308USK Posted Saturday at 11:38 PM Report Share Posted Saturday at 11:38 PM @98Z5V Hope you can give my brain some ideas...Reusing the lower from my fully running 10.5” 300 Blackout, which includes JP SCS H2 Buffer Spring & Otter Creek Labs Hydrogen L - 7.62 suppressor New upper 8.2” 300 Blackout barrel Gas Port originally 0.0855” Adjustable Gas Block set wide open 1 round, 200gr S&B subsonic Rifle cycles, but will NOT lock back on empty mag ****************************** Drilled out using #43 machinists bit, re-assembled and tested. AGB wide open 1 round, 200gr S&B subsonic Rifle cycles but BCG does not lock back on empty mag. Will not run more than 1 round though.. ***************** Drilled out using #42 machinists bit, re-assembled and tested. AGB wide open 1 round, 200gr S&B subsonic Rifle cycles but BCG does not lock back on empty mag. Will not run more than 1 round though.. Brass ejecting at ~4:30-4:45 ################# Opened up AGB to 28 clicks 1 round, 200gr S&B subsonic Rifle cycles but BCG does not lock back on empty mag. Will not run more than 1 round though.. Brass ejecting at ~4:15-4:30 ################# Opened up AGB to 38 clicks 1 round, S&B 200gr subsonic Rifle cycles but BCG does not lock back on empty mag. Will not run more than 1 round though.. Brass ejecting at ~3:45-4:00 ################# Closed AGB to 18 clicks (wide open) Swapped JP SCS H2 for a JP SCS REGULAR (no tungsten weights) 1 round, S&B subsonic Rifle cycles but BCG does not lock back on empty mag. Will not run more than 1 round though.. Brass ejecting at ~4:30’ish ***Video below (if you zoom in to 300% and you can see the ejection pattern more clearly)*** I have another upper and 8.2" barrel; measured the gas port 0.0880. Assembled with the upper and put an AGB on it and will take it out to the range to test it with my regular 5.56 M4 lower and see how it fares that way. I have another JP SCS regular weight (as stated above), but DO have 1 tungsten weight I could add to make it an H1. On the 8.2" barrel I have drilled out to #42 drill bit size, based on the data I captured so far, the rifle still looks like it is undergassed. Am I off my rocker? Should I drill out using #41 drill bit? Should I add the tungsten weight to my current H2 buffer to make it an H3 buffer? Quote Link to comment Share on other sites More sharing options...
shooterrex Posted Sunday at 01:22 AM Report Share Posted Sunday at 01:22 AM Put an actual buffer and spring in it and see how it works. The JP stuff is racegun parts. Is your gas system pistol or carbine length? Quote Link to comment Share on other sites More sharing options...
308USK Posted Sunday at 01:57 AM Author Report Share Posted Sunday at 01:57 AM On 4/12/2025 at 7:22 PM, shooterrex said: Put an actual buffer and spring in it and see how it works. The JP stuff is racegun parts. Is your gas system pistol or carbine length? Pistol length gas. I'll run my 2nd upper/barrel with my M4 lower and standard buffer (and bring an extra regular buffer weight) when I do my next testing late Monday afternoon. :) Quote Link to comment Share on other sites More sharing options...
308USK Posted Sunday at 08:16 PM Author Report Share Posted Sunday at 08:16 PM Updated the video to zoom in on each shot, to better discern ejection location. Have my 2nd upper attached to my M4 lower, so I can run a normal buffer weight & spring. Have an H1 buffer weight for that configuration as well, so I can gather all the data I can when I can get to the range. Was planning to go on Monday after work, but have two appointments I forgot about, so it'll likely be Thursday or Friday before more data can be acquired. Quote Link to comment Share on other sites More sharing options...
98Z5V Posted Sunday at 11:40 PM Report Share Posted Sunday at 11:40 PM Pete, here's what I see from the data above. Question first, though - is this gun ALWAYS going to live suppressed, and shoot the 200gr projos? That will simplify the process, if it's a Yes... Then, we don't have to worry about getting it operational for lighter supers, and no can... 8.2" with pistol gas. I don't even think Carbine gas would fit on this barrel length - not enough barrel. 0.0855" initial gas port diameter. Went to #43, 0.089". Went to #42, 0.0935". Buffer / recoil system never changed, in initial tests. Case dispersion never changed, all in the same area. ^^^ Data gathered. Observations. 300BLK can be a MFer to tune right. That's a fact. Too much recoil system, and not enough gas port diameter. Even though the ejection pattern fits the clock-mold that's on the internet - ejection didn't change, with increasing gas pressure through port diameter increases. Max you can go on a gas port diameter is 0.125". Eighth of an inch. That's because 0.125" is the max ID of a common gas tube, and the max ID that gas block ports are drilled to - for that very reason. Anything over 0.125" wouldn't matter, because then you'd have the gas tube ID as the restriction. When gas port diameters for a barrel config are starting to get close to 0.125", that's usually when barrel manufacturers up the size on the gas block journal dimension. 0.625 pencil journals grow to 0.750" journals, then 0.825", then 0.936" journals... Those are the 4 common sizes. Now, you DO have an adjustable gas block on there, already - so there is no penalty in doing what I'm going to recommend... First, I'd just put a standard AR15 Carbine buffer in there (3.0oz, 3.250" length) with a regular carbine buffer spring. Test the gun, and look for the cyclic operation to change, or the ejection pattern to change - see of that makes a difference. If that makes a difference, we got somewhere. If it does NOT make a difference, I'd go straight to blowing that barrel gas port diameter to 0.100", or as close as you can get to it, and rest with the lighter recoil system still in there - and look for changes in cyclic operation or ejection pattern. 0.100" should be a #39 (0.0995") or a #38 (0.1015"). There is zero penalty in doing this, and punching big, because the gun already has an adjustable gas block that can be dialed back, from wide open, in case that diameter is too much... Quote Link to comment Share on other sites More sharing options...
98Z5V Posted Sunday at 11:50 PM Report Share Posted Sunday at 11:50 PM Gas port diameter in the gas BLOCK... Check that for 0.125"... Quote Link to comment Share on other sites More sharing options...
308USK Posted Monday at 12:05 AM Author Report Share Posted Monday at 12:05 AM On 4/13/2025 at 5:40 PM, 98Z5V said: Pete, here's what I see from the data above. Question first, though - is this gun ALWAYS going to live suppressed, and shoot the 200gr projos? That will simplify the process, if it's a Yes... Then, we don't have to worry about getting it operational for lighter supers, and no can... 8.2" with pistol gas. I don't even think Carbine gas would fit on this barrel length - not enough barrel. 0.0855" initial gas port diameter. Went to #43, 0.089". Went to #42, 0.0935". Buffer / recoil system never changed, in initial tests. Case dispersion never changed, all in the same area. ^^^ Data gathered. Observations. 300BLK can be a MFer to tune right. That's a fact. Too much recoil system, and not enough gas port diameter. Even though the ejection pattern fits the clock-mold that's on the internet - ejection didn't change, with increasing gas pressure through port diameter increases. Max you can go on a gas port diameter is 0.125". Eighth of an inch. That's because 0.125" is the max ID of a common gas tube, and the max ID that gas block ports are drilled to - for that very reason. Anything over 0.125" wouldn't matter, because then you'd have the gas tube ID as the restriction. When gas port diameters for a barrel config are starting to get close to 0.125", that's usually when barrel manufacturers up the size on the gas block journal dimension. 0.625 pencil journals grow to 0.750" journals, then 0.825", then 0.936" journals... Those are the 4 common sizes. Now, you DO have an adjustable gas block on there, already - so there is no penalty in doing what I'm going to recommend... First, I'd just put a standard AR15 Carbine buffer in there (3.0oz, 3.250" length) with a regular carbine buffer spring. Test the gun, and look for the cyclic operation to change, or the ejection pattern to change - see of that makes a difference. If that makes a difference, we got somewhere. If it does NOT make a difference, I'd go straight to blowing that barrel gas port diameter to 0.100", or as close as you can get to it, and rest with the lighter recoil system still in there - and look for changes in cyclic operation or ejection pattern. 0.100" should be a #39 (0.0995") or a #38 (0.1015"). There is zero penalty in doing this, and punching big, because the gun already has an adjustable gas block that can be dialed back, from wide open, in case that diameter is too much... Thanks!!!!! I will be drillin' like a Texas oil-hand!! Quote Link to comment Share on other sites More sharing options...
98Z5V Posted Monday at 12:16 AM Report Share Posted Monday at 12:16 AM Quote Link to comment Share on other sites More sharing options...
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