I've got an 11.5" carbine barrel from Ballistic Advantage (5.56mm Modern Series model) and I'm told it has a .073 gas port. Because I'm running an A2 FSB that came factory installed and pinned, I won't be able to modify the port or use an adjustable block. I'm running it on a lower with an RRA stock, buffer, tube and spring. The buffer that came with the RRA stock assembly is 84.5 grams (2.98oz) - a stock carbine buffer.

I plan to run a mix of unsuppressed and suppressed (762-SDN-6). Although with the N6 being a .30cal can, the back pressure increase shouldn't be too high.

What weight buffer should I be using? Will the stock buffer I already have work well?

It will run fine most likely, but it will be fairly overgassed when suppressed.



I'd put an H2 in it if it were me.

I know on a 10.5/ 10.3 barrel the "desired" gas port is .070 - at least in the clone world. These do have some issues running low powered ammunition on occasion, so an 11.5 at .073 might not be too overgassed running suppressed. My personal rifle is a 10.3 with a .082 port and that IS overgassed. I have an H2, but am considering moving into an H3.

Looked for a port diameter per barrel length guide, and found this:

http://www.ar15.com/forums/t_3_4/550213_Gas_port_size_and_distance.html

Some links in this as well:

https://www.ar15.com/archive/topic.html?b=3&f=118&t=628590


While this doesnt come out and answer your question directly, it will give you a baseline of where your barrel currently lies. Shoot it, see how it feels with the standard Carbine buffer. Then add the suppressor. Note the difference and note reliability changes, if any. Doesnt hurt to have different buffers on hand to swap out. If you dont short stroke with an H2 unsuppressed though, I'd probably stick that in there and be done with it.

I just bought the same barrel.  It seems slightly undergassed.  Full power 5.56 and .223 runs fine with an H buffer.  However, I notice that with PMC bronze it does not lock back on an empty magazine 100%.  It is fine though with a regular carbine buffer.  I also tried some Monarch .223 (repackaged Russian steel case), and the bolt will NOT lock back on an empty magazine 100% even with the carbine buffer.  I think this barrel would work well with a silencer.

Edit: This is shooting unsupressed.

Thanks for the update. Looking at a couple of charts that recommend 11.5" port sizes (most being around .081-0.89), it does seem to be slightly undersized at .073" ...  so I had planned on running a standard Carbine buffer anyways just to try out first. Hopefully it runs well without my suppressor because half the fun of a shorty is being able to shoot it short... at least once my stamp arrives.

Are there any other indications that a rifle is under gassed other than failure to lock open?
What is generally considered to be the weakest common ammo around?

The failure to lock back was the only sign of being undergassed.  It feed and extracted just fine.

PMC bronze and the Monarch are the only low power stuff I've tried thus far.

Edit: It also appears that this barrel has a tighter chamber than my other rifles because it leaves more wear/scratches on the cases.