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Posted: 2/9/2024 3:12:49 PM EDT
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Here's the build: Used Colt 6520 upper, new Colt BCG/CH, new Colt H2 buffer w/stock carbine buffer spring, new Colt 10.3" M4CQBR barrel. Lower is Colt M4 Carbine w/LT MBT-2S. Muzzle device is a Surefire Blast Warden and not a suppressor (Surefire A2 FH underneath). Couldn't find a Colt carbine gas tube so used a new Wilson Combat carbine gas tube. Mags: Lancer 20rd, Surefeed aluminum 20rd. Ammo: Hornady Black 75gr Interlock HD SBR. 5.56mm Issue: With Lancer mag, will not pick up the next round. With SF mag, attempts to pick up next round but jams. Other data point: Suppressed Colt 6933 (11.5") w/H3 buffer runs fine during the session using the same ammo & mags.  |
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I did not try that as I ran out of time. Also, I'll need to find a range that's comfortable with me making those kind of changes on the line. Is the idea that I would need an H3 buffer (vs a standard H)? What if that doesn't work (or work well enough)? Trying to sort out all the variables so I know what I need to bring next time to see what I can do to get it working. |
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Quoted: Shoot with one round in the mag and see if it locks back. I'll try that. Swap uppers and lowers to see if the problem follows the upper. I'll try that. It always hand feeds without issue? It feeds fine if I manually rack the bolt. How much travel behind the bolt catch do you have? It goes all the way back such that I can't see it in the ejection port opening. Gas key feels tight? Yes and it looks staked properly. |
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Quoted: You should be able to pull the bolt back and have 1/4" to 3/8" of room between the front of the bolt and the bolt catch. Swapped over the entire lower (w/H3 buffer) and entire BCG/CH from the suppressed gun. I'll shoot this one unsuppressed & suppressed and see how it goes. I really want this setup to run. I'd get rid of the 6933 over this if I can get it to run. |
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Quoted: Hard to get a measuring stick in there, but eyeballing it, I'd say I have at least that much if not more. Swapped over the entire lower (w/H3 buffer) and entire BCG/CH from the suppressed gun. I'll shoot this one unsuppressed & suppressed and see how it goes. I really want this setup to run. I'd get rid of the 6933 over this if I can get it to run. Quoted: Quoted: You should be able to pull the bolt back and have 1/4" to 3/8" of room between the front of the bolt and the bolt catch. Swapped over the entire lower (w/H3 buffer) and entire BCG/CH from the suppressed gun. I'll shoot this one unsuppressed & suppressed and see how it goes. I really want this setup to run. I'd get rid of the 6933 over this if I can get it to run. Insert empty mag, rack bolt, let it stop on catch, pull bolt back and see how much movement it has behind the catch. Measure the charging handle movement if you want to be more accurate. |
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Quoted: Sounds undergassed. You may want to try a standard buffer or open the gas port a small amount. It's probably simply too heavy buffer though. I always start with a standard carbine buffer and increase weight only if needed. Out of curiosity why did you start with H2/H3 weights? |
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Quoted: Agree all the way up to opening the gas port lol. I'd check a lot of other things before I did that, especially if you may ever shoot this gun suppressed. I'd work from the gas block itself through alignment, the tube, gas key and interface, etc., etc. It's probably simply too heavy buffer though. I always start with a standard carbine buffer and increase weight only if needed. Out of curiosity why did you start with H2/H3 weights? Quoted: Quoted: Sounds undergassed. You may want to try a standard buffer or open the gas port a small amount. It's probably simply too heavy buffer though. I always start with a standard carbine buffer and increase weight only if needed. Out of curiosity why did you start with H2/H3 weights? 
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Quoted: Reading the internets. Quoted: Quoted: Quoted: Sounds undergassed. You may want to try a standard buffer or open the gas port a small amount. It's probably simply too heavy buffer though. I always start with a standard carbine buffer and increase weight only if needed. Out of curiosity why did you start with H2/H3 weights? Some guns do benefit from the heavier weight, but I'm at about 50% of my ARs that just need a regular carbine buffer even suppressed (which is about 99% of my shooting). A couple did exactly what you describe when I tested H1/H2 buffers in them. |
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I would look for gas leaks at the key, gas block, bell on the gas tube, and proper gas tube length. Then check for a misaligned gas block, check the gas port size and check for a misdrilled gas tube. If the port size is small and everything else is good, open it up a little. |
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have you tried different ammo , maybe some m193 55gr ball ammo. i've had issues with that bullet feeding smoothly in usgi mags in some guns if it does the same with m193 it sounds under gassed as has been mentioned above did the barrel come with the fsp already pinned , did you check alignment with the gas port does the wilson gas tube fully enter the gas key if you can i would go over the gas system to make sure everything is "all good". pull the fsb and pin the gas port to check it and alignment of the fsb with the gas port. measure the length of the gas tube and make sure it's going into the gas key far enough. for some reason that wilson combat gas tube is standing out as an issue to me. check your gas key to make sure it's tight and that it lines up with the gas tube good with that list of parts that thing should run like a top , it's an awesome little blaster. personally though i would have to put a set of carbine length m16 triangular style handguards on it |
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Quoted:  (Off to see how much more this parts build is going to cost me) shorty handguards i have a set somewhere around here i was going to cut down to work with pistol length gas fsb on a 7" kitty kat barrel |
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Got the upper back and went to the range today. Standard carbine buffer seems to work (3oz). Put an H buffer back in and it ran ok with the Lancer mag, but had intermittent problems locking back after the last round with the SureFeed mag. Otherwise, both fed multiple rounds without problems. It also ran with the H buffer on a mag of Mk262, but didn't run it all the way out to see if it locked back. I also had a brand new (literally arrived this morning) Colt 10.3" M5 CQC upper (complete) out today that ran perfect with both an H & H2 buffer, unsuppressed. Not sure why the 6520 upper won't run like that; not sure what I want to do with it. |
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Steve ran through all the components to make sure everything was sized correctly, in good repair and assembled correctly. I can run some IMI Razor, BH Mk262 or Aussie 69gr BTHP but that's about all I have for alternatives (can't run steel core at the range I go to). |
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