Does anyone know what diameter gas port colt used on their 20" barreled SP1?

Anyone have an old barrel with a non eroded gas port they can measure?

Thanks!!

.093"

That is incorrect, that is the size for a barrel that is .750 at the gasport.  The SP1 is .625 there, much thinner barrel.

Which is coincidentally what I've seen as well.

Another source puts the range for lightweight barrels at up to .093" and the range for heavy barrels starting at .093".

Also:

http://www.dtic.mil/dtic/tr/fulltext/u2/731218.pdf

If the M16A1 doesn't have a .092/.093" gas port they've done a good job hiding that fact from the internet. I just boxed mine up to ship or I would make an attempt to get it measured.

The first mass-produced AR-15's did use a gas port diameter of 0.092". Here is the proof on page 3 (page 6 of the .pdf) of the report Infantry and Aircraft Weapons Division Report on A Test of Rifle, Caliber .223, AR-15 dated November 1960.

Another thing to think about is, the difference in length of the gas port from the bore to the surface between a barrel with a diameter of 0.750" at the gas port, and a barrel with a diameter of 0.625" at the gas port is 1/16th of an inch.

Here is also a barrel blueprint for a M16 / M16A1 barrel which shows the port diameter of 0.092" + 0.001".

So does that mean the M16A1 is over gassed or the M16A2 is under gassed?

It would be neither for both. As I mentioned, the difference in length for the gas port between a 0.750" and 0.625" barrel diameter at the gas port journal is 1/16th of an inch. This will make no significant difference when you account for the entire length of the gas system from the bore side of the port to the gas cylinder in the carrier.

Barrel diameter and porting matters at lot more in shorter gas systems I believe.

Define over gassed and under gassed, with your understanding of what each is.
Here's my understanding. Under gassed would have the rifle malfunctioning on a regular basis, or, all the time. The action would be short cycling, failing to pick up a round from the magazine. Ejection would be "weak" or "limp", having the empty case barely fall from the ejection port, or not at all.  Thing is, just because your rifle is exhibiting these symptoms doesn't mean that it is under gassed. The rifle could be suffering from some other problem/problems.
Over gassed would have the bolt carrier traveling at an excessive speed, possibly causing the brass to be thrown to the 1-2 o'clock position. That may depend on if rifle in question has a Brunton bump. Recoil could be particularly harsh compared to a rifle that is gassed correctly, all other things being equal. On and on and on..........
So, is this discussion hypothetical or do you have a rifle exhibiting problems?  Most people wouldn't give a thought to the gas system assuming everything is working. Does the bolt lock back after the magazine is empty? Is it ejecting brass and picking up another round, etc, etc?
You can always change recoil springs, buffers, bolt carriers, to fine tune the entire system to your preference. As a final thought, I would say that Colt erred on the side of over gassing. That would ensure the rifle functioning under adverse conditions such as being dirty or weak ammo. Probably safe to say that most manufacturers of AR style rifles/carbines have done the same thing.
These are mostly my thoughts, and YMMV

I measured my cut down pencil barrel for my 605 and it was in that .093" +/- .001".  Looked like .094" to me.  It of course has the .625" journal, and it also runs well with M193.  I had the guy who cut it leave a little bit more barrel for dwell time to see if it would cycle, and so far it does.

This barrel came off an SP-1 that had been neutered during the Clinton Ban era, bayonet lug removed, and some type of muzzle break pinned and welded, so it was perfect for a 605 barrel.

I have a standard 16 inch barrel being cut to 12.75 and moderator permanently attached. Does anything need to be done to gas port?

If it's a .068", you will want it opened because of the dwell time loss of 3.25".  I don't know what the optimal port diameter is for your uses, but I'm thinking at least a .076".

Someone with a BCM 12.5" can look at that port diameter and then advise, as that is where I would look first.

.063 is a smallish size for a 12.5 but I would start there. .076 is larger than the port in a 10.5.

On our 10" and 11.5" USAF GAUs, we opened them up to .108" for reliability if they burped.  It was hard on gas rings, though.

You fucked those guns up bad. I'm surprised they worked.

For the record, This barrel being cut is a .625   if that matters............

Yikes.

My Rifle has a short-stroking problem - the gunsmith says it may be a gas port problem. What are the correct sizes for the gasport?

A: Courtesy of Rick McDowell of Competition Specialties, NRA Life, USPSA Life, Master ( 105 E. Cass, PO Box 451, Osceola, Iowa 50213 Phone: (800)369-4481 / (641)342-2011 )
"I have an Extraction Problem, is this related? How can they be fixed?The below gas port dimensions are the ones that I have accumulated while working on AR's for years. When someone is having problems with short stroking and their gas port is not within the below ranges, that is usually the main problem. On individual rifles I have varied both bigger and smaller with port sizes, but these were definitely exceptions, or there was no other choice due to poor quality parts we were working with.
For example;
11.5" barrel, .750" barrel diameter at the port, 3.850" from muzzle; Port should be approx. .086" to .094" in diameter. The smallest I have seen the port on these short barrels is .081" and it worked with some military ammo but not with some faster burning rate, commercial ammo. With the .086" port the military ammunition worked better but the commercial still wasn't getting enough pressure to work the carrier properly. The .094" port would work with both the high port pressure military ammo and low port pressure PMC ammo. Anything over .094" will cause short stroking/timing problems from too much pressure to the carrier and under .086" might cause short stroking for lack of pressure. To know if the port should be changed or not will depend on a few other things. Is the chamber narrower than it should be? Is the chamber rougher than it should be? Are the rings worn and allowing gas to blow by? Is the carrier key allowing gas to blow by? Etc., etc. Eliminate these variables first! With a smaller diameter barrel at the port, the gas port sizes will usually be a little smaller as well, given the barrel length is still the same."

That number is wrong.

Fucked them up bad?????   The idea is to make them 100% reliable since your life depends on them as an SP or PJ.  They actually worked 100% reliably even with a 10" or 11.5" -  both without a moderators (only A1 FHs).  .108 is only .014 inch larger than the .094" they were.  About once a year, we checked gas rings.  Every once in awhile we would have to replace one (usually the front one, IIRC)

14 thousandths is profound when you're talking about ports. Especially on short barrels.

Glad they work. Heaven forbid you break a bolt, though.

BCM ports 11.5s at .076" and that runs weak ammo.

My Colt Commando 733 11.5" barrel's gasport is .077 and it's factory, my measurement could be ever so slightly off but it sure looks .077 to me and I have quality calipers, so if there's a mistake it's my poor eyesight.

Oh well, I am sure they have been USAF Frankengunned out of existence with 14.5 M4 barrels now.  We did it, as it had come down from "on high" in the supply chain in a memo to our chief armorer.

Anyone know what the port size is on 20" AR15Sport.com 1/9 pencil barrels?  I tried to feed my 602 clone some 5.56 loaded with ammo loaded with 55 grainers and 21.5 grains of IMR4198.  I have used this load for 30+ years, and it always worked.  It works in my M4gery builds (including one I did with an AR15Sport 1/7 M4gery 16"), and I have used it in other builds in the past with 20" Colt barrels.  In the 602 clone with the 20" AR15Sport 20" barrel, it short cycles.  Some FTE, FTFs, and won't lock the bolt back. I don't know if the burn rate is too quick for the 20" or if it is under-gassed.

Not a big deal, as I switched to IMR4064, and am saving the 4198 for my 160 grain cast bullet loads for my AK.

I know shortened barrels might need sone work but what about a cut barrel that is lengthened back out to 16"? Seems that is the to t al length stays t hg escape, maybe no changes in port should take place?

A pinned on muzzle device doesn't change the needed port size unless it provides backpressure.