Trying to guess velocity on my AMD65 w/12" barrel. Load in question is Tula 154 gr at 2100 FPS from a 16" barrel. I'm guessing it's about 1700 FPS out of the 12" barrel. Anybody have an educated guess?

I know that regular 122-124gr doesn't lose too much velocity going from a 16" to a 10", from 2300-2400 down to 2100-2200 fps

Here are some good videos to give you an idea:
https://www.youtube.com/user/chopinbloc/search?query=7.62

With Tulammo 154gr SP, I got (20 round average):

2090 FPS - 16 inch (Arsenal SAM7R)

1942 FPS - 10 inch (PAP M92)

The 154gr had the smallest velocity change out of the 21 different types I tested.

As stated above very little drop. A 7.62x39 does much better than an AR with a short barrel

You don't get down to 1700 FPS with 123/124gr bullets until you hit 7"-ish barrel lengths.

Good stuff, gents, thanks! I was surprised at the 154 about how little velocity it loses with a shorter barrel. This tells me they're using a pretty fast powder in it.

Yessir. For most commercial 7.62x39 offerings, the majority of powder is burnt in the first 7.5" of barrel. You lose a little less than 100 FPS for most commercial 123/124gr loads going from a standard 16" barrel to a 12.5" barrel (AMD 65, AK-105, etc.), about 200 FPS going from a 16" barrel to a 10.5" barrel (M92), and about 450 FPS going from 16" down to 8" (107UR). Notice the rate of that dip? The Mini Draco is about as short as you can take a barrel shooting 7.62x39 to get practical use out of the available bullets.

For what the round is capable of and and factoring in portability & usability, I think the AK-105 / Draco SBR's 12.5" barrel is the ideal length for an all purpose 7.62x39 carbine. For a super compact rifle that's still getting practical legs behind the 7.62x39, the Krink-length 8-8.5" barrel is as short as I would go. Legal nonsense notwithstanding (looking at you, NFA), I see no reason for a barrel longer 12.5" on a round like this.

+1. 1921fps from Herter's from M92: