I currently have an AR chambered in 7.62x39 with a 16" barrel. I was considering building another rifle in 300AAC. I also had the thought to just build another 7.62x39 upper, but with a shorter barrel. I can swap it with my 5.56 pistol upper or SBR it. That being said what would be the optimal 7.62x39 barrel length to run suppressed? Is it worth it to run suppressed 7.62x39 without using subs? The subs are pretty pricey still(IMO) for both 300AAC and 7.62x39. I do not currently reload either. I don't have a real use for this other than just building the upper(or complete rifle) for fun and plinking. Which is why 7.62x39 is appealing(dirt cheap ammo).

7.62x39mm and 300BLK offer basically the same performance. In an AR-15 platform 300BLK is much better, more reliable, uses same mags and parts as the 5.56mm variant, only difference is the barrel and ammo. In and AK platform, obviously 7.62x39mm is better. You can buy factory subsonic 300BLK. If you reload you can make subsonic 7.62x39mm. Since you're wanting subs and are using an AR platform I'd suggest getting a 300BLK upper for it. The AR platform is quieter suppressed compared to the AK platform.

i run an 11 inch supressed X39 and the performance between it and blackout is not even close. i use the factory hornady SST

I always use a suppressor to shoot.  I shoot supers, not interested in subs.  My 16" is very pleasant with the can on, most all of the noise is the sonic crack downrange.  The 10.5" sbr is quite a bit louder, not as nice to shoot without hearing protection.  I think that 7.62x39 is a great caliber to suppress.

don't have a suppressor, but am running a 7 1/2" x39 pistol with 208 gr bullets and 10.3 gr of 2400, think subsonic, don't have a cronie. locks back on the mag.

What do you mean?

i mean considering equal bullet weights the X39 will always have more velocity and enegry, it has more case volume..   there is no magic to blackout that makes it go faster than it should

X39 is a lot of fun to shoot suppressed.  Full powered loads are excellent.  Only subsonic round I shoot is 22LR.

Ok, I was just curious as to whether you meant it suppressed better or something. I've been on the fence about building one, or the other, for a few years.

Because it has more case volume it is harder to suppress than the 300 blk.  Not that it can't be done, it can but some subsonic rounds won't cycle the action without a suppressor.

I love when people say the 300 blk has more or equal performance.  Truth is the x39 can run 125's 2400 to 2450 fps out of a 16" barrel.  No 300 blk will push 125's that fast.  With 150 grain bullets the x39 can get to 2100 to 2200 fps the 300 blk can only get them to 2000 fps.  Even Bill alexander who has put a lot of effort in improving performance for the 300 blk has said with careful handloading he can "ALMOST" make it a x39.    And this is one time that 100 to 200 fps can make a legit difference.  Most of these bullets only expand at 1800 to 2000 fps.  Meaning they might expand at 200 yards max.  The x39 can get you another 50 yards.  Adding 50 yards at shorter distances can be meaningful.