308USK Posted May 6, 2023 Report Share Posted May 6, 2023 Hey gang, my brother was sighting-in his 6.5CM that he just put a new barrel on...hoping y'all can help identify the issue: Background: My brother has built countless rifles (small-frame AR's, large-frame AR's, yada yada yada), so this one should not be any different; but, for some damn reason it is. The upper is not one he built, so there are some unknowns there. I can get the details, if it is important/needed, to identify the issue. Rifle used to cycle consistently and would not have any issues. The barrel was too damn heavy, so my brother bought a new lighter-weight barrel and put it all together. Rifle has an adjustable gas block (he has cans, so he uses AGB's just out of habit). Issue Description: Round chambers and will shoot the first round, but then will not feed the next round. He started with the AGB wide open, then down to standard open (18 clicks open), then +/- a small # of clicks, trying to balance gas vs. mass. More often than not, the rifle would require dropping the mag and clearing the jammed round (see attached pic). Hopefully the brainiacs on the forum can help narrow down what is causing the issue! Quote Link to comment Share on other sites More sharing options...
shooterrex Posted May 6, 2023 Report Share Posted May 6, 2023 What is the gas port diameter????? Probably the problem if the gun ran with the other barrel. Quote Link to comment Share on other sites More sharing options...
Armed Eye Doc Posted May 6, 2023 Report Share Posted May 6, 2023 Did it run soft point ammo in the last configuration? Some ARs do not like them. Perhaps the dent in the tip is catching on the feed ramps?? Quote Link to comment Share on other sites More sharing options...
308USK Posted May 6, 2023 Author Report Share Posted May 6, 2023 On 5/5/2023 at 7:53 PM, shooterrex said: What is the gas port diameter????? Probably the problem if the gun ran with the other barrel. Journal size is .750; I will have my brother measure the actual hole size with digital caliper (oh wait...I had to drill baby drill myself (lmao). DAMN why didn't I think of that?) and report back. (insert facepalm here) Quote Link to comment Share on other sites More sharing options...
308USK Posted May 6, 2023 Author Report Share Posted May 6, 2023 On 5/5/2023 at 8:45 PM, Armed Eye Doc said: Did it run soft point ammo in the last configuration? Some ARs do not like them. Perhaps the dent in the tip is catching on the feed ramps?? My brother will be measuring gas port hole size, but will also inspect the feed ramp area... Thanks to all helping...always appreciated! Quote Link to comment Share on other sites More sharing options...
98Z5V Posted May 6, 2023 Report Share Posted May 6, 2023 3 hours ago, 308USK said: Issue Description: Round chambers and will shoot the first round, but then will not feed the next round. Start with getting a picture up of that ejector face. I'll bet it's square as hell. Not rounded in the least. After that - I'm with the guys on the barrel. It ran. New barrel. Doesn't run now. Gas port diameter. Gas tube length. Both. Pics of parts are just about gonna be mandatory here. Quote Link to comment Share on other sites More sharing options...
308USK Posted May 6, 2023 Author Report Share Posted May 6, 2023 On 5/5/2023 at 11:39 PM, 98Z5V said: Start with getting a picture up of that ejector face. I'll bet it's square as hell. Not rounded in the least. After that - I'm with the guys on the barrel. It ran. New barrel. Doesn't run now. Gas port diameter. Gas tube length. Both. Pics of parts are just about gonna be mandatory here. Will ask my brother to get a pic of the ejector face as well as the gas port diameter measurement! Will reply once I have info...thanks again y'all... Quote Link to comment Share on other sites More sharing options...
308USK Posted May 8, 2023 Author Report Share Posted May 8, 2023 Okay gents, have some information. 1) Barrel length is 22" 2) Rifle-length gas 3) 6.5CM 4) Gas Port Size measured 5x with 2 different sets of calipers; consistently as seen in attached picture +/- a wee bit. What "should" the gas port size be for this configuration? (He's ready to drill, once he knows the proper port size). Quote Link to comment Share on other sites More sharing options...
98Z5V Posted May 8, 2023 Report Share Posted May 8, 2023 WHOA!!! WAY TOO SMALL!!! Give me a few minutes, and I'll work on a number that will definitely work. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted May 8, 2023 Report Share Posted May 8, 2023 On 5/5/2023 at 7:58 PM, 308USK said: Journal size is .750 3 hours ago, 308USK said: 1) Barrel length is 22" 2) Rifle-length gas 3) 6.5CM With those number for a barrel config, he needs 0.075" minimum, possibly up to 0.080". Do 0.075" first, and run the gun. I bet it works like a champ. Don't go big, to 0.080" unless you need to. Run it at 75 thou first. Quote Link to comment Share on other sites More sharing options...
Armed Eye Doc Posted May 8, 2023 Report Share Posted May 8, 2023 1 minute ago, 98Z5V said: With those number for a barrel config, he needs 0.075" minimum, possibly up to 0.080". Do 0.075" first, and run the gun. I bet it works like a champ. Don't go big, to 0.080" unless you need to. Run it at 75 thou first. He already runs an adjustable gas block for a can. Shouldn't he go large? Quote Link to comment Share on other sites More sharing options...
98Z5V Posted May 8, 2023 Report Share Posted May 8, 2023 1 minute ago, Armed Eye Doc said: He already runs an adjustable gas block for a can. Shouldn't he go large? Nope, no way. Make a working gun first, and confirm that you have a working gun. Priority #1. No can, adjustable gas block WIDE FUKIN' OPEN, and make a working gun. After that, if there's an issue with the can - you mess with the gas port again, to make the can run. Make a working gun, first. Don't Complicate Shiit With Complicated Shiit... Quote Link to comment Share on other sites More sharing options...
98Z5V Posted May 8, 2023 Report Share Posted May 8, 2023 (edited) He's gonna need 0.075" gas port diameter for the gun to run unsuppressed, no matter what. Bare minimum. AGB Wide Open. Once he adds the can, backpressure will be increased, and he can dial that AGB down, decrease pressure, and make the can bearable. No need to make a gas port bigger than it really needs to be, to run unsuppressed. Once you add the suppressor, dial it down. Edited May 8, 2023 by 98Z5V Quote Link to comment Share on other sites More sharing options...
Matt.Cross Posted May 8, 2023 Report Share Posted May 8, 2023 On 5/7/2023 at 11:32 PM, Armed Eye Doc said: He already runs an adjustable gas block for a can. Shouldn't he go large? Larger = more flow = less velocity & more pressure at the BCG, can is already adding more pressure, optimizing the flow via the gas port renders the AGB redundant. You want the gun to run properly via proper gas port size rather than via AGB restriction. ETA: Tom replied faster than I could, but we said essentially the same thing.... Quote Link to comment Share on other sites More sharing options...
98Z5V Posted May 8, 2023 Report Share Posted May 8, 2023 Well stated, also. You're the technical guy - I'm the guy with the hammer. Quote Link to comment Share on other sites More sharing options...
dpete Posted May 8, 2023 Report Share Posted May 8, 2023 12 hours ago, 98Z5V said: Well stated, also. You're the technical guy - I'm the guy with the hammer. Sometimes it seems you can intimidate $hit to work with the threat of using said hammer. Quote Link to comment Share on other sites More sharing options...
308USK Posted May 22, 2023 Author Report Share Posted May 22, 2023 Hey Gang...my Brother has not had a chance to get to the range yet. The barrel has been drilled (using a #47 bit) and everything put back together. Hopefully this coming week won't quite so hectic and he can get to the range to confirm proper cycling and whatnot. ;) Note: The original BCG was also a POS (just pulling out of the rifle while on the workbench at home required some substantial effort. We put the BCG from my large frame AR in (.308) just for fitment verification. It went in smooth as butter and came out just as easily...so he bought a new BCG and it went in smoothly as well, so that should eliminate that issue. Quote Link to comment Share on other sites More sharing options...
308USK Posted May 28, 2023 Author Report Share Posted May 28, 2023 Follow-up... My brother and I hit the range today. His rifle, post gas-port drill out is cycling and working 100%. He even put a can on and adjusted the gas block a couple of clicks and the rifle still worked 100%. He's happy... Thanks again, to the knowledgeable peeps here! Quote Link to comment Share on other sites More sharing options...
98Z5V Posted May 31, 2023 Report Share Posted May 31, 2023 (edited) Great news, man! Thanks for the follow-up. Glad that thing is running. EDIT - What number did he hit, to make it run? 0.075" or 0.080"? Just so I know for my own information. Somebody needs to keep me straight around here... Edited May 31, 2023 by 98Z5V Quote Link to comment Share on other sites More sharing options...
308USK Posted June 1, 2023 Author Report Share Posted June 1, 2023 On 5/30/2023 at 10:00 PM, 98Z5V said: Great news, man! Thanks for the follow-up. Glad that thing is running. EDIT - What number did he hit, to make it run? 0.075" or 0.080"? Just so I know for my own information. Somebody needs to keep me straight around here... He drilled out using a #47 bit, so just hair over 0.07850. HA! Keep YOU straight? Not a prayer in hell sir... (rotf) Quote Link to comment Share on other sites More sharing options...
98Z5V Posted June 2, 2023 Report Share Posted June 2, 2023 5 hours ago, 308USK said: He drilled out using a #47 bit, so just hair over 0.07850. HA! Keep YOU straight? Not a prayer in hell sir... (rotf) GREAT NEWS! I was only asking, trying to see where he landed for an operational gas gun, just to backstop the voodoo math I've come up with, to calculate all this craziness. So, really - it's keeping me straight... Well, as much as possible, in that regard... FINE work on that gun. I'm happy with that... Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.