View Full Version : Colt 11.5 inch fbi barrel gas port size
Is the gas port the same size on the Colt fbi heavy barrel as the standard Colt 11.5 inch barrel?
Grant measured one to be .069.
https://www.m4carbine.net/showthread.php?59364-The-Colt-M4A1-SOCOM-Barrel&p=2328220#post2328220
Post 150
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That is a good size.
I believe the 6933s are ported quite a bit larger.
Jwknutson17
08-30-17, 14:37
I prefer these 11.5s to be this, and even less. I cant see why anyone makes an 11.5 with a larger port. Even companies that offer (reduced) port sizes aren't even this small. Guess you need to cater to folks running garbage ammo these days.
Bob Sacamano
08-30-17, 14:58
I have been considering one of these barrels but am yet to get a confirmation of port size. If 0.070 is acceptable for 10.5, I would expect an 11.5 to be smaller.
titsonritz
08-30-17, 15:34
That is a good size.
I believe the 6933s are ported quite a bit larger.
IIRC, it is .082, way too gassy.
I just bought one and wanted to see if I was going to have to change my setup. My current colt upper has the standard 11.5 barrel on it and on full auto with a suppressor you get a face full of gas. I run a xtra heavy buffer to keep the rate of fire down.
I prefer these 11.5s to be this, and even less. I cant see why anyone makes an 11.5 with a larger port. Even companies that offer (reduced) port sizes aren't even this small. Guess you need to cater to folks running garbage ammo these days.
Its hard to imagine even trash ammo needs these blow holes. Its stupid.
PB
The last 6933 I measured (newer production) came in much smaller than .082. IIRC it was in the low .070's.
I have a 6933 in quarantine and while I've only been able to put a few rounds through it, it gave no signs of being over gassed
tango-papa
09-04-17, 13:11
A complete Colt 6933 URG purchased from G&R in late 2016 - sent to Iraqgunz for rail install and gas port measured at .074.
See post# 32:
https://www.m4carbine.net/showthread.php?192019-Colt-LE6920-quality/page4
tp
Still pretty big. I have an 11.5 that runs Wolf Gold with no muzzle device. And dirty/dry. .065" port. Standard BCG. It ran M193 and M855 at .0635". That's 30+1 rds and lock-back.
longshot2000
09-23-17, 10:37
The FBI heavy (SOCOM like) barrels are much smaller gas port than the 6933 profile 11.5." I see that Grant measured 0.069 with a pin set. I will take that. I used a caliper (much less accurate), and was getting 0.070 to 0.073. Colt generally runs smaller gas ports.
I have read technical specs that say that an 11.5" carbine should be closer to 0.080. I get lost in the math. If 0.070 is good for 10.3" and 0.063" is good for 14.5", I am not sure why one would want a larger opening in between the two (10.3" and 14.5")
I can tell you that the Colt 10.3" Gov't profile and the 11.5" HBAR shoot very nicely, both suppressed and unsuppressed. Not on topic, but I had an exchange with a SWAT team lead, who was looking to upgrade (replace) barrels, and I made the comment that the BCM 14.5" SOCOM was every bit as good as Colt, and he was very vocal, in a negative way, about BCM SOCOM barrel accuracy, and was replacing team BCM barrels with Colt. I was a little surprised. For me, 14.5" is about maneuverability, and point and shoot, not so much about MOA accuracy. I did not know Colt was that much better. I got schooled.
ETA: Just got a nice set of pin gauges. 0.069 or 0.070" is correct on the heavy barrel. I just have to learn how to use pin gauges. Mine are "minus," which I understand is normal. The 0.069" falls nicely with no effort. The 0.070" will not fit without a shove. I think that means the hole is 0.069" ??
I have the 6933 thin barrel as well, but I am not taking the FSB off just to measure. Keep in-mind that the early short barrels were cut-downs of 14.5" M4 profile, so those had a smaller port (0.063" off the top of my head). I cannot imagine Colt would have enlarged beyond 0.070" for 11.5."
Still pretty big. I have an 11.5 that runs Wolf Gold with no muzzle device. And dirty/dry. .065" port. Standard BCG. It ran M193 and M855 at .0635". That's 30+1 rds and lock-back.
Did it open up over time?
No, I opened it to .065" for reliability when very dirty and/or dry.
With a BCM Gunfighter MD it ran at .0595", but with an A2 FH in needed a larger port. So, MD makes a difference.
.063" on my 11.5 will only lock back with 5.56mm Black Hills ammo. .067-.068 is ideal for non-suppressed 11.5 in my experience.
So many variables, to include individual barrels. I have one 10.75" (cut down) that runs at .065". PMC, Wolf, suppressed and non. Another that I cut to 11.5 - same base barrel model- needs .0675. Go figure. I think that is one of the reasons manufacturers don't want to go too small with gas ports; they can't control the variables.
If you're one of those people (like me) who likes to test and try things, and attempt to optimize, there are options. Get a manufacturer to make you a barrel with no port drilled, or a very small one drilled and work up your own recipe. It's satisfying, if nothing else.
I know that we take the variables to try and understand our guns, however each gun is its own self. And we have to tweak and tweak and TWEAK, until it runs to our satisfaction. Not that data can't help , it surely can but in the end you have to make your gun run.
PB
Wolf Gold is fairly hot, and not at all like their polyformance or mil classic steel-cased brands. Just making sure folks understand the comparison. :happy:
Still pretty big. I have an 11.5 that runs Wolf Gold with no muzzle device. And dirty/dry. .065" port. Standard BCG. It ran M193 and M855 at .0635". That's 30+1 rds and lock-back.
TacMedic556
10-13-17, 14:17
Our team uses 6933's, many purchased in 2005 & 2006. We've run them for years with standard carbine springs and H buffers. Ammunition choice has evolved over time with duty in the past being a 77 grain Law enforcement load I cannot recall and now a 75 grain TAP. We used a variety of ammo in practice and drills for up close stuff where there was not a noticeable zero shift at all. Everything from 55 grain 223 stuff to 77 grain factory reloads from Freedom - the 6933 is incredible. I can honestly say that in 11 years I may have had 5 malfunctions - all of which were magazine related. Ditched the bum mag and moved on. Literally the most reliable black rifle I have ever shouldered, period. Nothing comes close. It is the reason I would bet my life on a Colt.
So many variables, to include individual barrels.
This is very true. We choked down several short AR barrels. We shot a bunch of them on the same day, and it's amazing how unique the recoil impulses are. Almost no two are alike when you get into really feeling the gun run.
crazy that the same gun from basically the 60s is still arguably the #1 cqb firearm
I always thought the 6943/6945 could be the coolest, monolithic upper and colt Canadian CHF barrel
Our team uses 6933's, many purchased in 2005 & 2006. We've run them for years with standard carbine springs and H buffers. Ammunition choice has evolved over time with duty in the past being a 77 grain Law enforcement load I cannot recall and now a 75 grain TAP. We used a variety of ammo in practice and drills for up close stuff where there was not a noticeable zero shift at all. Everything from 55 grain 223 stuff to 77 grain factory reloads from Freedom - the 6933 is incredible. I can honestly say that in 11 years I may have had 5 malfunctions - all of which were magazine related. Ditched the bum mag and moved on. Literally the most reliable black rifle I have ever shouldered, period. Nothing comes close. It is the reason I would bet my life on a Colt.
TacMedic556
10-14-17, 00:01
From my experiences using one issued to me it is a work horse like no other. It has seen rain, snow, sleet, below zero, dust, dirt, heat, lack of lube, lots of lube, crap ammunition, good ammunition, and it just chugs along churning out brass.
47997
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