18'' rifle length upper on a carbine buffer lower

You shouldn't, except maybe with low powered Tula ammo or some such. I've set up a few different ones like that over the years without any problems I was aware of and I'm currently running a Colt 20 inch rifle on a carbine lower w/ standard buffer.

good luck

M

I run my 18" Wylde chambered upper with a rifle length gas system with an H3 5.5oz. buffer. Works well for me.

No an h3 is heavier than a carbine buffer. They are the same size and work with the same system but different weights. In carbine systems they have carbine, H/H1, H2, and H3 buffers. The weights are different with H3 being the heaviest.

I build one of these for my wife, very accurate and soft shooting. Its a rifle length gas tube,18" barrel, with a milspec carbine buffer tube and standard buffer.

Don't over think it. Try the "standard" parts first and if you have issues fix them.

No. The Mark 12 series of rifles used in the military was often fitted with a collapsible M4 stock. It was a 18" rifle system that was often fired suppressed and not suppressed.

My space-DMR has an 18" rifle gas barrel and a plain-Jane milspec carbine buffer. Runs great and the recoil is ridiculously light, even compared to other ARs.

It's unlikely you'll have any problems. If you do, they will be easy to fix, so just shoot it and see what happens.

You won't have any issues with a standard buffer weight... but a heavier buffer will smooth out the recoil impulse.

If you want a heavier carbine buffer... I strongly suggest a configurable buffer kit... way cheaper in the long run.

Something like this...https://www.kakindustry.com/ar-15-parts/lower-parts/buffer-tubes-and-parts/ar15-buffer-system

It'll run fine. My BCM 18" MkI2 Mod 0 rifle runs great on a carbine lower.

You can get an H3 buffer and remove weights to get it right. It's not a big deal, I think I ran an H, maybe an H2 would be best.

Many different combinations of upper gas system length and lower buffer weight combination work perfectly fine.

I run H2 buffers in almost all of my AR's, with 7" through 29" barrels up top; pistol, carbine, mid-length, rifle, and extended rifle gas lengths, and in 17Rem through 458soc, even 243wssm. All of them run just fine.