Cutting barrel/gas port size

[blaster22]

If I cut a Colt 1/7 from 20" to 18", do I need to resize the gas port?

[K.L. Davis]

Do you have to? No.

Can not doing so possibly cause problems? Yes.

18" barrel with a rifle length gas system can have hiccups in colder weather, the Intermediate Length gas system offered by Vltor was developed specifically for this reason.

But how important is it to you? Will you ever be in cold weather? Is one FTF in 214 rounds acceptable to you? What is the cost of your rifle hanging up, really?

You will get a lot of people tell you that you can do it with no problem, and by most standards they are absolutely correct... but measure their advise against their standard -- I have seen people attest to "perfect reliabilty" of a rifle based on their shooting "nearly 500 rounds" from it in the 15 months they have owned it (safe to say that it was only shot in mostly ideal conditions too).

And you will get those that just refuse to admit that something could possibly be amiss with things -- I recently saw on another forum where a person was told that they did not need to open the gas port on a rifle length system cut to 16" (based on the advisors experience)... when I am sure that anyone that has extensive time on one of these systems will tell you that they are problematic unless the port is opened.

Bottom line: For most purposes, you will not see any problems.

[blaster22]

I can open it up, no problem. What size should I open it up to? Haven't found a spec anywhere.

[madryan]

I've always been under the impression that the longer the gas system the larger the gas port. So, if anything you would want to make it smaller. IMO this requires welding up the old port and IMO isn't worth it. My RRA upper has a .070 port and is a carbine length gas system. It should have a .062 or so port. IMO RRA does this either 1) out of laziness 2) so that you can shoot crappy wolf and other substandard ammo without it shortstroking. I just threw in a heavier buffer to counteract the extra pressure and slow down the bolt and it works great.

[LOKNLOD]

I'm sure one of the experts will chime in with more details, but I'm not so sure that is the full story, madryan. I've been working to understand this lately so here's my attempt at an explanation:

Gas port size seems to be dictated by a combination of two variables, overall barrel length and length of barrel past the gas port. (Three, really, if you include barrel diameter at the port but we're not monkeying with that so let's ignore it.)

I believe that gas port size increases with longer barrel lengths (using standard gas system/length combinations) due to the variation in rate of gas expansion from the combustion of powder in the chamber. There is a finite amount of gas that is given off by the combustion, and the rate (and pressure) it is expanding at declines as time increases (I'm sure there's an interesting equation for this relationship somewhere). Further out the gas port is, the pressure is lower, the distance back to the bolt is longer (and therefore the volume of the gas tube greater), and a larger gas port is needed to "encourage" more gas to expand back down the gas tube, fill that volume, and act on the bolt with sufficient pressure, before the bullet leaves the barrel and the system is opened to atmosphere. And that leads us to variable #2...

Given that you had the exact same barrel length, say 16", the gas port size would need to increase even more as the length of the gas system increased. Rifle > Mid > Carbine. This is due to the other variable, barrel length past the port, which manifests itself as dwell time. When the gas from the combustion in the chamber is expanding, it is acting in two directions: first to push the bullet down the barrel, and after it reaches the gas port, to cycle the bolt. The force on the bullet continues so long as the bullet is in the barrel (unless you've got a barrel whose length/volume is greater than that of the combustion gas expanded to the point it reaches atmospheric pressure, but that's just silly), but the time it can act to cycle the bolt is only during the brief period between the bullet passing the gas port, and the bullet exiting the barrel, the system has lost pressure (open to atmosphere). That's why the length of barrel past the gas port is so critical, as it determines the time limit the expanding gases have to operate the weapon.

As for the slightly oversized ports (relative to ideal) found on most carbines, it's a good thing, and not just laziness on their part. That extra diameter is insurance for when the ammo is slightly underpowered, or the weapon is dirty, or poorly lubed, or the weather is cold. Maybe it cycles a bit more violently and decreases the lifespan of some of the internal parts, but it's definitely a calculated tradeoff on their part. I'd wager the decrease in lifespan is much smaller than the increase in failure to operate caused by insufficient gas or bad conditions, and also that 95% of users will never "use up" the life of parts. And if they did, not many will complain about replacing some guts after thousands of rounds -- but you can bet any rifle that leaves the factory and fails to operate to expectations (even if it's the user's fault - dirty or unlubed), then that's going to come back to bite the manufacturer in the butt through bad reputation and word of mouth (especially these days).

Meanwhile, if I'm misunderstanding the way things work, somebody please correct me because as I said at the top, I'm currently self-educating on this...

ETA:

I recently saw on another forum where a person was told that they did not need to open the gas port on a rifle length system cut to 16" (based on the advisors experience)... when I am sure that anyone that has extensive time on one of these systems will tell you that they are problematic unless the port is opened.

I had an upper setup with a barrel in that config, and found it to be problematic even with the port opened up...

[K.L. Davis]

I can open it up, no problem. What size should I open it up to? Haven't found a spec anywhere.

The only real "spec" would have to be the SPR... it is opened up pretty big for particular applications that may not concern you really -- Without all of the low operating temps, ammo, etc questions... I would say just make sure you are 0.082 or maybe a little bigger.

[K.L. Davis]

8< --------- 8< -------- SNIP
Meanwhile, if I'm misunderstanding the way things work, somebody please correct me because as I said at the top, I'm currently self-educating on this...


for any given port size, if the dwell time (barrel length) is reduced, the port is generally enlarged to make up for the lost dwell.

[LOKNLOD]

for any given port size, if the dwell time (barrel length) is reduced, the port is generally enlarged to make up for the lost dwell.

That is certainly a much more concise way of putting it ;)