I was going to echo some of what mjkeat has posted.
The 7.62 x 39mm is a powerful, POWERFUL round inside of about 200 yards. So for a multifire weapon with multiple rounds on target, it's going to do the job.
The problem is, unless you are building a street to street, urban battlefield rifle, it's going to present "issues" in reliability and accuracy.
The ammo isn't "spec", which means it varies quite a bit from lot to lot and sometimes round to round in a single shot, precision category.
If you have a want for this type of rifle, your big issue is going to be tuning the weapon in the following categories:
1) Uneven ammo pressure
2) Feed ramps
3) Magazines, Magazines, Magazines.
Those three categories are going to be key to building a reliable, accurate and consistent weapon.
I would look at a different application.