if you were to put the 2 rounds side by side,you'd see a VAST difference in case length- and the russian 7.62 is not the same as ours in bullet diameter either, even though the numbers look the same- ours is a .308 diameter, theirs typically is somewhere about a .311- they don't make the 99 anymore, so if you do anything drastic, you'll never replace it- btw , you still get spare mags for it if you have one of the mag types- and i have used 7.62 x51 nato in mine- it doesn't like it very well b/c of the bullet weight- performs much better with 308/165 or 308/180