A little bit ago someone asked about the 7.62x39 imp.
It turns out, I still have the competitor barrel in that cal. I also found I had an identical 14" barrel in regular 7.62x39.
The reg 7.62x39 clocked 2107 fps from factory stuff. I then pulled a few bullets and used the powder and bullets in the imp version to get as close as I could to a fair shake. I used comercial brass that gave the same vol as the mil spec stuff. What I didnt have were any hard cup primers, CCI makes them I think for mil loads.
Anyway, I got 2276 fps and the cases would fall out of the chamber. Another .3 grains gave me 2309 average, but the primers had a slight crater starting, but would still fall from the chamber with a rap on the side of the gun without engageing the ejecter. I see why those flint hard primers are important with the hot loads they run in the original. What 6" more barrel would give you I dont know, but I was impressed with how well both did in the short 14" barrels I tried them in.

Interesting results. I'd expect a 6" longer barrel to gain 200-300 fps.

Aside from velocity, the big advantage of the improved version is case taper is reduced a lot and one can use straight magazines in AR type rifles, solving the feeding problem.