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Old January 13, 2013, 03:33 PM   #1
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AR15 gas port relocation.

I have a new M&P 15, mid length barrel with a carbine gas system, I have never had a setup like this, I don't really like the carbine gas system on the mid length barrel, it doesn't seem as "smooth" as the mid length set up.

I am thinking about plugging the carbine gas port and drilling a gas port at the mid length position, and relocate the FSB. I have the equipment and skills to do all this work myself so it won't cost much.

anyone have any tips about this, my only concern is drilling the new gas port, it's a chrome bore. I wonder what type of bit to use for this to make the new hole as clean as possible. 118 or 135 degree drill bit?
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Last edited by iraiam; January 13, 2013 at 03:51 PM.
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Old January 13, 2013, 09:29 PM   #2
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"midlength" usually indicates the gas block location is about 1/2 way between the carbine and the rifle length.
You have a "midlength" with a carbine gas block location?
An easier and likely better solution would be a "pigtail" gas tube. These have a curl that wraps around the barrel under the hand guard to make the extra length.
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Old January 14, 2013, 02:26 AM   #3
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Well, you're going to need a carbide bit to do the drilling. Chrome is hard stuff and you'll ruin a HSS bit trying to punch through the chrome plate.

118 degree bits are fine. You'll want to center drill the hole to prevent the twist drill bit from wandering off either side of the barrel.

Usually, I drill the hole under-sized by about .005 and then follow up with a chucking reamer to the desired size. This gets rid of any burr on the interior of the barrel.
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Old January 14, 2013, 04:33 AM   #4
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Mobuck; sorry, let me clarify. I have a 16 inch barrel with the gas port in the carbine position, I am not really liking this setup.

wyop; Yes I have carbide, and cobalt drills, thanks for the tip on finishing up with the reamer, I don't believe my reamers go quite that small, I'll have to rectify that.
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Old January 14, 2013, 04:44 AM   #5
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To do it right,the hole should end up centered in a groove.
I second the plan of using the pigtail gastube.It winds a couple loops around the barrel.By lengthening the tube,the gas pulse is delayed.You can also do some tuning with buffers,like H,H-2,etc.
I know someone with a tremendous amount of AR experience that uses them with success.It is also very easy to do.If it works out for you,great!!
If you do not like the results,you can still try your plan.
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Old January 14, 2013, 04:45 AM   #6
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have you considered just getting another barrel in mid length?
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Old January 14, 2013, 06:17 AM   #7
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I cast absolutely no doubt that you can do this. Heaven knows, I still have a lot to learn about machining.

That being said--I would probably not take the step you're describing. There are a few variables that you must be aware of.

1. Gas port pressure

Full length (20"), carbine (16") long target (24"), SBR/Military (10.5, 14" and everything in between) have different sized gas ports. This has everything to do with the proper cycling and timing of the firing cycle. With this, you must be very exacting and careful.

And, if that were not enough, you have different sized gas ports for suppressed and non-suppressed firearms as well.

Too big a gas port, and you'll have a lot more gas than you need roaring back to unlock that bolt carrier group before it should. Too little and the rifle won't cycle.

And, as someone above mentioned, you must time the port to be located in a groove, and NOT on a land.

If you want to do it, and you're confident that it will work--well, then, proceed with caution. However, I would simply buy the barrel/gas block configuration that you want.
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Old January 14, 2013, 06:33 AM   #8
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Powderman brings up some good points. I know Ruger drills the hole different sizes for different barrel lengths.
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Old January 15, 2013, 01:27 AM   #9
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In general, the issue of port size goes like this:

On the one hand, the closer you are to the chamber, the higher your pressure.

On the other hand, the closer you are to the chamber, the longer your pressure duration (assuming a 16" minimum barrel length). Said inversely, the closer you are to the muzzle, the shorter your pressure duration. At some point too close to the muzzle, your gas impulse won't last long enough to cycle the bolt reliably, so you end up having to increase the port size to really goose that bolt to the rear and then let inertia do the rest.

On the third hand, there's a difference in pressures between .223 Remington and 5.56 NATO ammo.

And on the fourth hand, it is possible to over-gas an AR action and cycle the bolt too fast and too hard. One symptom of an over-gassed action is the buffer hitting the rear of the tube (ie, completely compressing the spring and making noise). Another symptom is that the bolt doesn't pick up the next round from the mag.

So what you have to do when balancing these two issues is:

a) start small. Like 0.049 is a lower bound I'd start with when I know nothing.
b) assemble the barrel, gas block, gas tube, etc.
c) load a magazine with one round. Chamber from the magazine. Test fire. Observe where the brass goes - if it ejects at 3 to 4 o'clock out the side of the ejection port, things are looking promising.
d) remove magazine. Load two rounds. Chamber first round. Fire. Stop. Check to see if the rifle loaded the second round from the magazine. If it didn't, you might have short-cycled. Check the brass of the round in the mag, and try to determine whether the bolt got over the rear of the case rim.

You might need to open up the gas port a little more. If your ejected brass is flinging out of the port at 4 to 5 o'clock and it's ejecting fast and hard, your action might be over-gassed, and you'll have to put in a heavier buffer to slow down the bolt to pick up the next round in the magazine.
e) If your bolt did pick up the next round, fire it and see if the brass ejects where the first round did. If this is all OK, things are looking more promising. Load a magazine with five rounds and cycle through them. If you can rip through all five rounds, you're probably done. If you really want to test, then run five rounds of NATO ammo and five rounds of .223 SAAMI ammo through it, then inter-mix them to insure that the action cycles with both types of ammo.

If the rifle ejected at 2 o'clock or further forward, you're probably under-gassed, and you need to increase the size of your port.

If your port is closer than about 3.5 to 4" from the muzzle, NB that you might end up with an issue about the duration of the gas impulse to the BCG. Don't try to locate your gas port too far forward.

When I'm increasing port size(s), I go up by about 0.005" at a time. That moves the ejection direction (for me, on rifle and mid-length gas systems) by about "half-o-clock" or 30 minutes. I've never done one of those carbine-length barrels with a custom barrel where I had to drill a port. Custom barrel guys seem to want longer barrels.

Maximum port sizes I've ever seen have been about 0.090".

One way to slam-dunk this once is to drill a max-sized (0.085" or so) port and use a gas block with an adjustable gas valve. Then you test fire the rifle and fool around with the valve to get it right.
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Old January 18, 2013, 01:27 PM   #10
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Some things I haven't seen discussed but I think are relevant:

1. What is the expected outcome of altering the gas system?

2. In the location of the new gas port, is the barrel O.D. and surface finish compatible with the gas block? Does the gas block have the front sight integrated, and will the new location be compatible with the hand guards?

3. I haven't personally worked on the M&P 15 -- is the gas block bolted or pinned? In my opinion, if you intend to use a pinned gas block, notching the barrel for the pins will be the more difficult part of the job.

Personally, I think the carbine gas system is fine for a service (non target) rifle. It offers more reliable cycling with various ammunitions because the pressure curves of some ammunitions (largely depending on bullet weight and powder speed) drops off very quickly as the bullet goes down the bore. In these cases, the mid- and rifle-length gas systems have a higher chance of gas pressure insufficient to cycle the action.

Also in my (worthless?) opinion, I think problems with over-gassing the AR are overstated. It is not great engineering to have an over-gassed rifle, but the AR gas and BCG components are robustly built and the design minimizes loading.

But, if the thought of over-gassing really bothers you, and is the reason for your project, have you instead considered an adjustable gas block? JP makes one. I have set them up before and they work... and admittedly... do soften the recoil and make the gun feel smoother.

The drawback is that it's another moving part on the gun to wear and maintain. And the adjustment isn't detented, so transitions to and from suppressed aren't very fast if you intend to keep the gas pressure precisely tuned at all times.
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