 0.3082. I can only tell you to use what works in your rifle, if you are using brand X with good results, go with that same diameter. do not try to make something too exotic, remember we dont have any special rifles to try out the new bullets, and stupid... hurts...
here in mexico its very difficult for me to get nice barrels, and I for once dont want to mess with them. its also tough to get nice bullets, I know that when I run out of expensive bullets, I could still have a way out by "rolling my own".
joe