You're asking A LOT from a suppressor. There will be a ton of un-burnt powder that gets thrown out of the end of a 7" barrel in 7.62x39. It will indeed erode the baffels with enough time.
The best thing to do is to get a suppressor that has a muzzle break type muzzle device. The muzzle device will take the lion's share of the flame cutting/erosion and is much cheaper to replace than the suppressor.
The other thing to consider is a nice STAINLESS STEEL suppressor. I don't know of too many rifle suppressors that will cover use on a 7" barrel for their cans. I think 10" is the shortest I've seen with full warantee.