Wanted to grab a BRN-4 10.4 upper but they’re sold out. If I bought a 14.5/16” upper would it be as simple as cutting it to 10.4 and replacing the gas block?

I would say. Go for it.

Sending you a pm

They seem to restock the BRN-4 uppers fairly often, I’d just put a stock notification on the one you want and wait it out.

Seems like a lot of work and added expense otherwise.

Gotta agree with this sentiment. seems like a lot of hassles and extra money. You could look on the EE section while you wait for it to come back in stock and maybe pickup a used one for less.

Brn barrels are nitrided.

If you cut the barrel and use the same gas block, you'd need to open your barrel gas port significantly (somewhere around .090" if i recall correctly) which is made much more difficult than usual due to the nitriding (very hard material  destroys small drill bits).

Even if you cut down and got the appropriate 10.4" vented gas block, you'd still need to open the gas port to 1.6mm to work in a factory configuration (HK red spring and red dot buffer).

Or you could cut down, get a vented 10.4 gas block, and experiment with lighter springs and buffers until you get the desired function and not play with the barrel gas port at all .