I have two Howa's 1500 Mini Action in 7.62x39mm, one with a standard synthetic stock and the other with an EXCL LITE "tactical" chassis both 20" barrel. Also, I have a Ruger Ranch Gen.1 7.62x39mm with a16.12" barrel. I tested all three with a chronograph (Athlon Rangecraft):
Ammo: Ruag/MFS brass 7.62x39mm:
Howa Mini Action standard synthetic stock, heavy 20" barrel:
Av.muzzle velocity=2405.2fps (45 shots); SD=21.5fps
Howa Mini action EXCL LITE "tactical" chassis heavy 20" barrel:
Av.muzzle velocity=2418.3fps (56 shots); SD=21.9fps
Ruger Ranch Gen.1 16.12" heavy barrel:
Av.muzzle velocity=2306.0fps (39 shots); SD=25.5fps
Loss on a 16.12" barrel: 99.2-112.3fps; more or less about 25fps/inch.
P.S. Also, there is a pretty good study here: https://rifleshooter.com/2016/01/7-62x39-mm-russian-effects-of-barrel-length-on-velocity/