I have a new 106ur that I'm going to make into a 300 blackout.  Planning on leaving 2" of barrel past there gas port, then venting the rest of the 16" into the custom can I'm going to build.

How big should I drill the gas port?

I'm still trying to figure that out myself, for a different odd-caliber AK project. But unless you're willing to dig into calculus to get your answer, then I'd probably say start at .070, and slowly size it up until it functions. Maybe take a look at Krink gas port sizes, 2" isn't a lot of dwell time if you're porting everything after that.

I found one guy on the files that went all the way to 4mm on his port, but that was a 16" barrel build.  With the gas port so much closer on mine I'm hoping it's less of an issue.

Also, I'm using a full 16" barrel that's ported.  I'm going to have to hand load but the idea is to up the pressure up and have the threads keep the speed down.  Basically how the MP5-sd takes regular 9mm loads down (mine will be some where in between) and outs them out slower.  My suppressor is going to be a custom job.

How close to the chamber are you placing the gas port? That's a major consideration. That, and what loads are you intending on running? You mentioned using a ported barrel to (presumably) keep supersonic loads subsonic, but you're already starting with a boutique caliber designed to run subsonic, so that seems like kind of a moot point. See if you can get your hands on some pressure curves for .300 BLK and some other cartridges, and compare the pressure @ a certain barrel length, then correlate that with factory AK port diameters @ certain lengths, and see if you can find one that matches up. Seems like one of the Dracos in 7.62 is going to be the most similar to what you're looking for.

So, I'm using a 106ur and that puts the gas port about 6.5-7" (?) away.  The darco would be a good place to start , thanks for the idea.  Darco is a 12" barrel I think so that's probably going to be a good starting point, it would have extra pressure, but extra barrel too.

The thought of using the ported barrel is so that my hand loads would be just over super sonic in a regular 16" barrel, but the ports bleed off pressure so the threads drag it back down.  That's should help it cycle because it will have a little extra push.  I'm trying to cheat the system.

Right, there's 3 flavors of Draco that I know of: the standard, mini, and micro, with roughly 12", 8", and 6" barrel lengths, respectively, and then there's the Yugo M92 with a ~10" bbl. So figure all those are 7.62x39, which obviously has very similar internal ballistics to .300 BLK using supersonic loads, so we can reasonably assume they'll need similar port sizes to function. Assume the gas port is about 1" from the muzzle on those, so the one with the most similar gas port pressure to what you're looking for is going to be the mini Draco with the 8" bbl. However, it only has about half the barrel length beyond the gas port that you will have, so it's going to have about half the dwell time, meaning it's going to have a larger port than you will need. How much larger, I can't say. Also keep in mind that these pistols (except the M92) don't come with muzzle boosters, meaning they're probably grossly over-gassed, while real shorty AKs require boosters to run properly. But, you said it will have an integral suppressor, so there's your little bit of extra back pressure. This doesn't take into account subsonic loads, which run at significantly lower pressure than the supers, but also will have about twice the dwell time. That might even things out a little, but I would think it would still be a good idea to tune the gun around the subs, then load your supers to be safe and still function. Granted, all this is estimation to find a starting point.

As far as trying to knock supersonic loads down to subsonic velocity with just porting, I'm doubtful. A quick glance at Quickload shows that a sample of supersonic loads are already moving >1000fps at 2 inches from the chamber, and around ~1500 fps where your barrel porting begins. I don't see how 7" of barrel is going to produce enough friction to knock 500 fps off, but I could be wrong. I don't have the knowledge to run the numbers on that, but I'm sure someone here does.

Sorry if any of that comes off as me talking down to you, that's not my intention. I just want to cover all the bases, and explain my reasoning behind my thoughts. It also makes it easier to follow for anyone reading this later. One more thought: have you asked the butchers over at akfiles or gunco? They have a history of doing odd caliber conversions with AKs, and there will be more first-hand experience than my numbers-based speculation.

Doesn't sound like your talking down at all.  All seems like very helpful information to me.

On the friction, I'm going to hand load, so I'm looking at a load that would be, maybe, 1150-1200 FPS in a standard 16" barrel, just above supersonic.  Trying to drag down 1500 isn't going to work I don't think.  I want to function with factory Supersonic loads, but for sub's I'm going to go custom.  The goal is to game the system a little so I can boost pressure but keep below, also, if I'm using supers then its not going to beat the crap out of my gun.


Having said all that, the MP5SD takes a supersonic 9mm and drags it down to subsonic with a bunch of ports in a very short distance, so who knows what my results will be.

Just looked at an MP5SD barrel out of curiosity, and it confirms my suspicions: all the ports are drilled immediately in front of the chamber, meaning they're not letting the bullet accelerate past 1000 fps in the first place, not dragging it back down from supersonic velocities.

I think your best bet is going to be either dramatically reducing your unported barrel length and then porting immediately after the gas block, or figuring out an adjustable gas block solution, which would be preferable. You said you're going to build a suppressor for it anyways, so if you're monkeying around in a machine shop. Either doing the ghetto threaded set screw method or a more complicated, but more graceful method shouldn't be too hard.