My recipe for AR, Steel outer barrel, 4UAD hop chamber, 4UAD bucking (trim and sand smooth the side off), Unicorn barrel to the correct length 260mm ish maybe. Use some aluminium tape and perfectly wrap so that it's even all the way around the end of the inner barrel, might only need one or two wraps so that is just squeezes down the outer, or get a right sizes o ring, supporting the inner barrel in the outer is key.
For the AK, 285mm unicorn barrel, 4UAD bucking (trim and sand smooth the side off), 1 perfect wrap of teflon tape around the bucking where it connects to the barrel. Then again aluminium tape around the end of the barrel so it's supported in the outer (think it only needs one wrap in the AK).
I used .4s and doing this has made both shoot incredibly well. It's a much bigger upgrade for the AR than the AK but both can be improved with this.
FCW AK Triangle Stock With Rear Cushioned Butt Pad For VFC AV74 GBBR works well if you like the look of them, I like mine.