GP size for 11.5 suppressed AND unsuppressed

All the discussion about GP sizes got me thinking, is there a recommended GP size for a 11.5" 5.56mm barrel with carbine gas system and carbine receiver extension and buffer, if the user wants to run suppressed AND unsuppressed, without resorting to AGB/ABC, or the Vltor A5 system?

We have a numerous Colt 6933's, and I have tested one with a GT halo, and that gun runs 100% both ways and has around 750 rounds through it. Mostly PMC 55 grn fmj, with some Hornady 55 grn fmj, and about 100 rounds of 55 grn Gold Dot. Sample of 1, though, not a long term test, only the 1 can, etc. I have no hard data on the size of the Colt6933, GP but have seen reports varying from .069" to .093".

I have read of more than a few folks using BCM 11.5 barrels suppressed and unsuppressed and having no problems. And there are many reports of the BCM 11.5 using a GP of .076".

I understand that you are better off to set the rifle up to run optimally as dedicated for 1 condition OR the other, not both. As in, for a dedicated suppressed 11.5, the GP should be around same as a 16" carbine, with a .062" GP.

ammo: Speer Gold Dot .223 55, 62, 75 grn for duy, but also quality FMJ .223 and 5.56 for practice
Buffer: can vary but let's say H buffer as a baseline
Can: let's say full size like the Halo, M4-2000, Saker, or even a somewhat shorter can like a Sandman-k, or a Mini-4.

Your thoughts?
Generally 11.5" barrels are ported at .081" - .089", but you will also see where some are opened up to .094". As for what is optimum, tuning for suppressed vs unsuppressed, ammunition issues, this is something we cover extensively in our armorer courses. On most 11.5" guns set up with a carbine length stock and heavy buffer, you can get them to run suppressed and unsuppressed, though you might get better. You have to take into account the type of suppressor, as if it's a baffle type then you will probably need to slow the timing cycle down for reliable timing, as compared to suppressors like the ones from OSS where you don't have as much back pressure effects. I'm not a fan of adjustable gas blocks on duty guns, as adjustment screws can work loose and change how the timing on the gun runs, and if the gas block is hidden under a freefloat tube then it may be hard to get at the adjustment screws, which is whey on duty guns we build with fixed gas blocks. When suppressing a rifle, you should time the rifle to your duty loads, and the normal environmental conditions that you will be using the rifle in, taking into account that ammunition is effected by things like hot vs cold temps, and altitude, all of which effect the timing of the rifle. Also keep in mind that many LE duty loads are probably .223 and not 5.56, where most LE agencies that we see when teaching armorer courses are using the cheapest thing they can get their hands on for training of which a lot of time is surplus 55gr in 5.56, so you need to take into account that .223 generally has different internal ballistics as compared to 5.56.

Greg Sullivan "Sully"
SLR15 Rifles
(763) 712-0123


Regular Member
Network Support I
You may also want to reach out to Clint over at Black River Tactical, it is my understanding that he has messed with gas port sizing quite a bit.

As a random data point, I'm using a 0.063" BRT gas port insert on my 11.5"; it locks back unsuppressed on Federal Fusion MSR in fairly mild weather (I've not shot it below 50 °F), though I'm also using an LMT e-BCG, Sprinco Green Spring, and A5H2. Suppressed with a Surefire SOCOM556RC2, the gas-to-face is minimal.