From looking around I found that the AK-104/SLR-107 CR variants have 12.36" barrels in their intended length.

Is there any effective flash hiders available that could be used to pin to the barrel to make it to 16"?

I saw the CNC Warrior extended 4-piece Bulgarian style brake, but it's not what I'm looking for. It seems it would make the rifle front heavy which defeats the purpose of what I'm trying to do.

I have an SGL 21 and 103K style Saiga conversion (original front sight block pushed back to the gas block) that I would like to have converted to AK 104 configuration. I believe the only way to remove the pinned 74 style brake on the 103K would be to cut off the muzzle brake with the threaded portion of the front sight block and barrel underneath it. The rest of the sight block would have to then be pulled off. This should leave enough room for the 104 style gas block.

From what I found in searching for a flash hider was this:

240F

Is there any reason that this should not work? Besides having to have the barrel rethreaded?

Or is there anything else out there similar?

Im in for info also! Tag

You can use a CNC warrior M24x1.5 to 1/2x28 with an AAC Blackout 51T, which is similar to the B.E. Meyers. CNC Warrior just doesn't make an M24x1.5 to 15mm adapter.
http://www.cncwarrior.com/M24x1-5-to-5-8-24-Muzzle-Thread-Adapter-p/24934.htm

http://www.advanced-armament.com/BLACKOUT-51T-Flash-Hider-MITER-Mount_p_448.html

You could try a CNC warrior M24x1.5 to 14x1 LH thread adapter (1.7") and use a polish AKML flash hider (3.375").
http://www.cncwarrior.com/24mm-to-M14x1-LH-Muzzle-Thread-Adapter-p/23076.htm

http://www.robertrtg.com/store/pc/POLISH-AKML-FLASH-HIDER-154p1691.htm

Or Tantal flash hider, 3.2", has to be bored out to 7.62 though.
http://www.robertrtg.com/store/pc/TANTAL-FLASH-HIDER-NEW-154p1747.htm

Thanks,

Do you have any experience with the thread adapters from CNC Warrior? Can they be trusted?

CNC Warrior is GTG, other AK owners here will say the same.
With any long muzzle device on an AK, check for concentricity with a dowel.
I would spot weld the muzzle device to the adapter; as with most muzzle devices that aren't somehow retained, they could spin themselves off.

Kind of what Manticore Arms is talking about (not my rifle):

This is what I had originally intended to.

However, one of the rifles is cut down to a 103K configuration; it has the original sight block pushed back to the gas block, and then a 74 style muzzle brake pinned to the barrel.

Now, the thing I'm not sure about is whether the 74 style brake is pinned down to the actual barrel, or to the 24 mm front sight base threads.

If it's to the front sight base threads, then I'm pretty sure the front sight base can be pulled along with the brake, but if the brake is pinned to the actual barrel then I think the only way to get it off would be to cut the 24 mm front sight base threads along with the portion of barrel underneath them. In turn, I would still be able to have the 104 style gas block installed, but there wouldn't be much barrel length left to thread.

On another note: has anyone had the 24 mm front sight base threads cut off? I was thinking of buying this:

https://www.k-var.com/shop/AK-142B.html

And having the 24 mm threads cut off, and then just threading the actual barrel itself.

Is there any reason why this shouldn't be done? I don't have the tooling to do any of this myself, so it will be sent off to a gunsmith. I was thinking AKUSA would be able to do it.

Also, as an FYI, the AK in question is a DA AK103U built off a Saiga

if it was done "right" (meaning to ATF specs, from what I have been told anyway) it will be pinned through to the barrel.

These from E93 are optimized for install on shortened barrels. 2.50 AOL, .50 thread overlap allowing for a 14 inch barrel. Good time buy right now. They are 40% off for their Black Friday-Cyber Monday sale.