Posted: 7/23/2022 10:59:04 PM EDT
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Hey all- I pieced together a Super Duty with a Geissele 16" barrel, Geissele gas block and everything else G except a CMT bcg that headspaced fine with this barrel. Anyway, I was running a Geissele H2 buffer and spring and it was like there was no gas. So, I swapped out a standard carbine buffer and standard spring and spaced the gas block .025" off the shoulder and it is cycling now, but short-stroking a bit (not cycling far enough to pick up the next round and a couple cases didn't clear the ejection port). So, back to the drawing board. I put the gas block back to flush with the shoulder and I will test fire again tomorrow, but I haven't ever had a gun gassed so light that this was an issue. The gas key on the bcg is tight so I know I'm not losing gas there. Shit, I even staggered out the gas rings, knowing it isn't going to matter. BCG is lubed up good too. Anyway, I am using some Fiocchi 556, so I don't know if the ammo might be contributing, but I haven't ever had cycling problems with this ammo before. I'm sure it is loaded to NATO specs. Any thoughts? EDIT to add: I'll try the H2 buffer and spring again tomorrow as well, because as best I can tell this is what the 16" SD ships with from the factory. Also, I did clean the chamber with a GI chamber brush with some Hoppe's as well. |
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I would be very surprised if the barrel had a gas port so small that it wouldn't cycle using 5.56 with a standard carbine buffer on purpose. If you have a spare BCG or gas block I would try swapping those on just to test. If you have pin gauges to check out the gas port size to see if it seems too small that may also tell you something. Your screen name made me remember I have to shoot M4 and M9 next month 
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| A few people in the tech forums have had issues with the barrels they bought. G confirmed it's a pretty small gas port size and intended to run full power ammo. Anything known to be light/under powered will not run. Rifles from them all seem to run well, so they may have a different gas port size than they barrels they are selling. |
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Quoted: I would be very surprised if the barrel had a gas port so small that it wouldn't cycle using 5.56 with a standard carbine buffer on purpose. If you have a spare BCG or gas block I would try swapping those on just to test. If you have pin gauges to check out the gas port size to see if it seems too small that may also tell you something. Your screen name made me remember I have to shoot M4 and M9 next month Good luck at CATM. Easy shit! Yeah, I checked the port size and it must be .061", because a .062" drill I had on hand barely didn't fit. I checked out the gas block and its port is pretty large in comparison to the barrel port, plus the Geissele block has an alignment pin that makes it pretty hard to screw up. I checked with some weedeater line to make sure the gas key port is unobstructed and that isn't a problem, but just to be on the safe side I'm going to grab a spare BCG to take with me to the range. When I initially test fired it, it had a light film of oil in the chamber. So, hoping maybe I was getting a bit of hydrolock due to the oil in the chamber and the brass didn't want to extract and maybe that was slowing up the cycling of the bcg. BTW, I double checked, and it is GECO I was shooting, not Fiocchi. I do have some Lake City M855 so I'll try it and see if it is just an ammo problem. It probably won't be an issue once my suppressor mount for it gets here, but I want the thing to run unsuppressed as well! Thanks for the inputs! |
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Quoted: A few people in the tech forums have had issues with the barrels they bought. G confirmed it's a pretty small gas port size and intended to run full power ammo. Anything known to be light/under powered will not run. Rifles from them all seem to run well, so they may have a different gas port size than they barrels they are selling. Thanks, good input! Yeah, I am going to rule out ammo today with some LC M855. I am hoping that is the issue. It was GECO 55 gr that I used, not Fiocchi, by the way. Should still meet M193 specs, but maybe it's light. |
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Quoted: Any luck? If I remember correctly the Geco stuff is 5.56 and should cycle things just fine in pretty much anything. I doubt Geissele is selling these things with the thought that everyone will be running M855A1 at all times. That was their intent per Bill in the link a couple posts below yours. |
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Quoted: That was their intent per Bill in the link a couple posts below yours. I saw the part about it being intended for M855 or M193 which I believe the Geco he was shooting is a "clone" of M193. What I meant by my comment was that I don't think there should be a problem with what OP was shooting to make his gun run. M855A1 is about the hottest thing I could think of and for civvies is something like $2+ a round. That's why I was saying I doubt they would make it not work for something like those two common rounds, and instead for a higher pressure round that is likely rarely to be used. |
| Yeah, ran some LC M855 and some Win white box M193 today and it won't run with the Geissele H2 spring and buffer. With a standard spring and carbine buffer it will half-assed run, but still short stroking every couple rounds and not picking up a round out of the mag. I'm at a bit of a loss, and I've been working on these things a long time. Only thing I can think of is the port size is a hair too small. But why it would run with most set up the same and not this one is a bit of a mystery. In fact, I headspaced and swapped in a Colt BCG of known quality from a friend's rifle and still the same issue. It was throwing the brass to about 5:00 to 5:30. So, it is almost certainly a light-gas issue. So, I guess I should call Bill... @GACS |
@GACS this rifle is almost entirely Geiselle. I just priced it out that I could put it together for less piece by piece. I'm a trained small arms repairman, with 30+ years of experience on the AR/M16 series, BTW. So, I'm not a COMPLETE moron, although my wife would probably argue that. From front to back, it consists of: " />  -properly indexed A2 flash hider w/ crush washer -New Geissele 16" barrel I got from Aim Surplus. -Geissele gas block. I have tried it flush with the shoulder and .025" off. -New Aero or Anderson gas tube. Can't remember which. It was a spare mid-length gas tube I had on hand. -Mk4 rail -Geissele upper/lower set -CMT BCG (sorry, couldn't afford a REBCG yet). Headspace checks out plenty good with a FIELD gauge. -Tried with a Geissele H2 with twisted spring (doesn't even try to unlock and cycle). With a spare carbine weight buffer and standard spring it cycles but short strokes every couple rounds and throws brass to around 5-5:30. -Lower group consists of Geiselle LPK with SSA-EX trigger group. -Geissele buffer tube Also, When I had the gas block off I checked the port size with a .062" drill. It barely wouldn't go in, so it has to be somewhere very close to .062". I don't have a pin gauge set, and didn't have a drill that measures .061". So, it ain't exact, but I'm pretty sure we're in the .061-.062" range. |
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Quoted: Is the barrel not dimpled for the gasblock? I thought they came with one dimple opposite the gas port. Yes, it is dimpled. The dimple is large enough to tighten the set screw flush and at .025". The instructions for the gas block say flush, but when it didn't run I figured I better try the gas block at the more traditional .025" off the shoulder as well. Didn't matter, either way. |
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Quoted: Pretty sure they confirmed .062 for 16" gas port size. Would be sweet with a suppressor. I agree it probably would be soft shooting with a can. I haven't tried with my can yet because: 1) it should run suppressed and unsuppressed, and 2) I have a YHM QD mount for my R9 getting delivered this week for this rifle, but it hasn't arrived yet. Either way, it has to run unsuppressed. Period. |
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Quoted: I agree it probably would be soft shooting with a can. I haven't tried with my can yet because: 1) it should run suppressed and unsuppressed, and 2) I have a YHM QD mount for my R9 getting delivered this week for this rifle, but it hasn't arrived yet. Either way, it has to run unsuppressed. Period. Quoted: Quoted: Pretty sure they confirmed .062 for 16" gas port size. Would be sweet with a suppressor. I agree it probably would be soft shooting with a can. I haven't tried with my can yet because: 1) it should run suppressed and unsuppressed, and 2) I have a YHM QD mount for my R9 getting delivered this week for this rifle, but it hasn't arrived yet. Either way, it has to run unsuppressed. Period. Bill starts chiming in here: https://www.ar15.com/forums/AR-15/Geissele-16-CHF-Barrel-A-Quick-Look/12-772048/?page=3#i8434970 I'm thinking they went a tad too small for unsuppressed use. ETA: Their intent was for suppressed use while being reliable with hot ammo. I by no means want them to open the port up for cheap steel ammo. Reliable with 855 or 193 would be nice, while keeping suppressed from being harsh. |
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@CATM some of the early stand alone barrels did have tighter ports more optimized for hotter ammo and and shooting suppressed however we will take a look at your barrel top to bottom to see what's going on and make sure you are 100%. If you'd be good sending it in to us for an RMA we will inspect and take care of you. Please reach out to our CS people at our tech support portal and tell them GACS sent you. We'll get you RMA info and a prepaid return tag to send your barrel into us, or if you want to send the entire gun in we can check that out too. Just let them know when you speak with them how you'd like to roll with it ad we will support. -GACS |
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Well, I fixed it. Worked my way up progressively to a #50 drill and the port now pin-gauges to .069" and it throws brass unsuppressed at 4:00 and suppressed at 3:00. It is VERY soft shooting. It was even better when I had the port at .0635" and suppressor on. But unfortunately, it was still short-stroking unsuppressed. Right now it is still very soft suppressed, and it is reliably running through the cycle of operations and locking the bolt back on an empty mag unsuppressed. So, I am good to go! PS: Geissele was absolutely 100% willing to fix it, but I hated the thought of sending it off to PA for something as simple as port size. |
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Quoted: Well, I fixed it. Worked my way up progressively to a #50 drill and the port now pin-gauges to .069" and it throws brass unsuppressed at 4:00 and suppressed at 3:00. It is VERY soft shooting. It was even better when I had the port at .0635" and suppressor on. But unfortunately, it was still short-stroking unsuppressed. Right now it is still very soft suppressed, and it is reliably running through the cycle of operations and locking the bolt back on an empty mag unsuppressed. So, I am good to go! PS: Geissele was absolutely 100% willing to fix it, but I hated the thought of sending it off to PA for something as simple as port size. Glad to here it worked out. Which buffer are you using with the now .069 gas port? |
| I ended up having to go to .073". At .069" I was getting occasional failures to lock on an empty mag. At .073" it is running 100% both suppressed and unsuppressed. I put an H buffer in it, with a standard spring. The Geissele H2 with their spring is a bit much. I think I am going to order a Geissele H buffer and try it with their twisted spring next. But, with a standard H and spring it is good to go. The SSA-EX trigger is superb, by the way. Today was the first time I got to really put it through its paces and the trigger is excellent! |
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Quoted: I ended up having to go to .073". At .069" I was getting occasional failures to lock on an empty mag. At .073" it is running 100% both suppressed and unsuppressed. I put an H buffer in it, with a standard spring. The Geissele H2 with their spring is a bit much. I think I am going to order a Geissele H buffer and try it with their twisted spring next. But, with a standard H and spring it is good to go. The SSA-EX trigger is superb, by the way. Today was the first time I got to really put it through its paces and the trigger is excellent! Good info thanks. Did you try anything in between.069-.073? I ask cause my Colt 6960 16” mid has a .071 gas port and it runs great with a H buffer, haven’t tried a H2 yet. |
| There wasn't one in between. I was using numbered drill bits. A #50 is supposed to be .070, but I pin gauged the port after drilling and it gave me gave me a .069" port. Just under .070. The next size larger numbered drill was a #49 which is supposed to be .073", and it gave me an exact .073" port. |