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Posted: 12/31/2020 7:08:59 PM EDT
I have built 5 rifles over the last 6 months all using similar high quality parts. All 5 used Criterion Barrels. I'm in the process of breaking them in but experiencing malfunctions with 3.
Rifle 1: BCM upper, BCM BCG, PSA lower and Parts, Geiselle SSA trigger, BCM/Vltor A5 buffer tube, buffer and spring. Criterion Barrel is 12.5" carbine gas w/ Surfire Muzzle Brake. Port is appx .064. BCM gas block and tube. Bolt was purchased from Criterion fitted. Headspace has been gauged. Brass measures near minimum headspace. Rifle 2 and 3 are nearly identical PSA Upper, PSA bolt Carrier Group, PSA lower, CMMG parts kit, Geiselle SSA and Milspec PSA Triggers, Buffer Tubes from SB Tactical, BCM Springs and Buffers. Barrels are Criterion 13.9 midlength gas system w/ .073" ports. Pinned and welded Surefire 3 prong flash suppressor. Headspace has been properly gauged w/ go/no go gauges on both. Brass measures moderate on headspace. All 3 rifles experienced failure to feed and failure to Lock back on last round. Initially ran with oil on buffer/spring and grease on BCG bearing points. Barrel break in was w/ American Eagle 55gr. Shoot 1 clean for 5 shots, Shoot 5 and clean. Up to 25 shots while zeroing. I've disassembled and verified gas port alignment with block, verified minimal interference at gas tube/key. Verified buffer tube vent was clear, experimented with known lowers, swapped Bolt Carriers etc. Magazines used were an older colt 20 round, 20 and 30 round Magpul gen 3 and a USGI green follower 30 round. I've tried VERY lightly polishing the chambers with mothers mag and aluminum polish on mop. Lightly sanding the port with 400-600 grit paper rolled up (looing for bur or obstruction, ran pipe cleaners through the gas tubes etc. |
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[#1]
I am hesitant to drill/ream the gas ports up but can... I have a mill and proper drills but would probably use a chucking reamer if increasing less than .005. Concerned about chrome lining being damaged. I suspect that parts need more time to wear together and or there are burrs at the gas port that are fouling with copper and restricting gas as round count increases. Want the rifles to be 100% reliable. Round count through each rifle is 100-150 rounds. I've tried standard weight buffer up to H2 (A5H0 to A5H2 in the case of the 12.5). I've swapped around buffer springs as well based on length to see if it had any affect. Gas rings are offset and are fitting tight in carrier. Gas key was checked with oil around the key and compressed air.
Current thoughts are to clean everything thoroughly again after last trip when I shot wet with motor oil. Clean chambers and barrels thoroughly. (I've been using hoppes, followed by sweets and a rinse of alcohol to neaturalize the sweets. I was only using patches on dewey rod/jag but have recently ran a bronze brush through with the hoppes hoping to dislodge any copper building up at port) and shoot more to see if more break in is all that is necessary to wear parts together and reduce friction or shoot burs out around the gas port. If it is just a break in issue that is still concerning... most factory rifles I've owned functioned from the start. |
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[#2]
Each trip back out yeilded minor improvements. Sometimes they would work correctly for 10-20 rounds before exhibiting problems but eventually they all started acting up and short stroking. Brass appears to be ejecting at 4 oclock on all. Non are running suppressed yet. Most recent trip I ran them VERY wet with some used motor oil (soaked the bolts with the oil I had sufficient quantities of). After 30-50 rounds through each they were firing without short stoking but were loking back on the carrier not the bolt face. Again running well until minor fouling caused what appears to be an undergassed condition.
Thoughts? |
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[#3]
Failure to lock back is insufficient recoil energy, usually.
Not enough gas, or not enough shoulder. I guess also possibly weak ammo - but you didn't specify Wolf/Tula or reloads so I didn't bring that up as a primary possibility. |
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[#4]
You indicated swapping buffer springs based on length. Have you considered putting in a lighter spring?
RTB Reduced Power Carbine Spring |
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[#5]
I've used primarily AE223 (Federal American Eagle Bulk Box) 55gr. I did use the 12.5" doing some load development. 77gr SMKs w/ 23.5 8208 XBR... a fairly hot load. 12.5" was 2530fps.
I have looked into reduce power recoil springs. Specifically the Springco Yellow... From the MFG website "It’s approximately 20% lighter than the White spring, and was the consensus choice out of 4 different evaluation samples provided to all parties. They all added in their final evaluations that if a particular gun needed a lighter spring, that the gas port should be enlarged for overall reliability." |
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[#6]
Quoted: Initially ran with oil on buffer/spring and grease on BCG bearing points. View Quote Not no, but hell No. Get a large spray can of BreakfreeCLP, and use it to not only clean the rifles, including a chamber brush to scrub the bores by hand with CLP, then remove all the fouled CLP and lube just the upper Receiver bearing areas with Clean CLP. For starters, CLP has a cleaning agent it it that will get all the old oil and grease out that may be collecting fouling to cause problems, but used as a lube, cleaner continues to work to keep the rig from fouling choking out as well. As for solvent to clean the rig like Hoppes, do not belong in the same room as the rigs, since Hoppes leaves behind a protective coating that will foul choke out an auto loader quickly as well. If you need a solvent to clean the copper out of the barrel alone, then sweets cleans copper fast, and does not leave behind a protective coating that will attract fouling. Next, on your testing of the CLP cleaned and lubed rigs, Single round in mag, insert mag, charge round and leave empty mag in well, and fire the round. The bolt should lock back on the bolt catch (in front of the face of the bolt), and spend case should be ejected 10'~15'. If bolt does not lock back with the catch in front of it, or spent case is not ejected this correct distance, go police the spent case to take a look at its spent primer, the outside of the case, and even the rim of the extractor. Primer pressure flowed all the way to the edges of the primer pocket, round when over pressure. Sides of the case all marred to hell with scratches, chamber is less them mirror smooth/flat as a mirror. Spent case rim bent backwards, bolt is unlocking too soon, and will cause a form of short stroking, since spent case is still too residual bore pressure bound to the chamber walls, for the bolt/extractor to pull is cleanly. Now if spent case is not showing signs of any of the above, then either problem not enough gas pressure to the bolt to unlock it at the correct time, and it a mater of finding the leaks, buffer mass/weight to great for the load, or there is binding going on in the action that is slowing down the bolt unlock speed isntead. Hence don't screw with the gas port yet, but instead make sure that is not gas system action leakage (including key to top of carrier), and no binding of the B/C coming all the way back. To check the gas system, CLP around the front and back of gas block to barrel, and around gas tube to block, then use an air compressor gun with rubber hose to send 100lbs of air down the gas tube from the inside of the receiver. You will get air flow through the barrel gas port, but will be enough resitance to show you if you have leaks at the gas block to barrel and tube. On the B/C, CLP around the Base of the key to top of carrier, and them hold the bolt in while you send 100lbs of air down the key entrance. To add here, after you check the carrier key for leakage to the top of carrier, pull the bolt and dry fit the carrier with key in the upper receiver, to make sure you have the gas tube tweaked correctly to align with the key. Hence gas tube will enter key when face of carrier is about 1" off the barrel extension face, and should only take a light pull with your finger inside the carrier to pull it back off the gas tube, if the tube is correctly index aligned tweaked from the center of the barrel. Also, from the start, do a full charging handle pull on each rig, and make sure that the face of the bolt stops 1/8"~1/4" in front of the back edge of the ejection port window, and the last inch of CH pull is the same tension as the middle of the pull. Get us this far, and trouble shooting the problem should be simple, on what problems you found above. Also, short of the 69gr plus ammo, all the rigs should be running standard weight buffers. So make sure that you do have standard weight buffer for testing. If you are finding that the only problem alone is spent case rims bent backwards from the bolt unlocking too fast, then you can drop in a heaver buffer to slow the bolt unlock down isntead. |
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[#7]
If you are experiencing the same issue on all 3, then I would have to think that the cause is the same on all 3. I would suggest trying a few different things for process of elimination.
Have you tried any factory 5.56 ammunition like Federal M193 or M855, as the extra pressure may give you different results. Try inserting just the bolt carrier (No bolt, cam pin, or firing pin assembly) into the upper receiver, and see if it freely moves forward and backwards inside the upper receiver with no resistance. The bolt carrier should basically free float inside the upper receiver, as the bolt carrier should only be making contact on its raised rails (2, 4, 8 & 10 O-Clock if you are looking at it as if the bolt carrier is at 12:00), the only resistance you should feel is when the gas tube mates with the gas key but it should be very little. You mention that you verified that the gas blocks were mounted correctly, but again check to confirm that the gas port inside the gas block is perfectly centered over the gas port on the barrel. I've had several people bring me uppers and rifles that had the same issues you are experiencing, and when I removed the gas blocks I could see from the carbon residue left behind that the gas blocks were just barely off center, and when I used a gas block alignment fixture and got things properly aligned, the guns ran 100%. Along the same lines as checking gas block alignment, check your barrel nut alignment, to make sure the gas tube has side to side free float movement. A good way to look at this is to look at the back end of the gas tube where it sits inside the upper receiver charging handle track. What you want to see is that the gas tube is centered inside the charging handle track area, and not off to one side, you should be able to push the gas tube slightly side to side with the same amount of play of which will show that the gas tube is centered correctly so it can mate correctly with the bolt carrier key (gas key). You state that you are lubricating everything, make sure that you soak the 7 bolt lugs, as if these are dry you may experience short stroke issues. Another thing you can do for process of elimination, is try swapping bolt carriers and then just the bolts from know working rifles, make sure you are checking headspace to make sure things are within proper spec, and see if you can find any possible combination of bolt and or carrier that may work differently. I have had a few rifles cross my path that wouldn't run on one bolt, but would on another, and it came down to a stacking of machining tolerances and the angle of the back of the bolt lugs that were causing issues on the unlocking cycle. You do mention that you are using a Vltor A5 set up, and also mentioned that you tried different buffers and springs. I would suggest trying to get things to run off a standard carbine buffer and spring, and not something heavier until you get it running reliably. Keep in mind that if you add a heavier buffer or stiffer buffer spring, that these add more weight and resistance, which will result in the rifle running slower. So I would suggest starting with less resistance, and get it running reliably, then add more resistance if you need to slow things down. CY6 Greg Sullivan "Sully" SLR15 Rifles TheDefensiveEdge.com (763) 712-0123 |
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[#8]
Thank you to Sully and Dano. I will follow your advice and report back ASAP.
Dan |
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[#9]
Sully nailed it.
I also suggest break in with full power 5.56 ammo. And double check the gas block location. If you installed them against the barrels shoulder.... you might "need" to take into account the space a "Handguard Cap" ( the front bracket used to hold a handguards in place ) would have used... so your gas block location might be to far towards you. |
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[#10]
Got the rifles thoroughly cleaned with CLP. Leaving them dry for tonight and will apply CLP on bearing points, bolt lugs and top of hammer right before firing. I checked the keys as described by Dano with 100psi holding the bolt in... all three leak to some degree. Gas blocks did not leak except at tube... very tiny bubbles but not a significant leak. The barrels are dimpled for set screws and when the blocks were previously removed the carbon ring around the ports appeared to be centered. Key to gas tube interference is minimal. One has slightly more resistance as carrier goes into battery but I have tweaked it the best I can. I have a replacement gas tube if it comes to it. The keys are staked but I can replace if that is your recommendation. I'm going to shoot them and segregate brass tomorrow. I'll get photos of brass.
The rifles all have the shortest/smallest wire diameter buffer springs I have on hand currently and standard weight buffers. When charging handle is drawn fully to the rear the bolt face is showing 1/8" and there is significant space between bolt face and bolt catch. |
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[#11]
I've used a light grease like Cherrybalmz or Slip EWG countless times on AR bcgs. It's fine.
I bet it's the gas block alignment. |
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[#12]
Barrel is dimpled for set screw to ensure proper gas block alignment. Screw impressions show I was in the dimple and the carbon ring around the port confirmed alignment. Gas blocks are not leaking.
Grease used initially was lubriplate. Grease has uses but I'll stick with oil on AR15 in the future. |
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[#14]
Quoted: Grease used initially was lubriplate. Grease has uses but I'll stick with oil on AR15 in the future. View Quote Not oil, but CLP that has an active cleaning agent in it, to keep the fouling from building up to the point that is chokes the rifle out. Short of the barrel bore that may need a copper solvent like Sweets to clean out the copper fouling quickly, think of the cleaning of the rig, just a CLP change from fouled to clean CLP. Hence what you do miss during cleaning, the pre lubing of the upper bearing areas will add enough new CLP to clean what you missed during the last cleaning. Hence on barrel cleaning, start with Sweet and clean the bore (always push from chamber to muzzle from the chamber and will take less then a few mins to get the fouling out of the bore, since Sweets dissolves copper that fast). Once the bore is cleaned and dry (patches not longer coming out blue), then go to the chamber to scrub it out with CLP and chamber brush by hand, and push the fouled CLP from chamber out the muzzle. When you think that you have patched the chamber and bore dry, there is still a trace of CLP on the barrel chamber and bore surfaces that will prevent rusting for short term storage of the rig. |
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[#15]
I would just open the gas port slightly, one size larger at a time. Reassemble and shoot. Once it locks back on an empty magazine I would stop. I use a hand drill and plenty of oil. I go slow making sure I don't angle the drill. With care and little pressure you can punch through without touching the other side of the bore.
Break-in will loosen things up from that point forward. |
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[#16]
If I understand correctly your fix would re-use the existing keys? Remove, use a counter sink to de-burr, lap and reassemble with new screws and threadlock gasket...
Where can I get just the screws? Dan |
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[#17]
Quoted: If I understand correctly your fix would re-use the existing keys? Remove, use a counter sink to de-burr, lap and reassemble with new screws and threadlock gasket... Where can I get just the screws? Dan View Quote Yes, use the old carrier keys, and once you have chamfered the mating entrances to clean up any ridge burs before lapping them in together, make sure that the gas ports to bolt channels are matching as well. Hence been a while, but was a batch of keys that there gas port location on the bottom was wrong, and would not mate up to the carrier gas channel correctly when bolted down. Since you have a few B/C's that needed to be corrected, here is a 10 pack of new carrier key bolts for $6 . https://doublestarusa.com/bolt-carrier-key-screw-10-pack-ar105 And again, use the old bolt spec of 37in lbs to tighten the bolts, and make sure to peen key metal into the side of the bolt head with a pointed punch; not trying to peen the hardened bolt head material into the sides of the key channels. |
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[#18]
On one of the 13.9" rifles I removed the bolt carrier gas key, lapped, sealed and reassembled (as described above). That rifle appears to be functioning fairly well now. It can still fail to lock back if I intentionally give it less shoulder. I shot maybe 30 rounds one at a time and got lock back each time. I'm not 100% confident in it yet but will update after a few more range visits.
I used this carrier to test the other two rifles as well. Shooting 5-10 with original carrier and then carrier from rifle above. The 12.5" started to short stroke/fail to lock back within the first five rounds with its original carrier. Swapping carriers to the repaired one seemed to fix the problem. It was pretty definitive on this rifle and I'll be lapping and sealing the gas key on this one as well. The second 13.9 was more fickle. It seemed to work fine with its original carrier and then would short stroke (Fail to lock back) occasionally with the fixed carrier. As round count went up though I think I was getting more reliable function from the lapped and sealed one. I'll seal it and double check everything else again. In general I observed that the 13.9" mid length rifles are very prone to short stroking (lock back) if given less shoulder and require more attention to technique than any AR I've previously owned. I was shooting clean lubed rifles, in moderate cold (upper 40s), standing, off hand. Under less than ideal conditions or shooting from compromised/non conventional positions might be an issue. FYI for others that may be considering this barrel gas length combination and just my humble observations. |
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[#19]
Verify that the gas blocks are not start to leak as they heat up (their set screws should be 272 loctite in place), and always police the brass to make sure that is not a over gas problem with bent rearward spent case rims.or rough reamed chamber marking the hell out of the sides of the cases to start with.
If we can weed out the above of any leakage or binding problems, and still an slight under gassed problems, then may be time to either go with a lighter buffer if possible for the ammo at hand, decreasing the tension strength of the recoil spring, or increase the gas port by 1 number size bit. https://www.custompartnet.com/images/size-charts/drill-size-chart-english.png Also, I bring up ammo at hand, since some of the import 223 ammo is weaker in strength than most of the USA made 223 55KPSI working pressure ammo, while USA 5.56 nato ammo is loaded to 62K PSI working pressure instead. |
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[#20]
Quoted: I would just open the gas port slightly, one size larger at a time. Reassemble and shoot. Once it locks back on an empty magazine I would stop. I use a hand drill and plenty of oil. I go slow making sure I don't angle the drill. With care and little pressure you can punch through without touching the other side of the bore. Break-in will loosen things up from that point forward. View Quote 100% this! Don't need to tear anything else apart. Bolts are not air tight. Ever notice how the gas tube easily fits into the gas key? (Now if the gas key is loose, thats a different story.) They don't need to be, there is several thousands pounds of pressure being fed thru the gas tube in a very fast pulse. Open the gas port. You won't hurt the chrome lining... that is old rumer. That is kind of like worrying about the chrome being peeled out of the barrel by shooting. Don't drill into the far side of the barrel, it will not do you any good for accuracy! Unfortunately this is a long trial and error process.... you will have to work for it. Run it wet when new, oil not grease! |
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[#21]
as an addition to this thread I just finished a build with a Criterion 14.5" CORE barrel. Put it on an existing lower using a Vltor A5 RE, Sprinco green and A5H2. Wasn't picking up the next round my first range trip and I failed to bring alternative parts with me.
Went back out with a milspec RE, 2x milspec buffer springs and a sprinco white, plus H and H2 buffers. Ammo was mostly Magtech 62gr 5.56 plus some hornady .223 60gr TAP. No combination would lock the gun open. Tried the BCM headspaced bolt in a Hodge carrier and the LMT carrier I built it with. Also tried the complete Hodge bcg. Same results. Took the upper apart over the weekend and bought some pin gauges. The .073" pin goes in but catches on something before entering the bore. The .072" pin goes all the way through and feels pretty snug all around. I cant find any other examples with my Google fu of a 14.5" midlength + .625 gb journal with a port that sized. My Hodge 14.5 is supposedly a .075" port but that's on a .750" gb journal. The hodge 14.5 was previously running on this lower with the vltor a5 setup as a comparison. I'm stumped. |
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