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Posted: 3/28/2025 6:02:33 PM EDT
[Last Edit: TonyRumore][Edited]
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I have two barrels with this same gas regulator on them. I took one apart to see how it worked, and it doesn't appear to work at all. It appears the design intent was to bypass/vent some gas out on one of the settings, but there are no holes on the regulator collar that you turn. So regardless of the collar position, the bypass hole is covered. Am I missing something here? Tony  |
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Originally Posted By TonyRumore: I have two barrels with this same gas adjuster on them. I took one apart to see how it worked, and it doesn't appear to work at all. Those barrel assemblies don’t have any adjustment option. (None of the ones that look exactly like that do.) It appears the design intent was to bypass/vent some gas out on one of the settings, but there are no holes on the adjuster collar that you turn. So regardless of the collar adjuster position, the bypass hole is covered. Am I missing something here? Only the first gen M249 barrel assemblies had that option. The early barrel assemblies are identifiable by a flat protrusion on the side of the regulator. The rest of the regulator is smooth. It’s made to use like a lever to flip between the normal and adverse gas positions. The US Army had FN start sending them barrels without this capability. Idk if that happened with the first PIP, or before, but by the mid 90s they weren’t receiving new adjustable barrels anymore. The earlier (adjustable) barrels also had fixed carrying handles which couldn’t fold. For whatever reason, the new part is serrated, but rotating it doesn’t adjust anything. Apparently the overall gas regulator design change was the easiest way to “delete” the gas adjustment capability. It allowed them to use the existing gas block design. Many years later, they eliminated those additional parts and went to a solid one-piece gas block. The US Army wanted the gas adjustment capability removed because they didn’t want M249 gunners using the adverse position when it wasn’t necessary (which allegedly they were doing, to increase the rate of fire). Google Images isn’t doing well for me, but here’s one on an M249 in the originally-adopted configuration from the 1980s. Attached File |
| The “new” non adjustable knobs are the same as what the m240 has… they can still be easily turned into adjustable ones though by simple drilling the holes in them. If you want you can also make them into more than 2 position ones by simlply putting extra grooves into the gas block assembly itself. |
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Originally Posted By M4-Pilot: they can still be easily turned into adjustable ones though by simple drilling the holes in them. That wouldn't really work to get the high speed setting because you would be starting at 750rpm then going slower or not all. For it to work as intended, you would need to first enlarge the gas port in the barrel to get your max desired speed, then drill holes in the bypass regulator to slow it down by varying amounts. Or maybe I have this wrong if the newer barrels are running at the high speed setting????? Internet search shows huge variations in RPM depending on who posted the info. So now the question is.....what size are the gas ports in the barrels on the original early guns? Tony ETA: Yes, I know this is the semi-auto forum, but I'm talking about automatic fire here. |
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Yes, you have to change or drill out the set screw that iis what controls the gas port sizing in those style gas blocks. (Yes there is a ported set screw embedded deep inside that gasblock that is usually so carboned/coppered in that it’s almost imperceptible). I’ve been doing 3 position mk46 adjustable gas blocks that way for a long time and I’ve posted about here before, they work great. Edit: (not using mk46 gas blocks but these gas blocks on suppressed fa mk46s) |
| No, but you really need to soak it a ton to break through the carbon and copper if it’s been used a lot. Try to wiggle an Allen wrench inside the bottom hole of the gasblock and see if you find one that fits the hex of the set screw, sorry don’t remember what size it is) The gasblocks *can* be removed but they are very VERY tight so realigning them onto a new barrel can be pretty hard even if you freeze the barrel and heat the gasblock. If you have a borescope or decide to take the gasblock off you will see that they drilled the port in the barrel itself WAY oversized and then it’s choked way down with the set screw embedded in the gasblock. |
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Originally Posted By M4-Pilot: Sorry, the gas port screw is actually a flathead bit not a hex, and if you want and you're careful you don't have to go through all the trouble of removing it to drill it out. Just be careful bc the actual gas port screw is hard as hell. What size do you drill it out to, when you do the conversions? Tony |
| I’ll just say *bigger*… there was a lot more being done than just the mods to make the gas system adjustable. The 20” barrels were cut to 14.5” and were primarily set up to run about 700 rpm with a surefire RC2 on the highest gas bypass, so as you selected the positions that bypassed less gas (ie smaller hole in pos2 and no hole in pos3) ROF would speed up significantly. RC2s had a shit ton of backpressure though so with modern high flow cans it isn’t as necessary. |
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Originally Posted By R0N: 2d port was removed because too many people left them in adverse to boost the ROF to around 1100, same thing happened to the M240G when it was converted to the. B. That’s because at least in 06 era during infantry school, you were taught by your drill Sgt always to just ran em wide open. |
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I pulled the jet out of the gas block. (18.5" Bbl with the fake gas adjuster) The hole through the jet is .089" and the gas port in the barrel is .113". M6 x 1.0 right hand thread. Tony ETA: I also got my hands on an original gas regulator and the gas bypass hole measures .076".   |
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Originally Posted By R0N: 2d port was removed because too many people left them in adverse to boost the ROF to around 1100, same thing happened to the M240G when it was converted to the. B. What's a joe to do? 
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Originally Posted By KitBuilder: You’re correct. It doesn’t. Those barrel assemblies don’t have any adjustment option. (None of the ones that look exactly like that do.) It’s literally missing the 2nd port and the adjustment capability. Only the first gen M249 barrel assemblies had that option. The early barrel assemblies are identifiable by a flat protrusion on the side of the regulator. The rest of the regulator is smooth. It’s made to use like a lever to flip between the normal and adverse gas positions. The US Army had FN start sending them barrels without this capability. Idk if that happened with the first PIP, or before, but by the mid 90s they weren’t receiving new adjustable barrels anymore. The earlier (adjustable) barrels also had fixed carrying handles which couldn’t fold. For whatever reason, the new part is serrated, but rotating it doesn’t adjust anything. Apparently the overall gas regulator design change was the easiest way to “delete” the gas adjustment capability. It allowed them to use the existing gas block design. Many years later, they eliminated those additional parts and went to a solid one-piece gas block. The US Army wanted the gas adjustment capability removed because they didn’t want M249 gunners using the adverse position when it wasn’t necessary (which allegedly they were doing, to increase the rate of fire). Google Images isn’t doing well for me, but here’s one on an M249 in the originally-adopted configuration from the 1980s. https://www.ar15.com/media/mediaFiles/13082/IMG_6713_jpeg-3502287.JPG Originally Posted By KitBuilder: You’re correct. It doesn’t. Those barrel assemblies don’t have any adjustment option. (None of the ones that look exactly like that do.) It’s literally missing the 2nd port and the adjustment capability. Only the first gen M249 barrel assemblies had that option. The early barrel assemblies are identifiable by a flat protrusion on the side of the regulator. The rest of the regulator is smooth. It’s made to use like a lever to flip between the normal and adverse gas positions. The US Army had FN start sending them barrels without this capability. Idk if that happened with the first PIP, or before, but by the mid 90s they weren’t receiving new adjustable barrels anymore. The earlier (adjustable) barrels also had fixed carrying handles which couldn’t fold. For whatever reason, the new part is serrated, but rotating it doesn’t adjust anything. Apparently the overall gas regulator design change was the easiest way to “delete” the gas adjustment capability. It allowed them to use the existing gas block design. Many years later, they eliminated those additional parts and went to a solid one-piece gas block. The US Army wanted the gas adjustment capability removed because they didn’t want M249 gunners using the adverse position when it wasn’t necessary (which allegedly they were doing, to increase the rate of fire). Google Images isn’t doing well for me, but here’s one on an M249 in the originally-adopted configuration from the 1980s. https://www.ar15.com/media/mediaFiles/13082/IMG_6713_jpeg-3502287.JPG Originally Posted By KitBuilder: You’re correct. It doesn’t. Those barrel assemblies don’t have any adjustment option. (None of the ones that look exactly like that do.) It’s literally missing the 2nd port and the adjustment capability. Only the first gen M249 barrel assemblies had that option. The early barrel assemblies are identifiable by a flat protrusion on the side of the regulator. The rest of the regulator is smooth. It’s made to use like a lever to flip between the normal and adverse gas positions. The US Army had FN start sending them barrels without this capability. Idk if that happened with the first PIP, or before, but by the mid 90s they weren’t receiving new adjustable barrels anymore. The earlier (adjustable) barrels also had fixed carrying handles which couldn’t fold. For whatever reason, the new part is serrated, but rotating it doesn’t adjust anything. Apparently the overall gas regulator design change was the easiest way to “delete” the gas adjustment capability. It allowed them to use the existing gas block design. Many years later, they eliminated those additional parts and went to a solid one-piece gas block. The US Army wanted the gas adjustment capability removed because they didn’t want M249 gunners using the adverse position when it wasn’t necessary (which allegedly they were doing, to increase the rate of fire). Google Images isn’t doing well for me, but here’s one on an M249 in the originally-adopted configuration from the 1980s. https://www.ar15.com/media/mediaFiles/13082/IMG_6713_jpeg-3502287.JPG USMC Saw gunner here from early 90s... We were actually taught to use the adverse setting with a clean gun to increase ROF. Supposedly it was 1100 RPM. |
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