If you've been reading along, I've spoken about finally putting my SBR (Short Barrel Rifle) together after having my paperwork approved for a year. I ended up going with relatively fancy components since it was an effort a couple years in the making. Since I was planning on running my SBR suppressed some of the time, I chose a Innovative Arms W.A.R. Gas Adjustable Upper Reciever.
The idea is similar to Noveske's Switchblock. Since you get X amount of gas to operate your rifle when it's unsuppressed and some amount greater than X gas when you have a can affixed, in theory you can regulate this with a gas port so you aren't overgassed when the can is on. My other suppressed AR-15 is certainly overgassed. Even with some sealant at the charging hande, if you dump 20 rounds fast through it it sprays oil on your face. It's not harmful, just annoying. And probably bad for one's pores.
So the idea is you get the right amount of gas when you run unsuppressed and when you put a can on you rotate a switch and it restricts the gas a bit so you don't have lots extra. I fully support this idea! It does this by using a special gas tube (carbine length gas systems only!) that attaches to a valve in the front of the upper reciever, which meters the gas to a short tube section that sits where the standard gas tube meets the bolt carrier.
Since this wasn't going to be a standard built (14.5" or 16" barrel with a carbine gas system) I figured it may need some tinkering. I was right! So far it's not operating as I'd hoped.
For matter of comparison, my 12.5" carbine with a can permanently attached is running a H3 buffer (about as heavy as you see, excepting the 9mm buffers) and an extra power action (buffer) spring from Springco. It cycles perfectly and it probably still overgassed. It's using a heavy BCM barrel.
My SBR is using a lighter Daniel Defense 12.5" barrel. It's entirely possible the gas port is cut smaller than the BCM barrel. But running a extra power action spring and a heavy buffer was a no go. I was getting short cycling. So I switched down to a standard carbine buffer and spring. On the wide open gas setting it's cycling reliably, but it feels like barely. It's not a brisk cycle, it takes some amount of time (very fast but perceptable) to complete. This tells me it's on the verge of not cycling. When I install the can and flip it to the restricted setting, I've got a single shot rifle. It's doesn't even eject the empty case reliably. If I flip the switch to open with the can on, it runs like a top.
This testing was done using a .30 caliber can, so it's possible it's not creating enough back pressure to meet the design parameters. I'd love to switch my cans and/or barrels back and forth, but that can is not coming off my other carbine it's soldered to. And even if I could get it off, I can't have a short upper sitting around without a pistol lower or another SBR lower to put it on. So that's not an option.
The other thing I need to try is 5.56 ammo. For my first test I used PMC .223 ammo, I should see if I get different results with 5.56 ammo that runs at a higher pressure. (or my .223 reloads that do as well) I hate burning good ammo in the middle of an ammo drought though, but I'll never figure it out if I don't. Even so, if it takes 5.56 pressure ammo to function right that doesn't make for a very utilitarian carbine. One day I may want to run crappy steel cased ammo in my gun, and I bet even on the wide open setting it won't cycle the rifle.
I'm going to test some more, then maybe write the manufacturer. I think perhaps the solution is to open up my gas port a smidge, but that's sorta a irriversable step. I want to explore other options before I do that. But for now it's a fun project, and I'm learning quite a bit through this whole process.