Okay, I've been musing about getting a Romanian RPK (AES-10B), which caused me to wonder about the effect of barrel length on velocity with 7.62x39mm ammunition.
I found this article from 2016 which was a well executed study, although with only two types of ammunition. In any case, I thought it was interesting that they used a single barrel which controls for differences in land/groove diameters and wear, and any other differences when using multiple different firearms. It also enabled collecting data on many more barrel lengths. (Nevertheless, it would be interesting to see a comparison of many types of ammunition with an AK, SKS and RPK.) They cut it down by successive 1" increments (2.54cm for the rest of the world) and fired 5 shot groups with each type of ammunition at each length of their fairly heavy barrel.https://rifleshooter.com/2016/01/7-62x39-mm-russian-effects-of-barrel-length-on-velocity/
A couple of things that stand out to me:
1. The RPK barrel length seems to be at the maximum of the velocity curve; added barrel length is of no benefit in velocity or energy. The SKS certainly has a bit more velocity than the AK. (Parenthetically it's irritating that they chose inches when the standard barrel lengths of the common firearms are in centimeters).
2. Powder choice makes a big difference. It looks like that old Russian TCW white box was optimized for AK length barrels based on the tight standard deviation at those shorter lengths. The TCW also has a flatter curve, perhaps due to a faster deflagration rate for the powder used in this ammunition, such that maximum pressure is reached earlier in time, thus propelling the bullet to higher velocities sooner during its travel down the barrel. The TCW has a slightly smaller powder charge in terms of weight, but higher ballistic energy across all lengths. Again, this points out the big differences in powders formulations and the shape of the powder
The percent difference between the velocity of the 23" vs. 16.5" barrel length for the TMC was just 4.3% while it was 6.6% for the Brown Bear. In terms of ballistic energy, this percentage difference is 8.8% and 13.7% since E is dependent on the square of velocity;
E = 0.5 x mass x velocity2
Here are a table and chart drawn from their data. I converted the fps numbers to make the ballistic energy calculation easier. The old British feet/inch/slug/ft-lb system that we use makes the conversions a bit cumbersome
Pardon the mixed measurement systems for barrel length and velocity but they used inches so it was better to present that in inches to get even increments on the x-axis.