7.62x39 is one of the easiest conversions, just set back the barrel and rechamber. One thing to bear in mind is case length. .308 size and shorter rounds are fine. 30-06 rounds have trouble fitting in the clip. I have seen a few 06 conversions and the first and last rounds are the only ones that load. So effectively you can have 1 in the chamber and 1 in the clip for reliability. In the long run it would just be cheaper to get a starter reload kit and load for the 7.7. I use trimmed 06 brass then fire form it in the chamber.
I have a type 99 sniper rifle my grandfather brought back from Korea. Stock it did about 2.5". Bedded, trigger polished and recrowned it does a hair under an inch still in original form, not sporterized.