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Posted: 6/10/2005 5:32:25 PM EDT
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Okay, this is my second trip to the range with my new Leitner-Wise upper gas piston conversion. M193 and M855 have functioned flawlessly both times. The Cabela's factory seconds version of Black Hills Mk262 77gr OTM rounds have had NUMEROUS failures to extract both times---even after I installed a D-fender ring around the extractor for this trip. The extractor jumps the rim and the bolt attempts to pick up another round, ramming it into the rear of the still-chambered spent cartridge. I then remove the magazine, let the rammed round fall out, and let the bolt close again on the spent cartridge. Then I have to use a rubber mallet on the charging handle to get the bolt to finally extract the stuck brass. I do not think it's the ammo. I've fired this stuff through my A4gery and my Colt M4 (6400C) with no problems. I also do not think it's the conversion itself. The gas piston is doing it's job of knocking the bolt back to try and pick up another round. The round is STUCK in the chamber. The M193 and M855 work fine---zero problems. I sent my upper in for conversion, so it's not a proprietary L-W complete upper. I did not fire any Mk262 through it before I had the conversion done so I can't say it worked fine before. It is a chrome lined chamber and bore, 1:9 twist, M4 barrel from J&T. At first I thought that the hollow point might be catching on a slight ridge below the feed ramp and pushing the round back in the case upon chambering, thereby increasing pressure and getting the case stuck. I chambered and then manually extracted a few rounds (without firing) to see if bullet setback was occuring. It wasn't. The manually extracted rounds had the same OAL as unfired and unchambered ones. Then I thought it could be a chamber burr. There is a little "rough" area on extracted brass that extends about 1/4 of the way around the rear of the shell (on M193 and M855 also). I've seen similar marks with other weapons so didn't give it much thought. But why doesn't M193 and M855 get stuck if there is a chamber burr? It should affect ALL ammo. Any input is GREATLY appreciated. |
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bump wag, the only time I had a stuck case in 2k rounds of various brands, was with a dirty, bone dry gun, (while bumping wolf) had to put feet on FSB and pull on the charging handle harder than wanted to, but the extractor held, some chew marks on rim. can you swap bolt groups for some diagnostic? |
No spare carrier or bolt. Did install the D-fender ring before shooting this time. No access to what is considered "unblemished" Mk262. I've used the exact same ammo (all 200 rds I bought dumped ino a gallon ziploc baggie minus obvious screwups) in 2 other weapons with no problems. Chamber irregularity SHOULD affect ALL tpes of mil-spec rounds. Of course that's "in theory"...... |
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Yeah, it was me who posted in the L-W forum after my first range trip. This thread is in response to my SECOND range trip with the D-fender ring installed. Same problems. If you have to use a mallet or rawhide hammer to knock the stuck round out of the chamber then I'm going to guess that a D-fender or Advanced Combat Bolt would not be sufficient for extraction (I KNOW the D-fender isn't in this case). Something is making the round stick in the chamber---of course ONLY the Mk262. I'm totally perplexed by this. Two of the mil-spec rounds work fine but Mk262 does not. However, the same Mk262 ammo has worked fine in 2 other weapons I've used it in. The gas piston is working correctly, so it's not that. If there is a chamber irregularity, why isn't it affecting the M193 and M855? Judging by the amount of force needed to finally extract the spent brass I'd say no existing product would fix this problem. Is Mk262 ammo loaded to a higher pressure level or something? Maybe if I have an unusually tight chamber and Mk262 has higher pressures, it may cause it to stick whereas M193 and M855 doesn't (?). |
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You may have a shorter throat in this barrel and the bullet may be forced into the lands of the rifling causing higher pressure. Usually bullets should be seated about .010" away from the lands to allow some movement on ignition. Just because you do not see any bullet set back does not mean everything is ok with the overall length in relationship to the bullets ogive. You need to look at a round that has been chambered, and see if the rifling has engraved marks into the bullets major diameter at the ogive. Joe |
| Okay, I chambered a Mk262 and an M855. Upon close inspection I can see 2 lengthwise scratches (light ones) on both pieces of brass, probably from going over the feedramp and chambering. On the opposite side of the brass (I guess it would be the top part of the chamber would cover this area) I see numerous side to side light "hash mark" scratches on the brass---5 groups on the Mk262 and 3 groups on the M855. Obviously more of the Mk262 is coming into contact with whatever is causing these "hash mark" scratches than is the M855. There are more scratches on the Mk262 bullet itself than on the M855. The M855 has a narrower profile while the Mk262 has a ogive that extends farther out along the length of the bullet. I don't see what I can call uniform and concentric "rifling marks" on either round, but the Mk262 has more marks on the bullet itself than does the M855. |
I looked at the spent cases while at the range. No flattened primers or backed out primers. No obvious bulges in the cases. Those hash marks become real pronounced at the rear of the fired cases, more so on the Mk262. Whatever is making those little side to side hash mark scratches must be at the rear of the chamber and makes several series of them as the round moves forward while chambering. Then after firing they really stand out at the rear. However, if this is the irregularity possibly causing the sticking then why does only the Mk262 get stuck? Is it a higher pressure round? I just miked the rear of the unfired brass as close to the primer taper as I could get (above the extractor groove). The Mk262 is about .3735 and the M855 is .3725---this is doing it twice and eyeballing it with my micrometer mind you. That is what, one thousandth of a difference? Is that enough to cause problems if the chamber is tight? The barrel is a J&T 16", M4, 1:9 twist, chrome lined. |
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You need to mike the case head before and after firing. The increase in case head expansion should only be a couple of .0001" (ten thousandths inch). If the increase in diameter is larger than this it is a sign of high pressure. Comparing two different rounds head diameter tells you nothing. Joe |
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Just thinking out loud here. You keep asking if the BH is loaded to higher pressure. My guess is "no", but I'm sure it's loaded with a slower powder (Ramshot TAC, 4895 or equivalent) so the presure at the port will be higher than with a lighter weight bullet using something along the lines of a quicker H335 or equivalent. In addition to seeing higher port pressure, the brass will also see max pressure later in the bullets travel down the barrel so it's grip on the chamber walls will be at max later in the firing cycle. It may be just enough later that you see this in heavy weight/slow powder loads, but in a light weight/quick powder load there's enough time for pressure to decrease and for the brass to spring back and release its grip. Reiterating, the slower powder results in a higher port pressure. In a direct gas system that would also add to the problem, but from what I understand about the LW system, it uses just enough gas to cycle the rod and the rest is bled off. So from a theoretical standpoint there should be no negative effects from more gas at the port, at least that's my initial thought. You said these are BH "seconds". Do you know what aspect of their assembly/manufacture earned them the "second" label? Is it cosmetic defects or something else? Are the case rims beat up at the extractor? The more I think about this I'm wondering if it's soft brass expanding and not shrinking back. Any idea what brand of brass BH uses? If you have access to a heavier carrier or a Tubbs carrier weight insert you might want to give that a shot -- normally use of those delays unlocking a bit, but since this is a piston-driven design (solid, uncompressible mechanical unlocking mechanism) it may not offer any benefit, I'm not sure. The reason I say that in a direct gas system, it's not a solid uncompressible rod driving the carrier to the rear, it's compressible gas so it's not as if there's a direct mechanical linkage connectiong the two together, forcing immediate movement. Still, part of me thinks even with a direct, mechanical linkage, if it's trying to get a stationary object moving, the greater the mass of the stationary object, the longer it will take to get it started. Regardless, if it is soft brass, anything you can do to delay unlocking will allow more time for the body of the brass to shrink, or spring back, to it's original dimension. Another option would be for a smith to polish your chamber. That would smooth out any irregularities and make everything a bit slipperier. Let us know how you end up resolving this. |
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jmart, Thanks for the in-depth reply. The L-W system is a short stroke piston with an intermediate rod, not a solid piece like an AK. I think there are some blemishes that earn this ammo a "factory seconds" rating. I went through all 200 rounds and picked out any obvious defects. The rims seem to be uniform. The extractor is NOT ripping the rims, just jumping off. This ammo, as stated above, has worked fine in 2 other impingement AR's. I'm beginning to think that it is a combination of 2 factors at play here: "pressure curve" of the load (as you described above), and a tight or rough chamber that only serves to magnify the latter problem. I do NOT think it's the piston conversion. YMMV. Perhaps a good polishing of the chamber, as mentioned, would help. But I'm running out of Mk262, and at $130 for 200 rounds it ain't cheap!  ETA---the L-W carrier IS heavier, being beefed up at the rear and tapered to prevent carrier bounce. |
No I haven't, but that's not a bad idea. I'll do it later. Maybe there's a little rim difference between M193/M855 and Mk262? One thing to consider, though: the way I cleared the stoppage was to let the bolt go back forward and engage the spent brass. It then took moderately hard taps on the charging handle with a rawhide mallet to get it to extract. This brings up 2 points. First, the extractor holds onto the rim when I clear it this way. Second, if it takes whacking with a hammer to get it unstuck then perhaps NO strength of extractor/spring would fix it. (?) |
I honestly do not think it's the piston system. It works fine with M193 and M855, as it did BEFORE the conversion. However, I did not fire Mk262 before to be able to comment on it's reliability before AND after. I'm narrowing it down to 4 areas: extractor issues (poor lip or something), Mk262 issues, chamber irregularities, or a combination of any of the above. |
Good info. I'm contemplating the extractor may have "issues". Since it isn't ripping through the rim with the D-fender ring installed it leads me to believe it could be "lacking". Of course it works with the other ammo (as do all the other components)........ If it takes a mallet and moderate force to extract the MK262 spent brass then would ANYTHING work well? Is it normal for force to be needed to extract a round? |
I think I'll do that tomorrow. Since this one is from J&T I think I'll try Bushy or DPMS. Probably not much of a chance of finding a Colt one with the new mil-spec black insert. |
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Quoted: Abnak, can you send me a few rounds of that ammo so I can try it? Is is a .223 chamber, a Nato chamber or a Wylde chamber? Is the ammo .223 or 5.56? [/ quote] 48th, It's Black Hills 5.56mm NATO (not .223) 77gr Mk262. Purchased from Cabela's. Email me an address to send it to and I'll send it UPS ground, maybe 50 rounds or so. If it's going to happen it'll happen in that many rounds, trust me! I really do think the problem is related to MY gun, not the conversion per se. Can a non-commercial entity ship ammo? How about using the USPS? |
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Been tryign to ask the same question as 48th for a coupel of days now but the AR15 server seems to balk every time. What sort of chmaber does your 'M4' barrel have? Is it a 5.56NATO chamber or a .223 chamber? They are not the same and the Mk262 is on the limit for 5.56 would probably not quite fit into a .223 chamber. This could result in the round effectively getting wedged in and hence the mallet extraction technique. You should not shoot Mk 262 out of a .223 chamber. Is it Mk262 Mod0 or Mod1? Look at the bullet....Mod1 has cannelure. Simon |
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It's Model 1 (with cannelure). I bought the upper from J&T. I would assume that it's a 5.56mm chamber. But we all know what happens when you "assume". Just looked at the catalog that came with my upper and can't find any reference as to whether it's .223 or 5.56mm chambered. It is chrome lined. |
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Get in touch with J&T. Over int he Industry Forum, someone asked about their barrels and they said that theirs were nade by ER Shaw who also supply Colt and LW.... Now, I have no idea if ER Shaw does .223 chambers that's why you should ask J&T. If is a .556 chamber, you shoudl get them to look into replacing yours, wince a blemish in the chamber is a manufacturign defect. Simon |
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Not knowing much about different manufacturers barrels or chambers, I would ASSume the JT is likely a .223 chamber. Especially in light of the problems you are having. It makes a bit of sense to a sherlock holmes type anyway. I don't know much about the ammo you speak of. If you think there is value in sending me rounds to fire through my conversion, I will do it. However, it may be of no value because my rifle has a 1-7 5.56 barrel and is marked as such. My rifle closes on a no-go forrester .223 guage but not a 5.56 no-go. My biz partner Jesse has a JT barrel. At least we assume that is what it is because it is not marked. Checking his headspace wont reveal much to solve your head scratcher as they are two different rifles, barrel extensions, bolts, etc. |
exactly what i was thinking.., in fact i was reading somthing about the BH 77 Gr. loads causing high pressures & hard extractions, i'll try to find the article. |
If you could find that article it would be great! I called J&T yesterday from work and they said that their chambers are 5.56, not .223. Keep in mind that this ammo worked in 2 other guns, but both have Colt barrels FWIW. Perhaps a chamber imperfection? As far as having the barrel replaced, 48th and Buttplate can both attest to the fact that it is not as simple as that with an L-W conversion. That would entail a trip to J&T, then having it shipped back to L-W to be re-spec'd for alignment and either the same FSB would have to be used or a new one bored out. Major pain in the ass and use of UPS much more than I'd like! Plus there's no telling how long it would take L-W to get around to it aside from their normal production responsibilities. I may have to just stick to M193 and M855.  It could be worse. At least those 2 types are the most prevelant and obtainable. At $130 for 200 rounds, Mk262 is a little pricey to stash in bulk for SHTF! |
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ABNAK, Any updates? -Justin ETA: Leitner-Wise is already taking orders for the Advanced Combat Bolt and improved cam pins. Here are a few active threads: Commercial 5.56 ACB's Limited Availability. First Photo's: Advanced Combat Bolt. LW cam pins. Advanced Combat Bolt and Cam Pins. |
+1 - I would do this before anymore testing
Boy, that was helpful  If there is any chance that it's the gas system (and from you description I would say no) you can always remove the specific L-W components and replace them with a standard bolt, carrier and gas tube and try it that way but I think you will find that a waste of time in this case. |
The Blackhills 77 gr 5.56 Nato "blems" or "seconds" (in the WHITE box) that are being sold thru Cabelas and Fulton Armory are not reloads. They are new manufacture that got rejected by the Army due to "cosmetic" reasons (discolored brass, grains of powder stuck between the case neck and bullet, etc). Those cosmetic reasons have no impact on function or reliability. A representative from Blackhills Ammunition has posted info on this ammo in the Ammo forums. I have bought three batches (500 rds each) and all have run thru flawlessly. Having said that, the stuff in other colored boxes may be reloads or remanufactured, but the white box stuff is new. |
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Since this lot of the 77gr worked flawlessly in your other ARs, that rules out ammo as the problem. My feeling is the problem is the chamber on your J&T barrel. The solution is don't fire BH 77gr from the J&T. You could try some 69gr, but probably should stick to 55/62 gr. Not trying to be rude, but why on earth are you shooting 77gr from a 1-9 twist barrel? Forest, that was a cheap shot. 
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I've read conflicting things about using 77gr ammo in 1:9 barrels. Some say it will work, others say it is too unstable. Either way it's a moot point on mine! Yes, I agree it's probably the chamber. With it being chrome-lined I doubt the effectiveness of polishing. Correct me if I'm wrong on that, though. |
If I was a betting man (and I'm not) I'd wager that you are 100% correct. There are the same marks on ALL brass fired through it but only the Mk262 sticks---maybe due to slightly higher pressures causing a bit more case expansion? |
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Just my hunch: It's a combination of the ammunition (along the lines of jmarts' first paragraph), the JT chamber being rough/incorrect/whatever and the LW piston system (attempts to extract slightly earlier than direct gas?). Remove any one of those things and it works (albiet with very little margin for error). Not knocking LW's piston system, I think it's the best piston system for the AR invented to date. Had this been one of their uppers and not a conversion, I'm sure it would run like a sewing machine. Can't blame them for a bad chamber, but it would be interesting to see if the direct gas is a little more tolerant of your JT barrel. On the flip side, if it doesn't work with direct gas either (what is considered a long-term tested/known-good config) your JT upper has bigger problems than we thought and you should direct your attention to it and them, not LW. Again, want reiterate that is just my hunch. Just an observer, trying to give objective input, not start shit with LW (I am a fan!). They can't be expected to work magic on faulty parts. You've already tried switching ammunition and that works, can't exactly switch chambers, so try switching the gas system. Regardless of the result (works or not, let the chips fall where they may) you have more information to work with and can narrow down where your attention should be directed to. |
I never intended for anyone to get the idea that I was somehow linking this to L-W. They only converted what I sent them. If it is a chamber problem then it falls squarely on J&T, not L-W. Maybe someone can answer this for me: does the Mk262 operate at a higher pressure than M855 or M193? This would be the only way to explain why a chamber irregularity would handicap Mk262 and not the other ones. A case not expanding as much would not be as inhibited by a small burr or something as a case that was subject to higher pressures. The little scratches or "hash marks" are present on all of the fired brass but only Mk262 sticks---and by stick I mean the charging handle must be tapped with a mallet or a cleaning rod inserted and the brass tapped out (in other words, stuck pretty good). |
Mk262 operates at the same average peak pressure as M193 & M855 (52k-55k PSI). When you fire a round, the case expands and sticks to the chamber walls, then shrinks away. With Mk262, residual case pressures take longer to subside. So if you measured residual case pressure at a given time point after the firing, it will be higher on Mk262 than M193 & M855. In other words, with Mk262 the case takes longer to shrink away from the chamber walls. This coupled with a bad chamber might be too much for a gas piston system (which generally start unlocking & extraction earlier than direct gas impingement) to overcome. The direct impingement system may or may not work, as it should allow more time (talking milliseconds here - but the impact on difficulty of extraction is exponential) for the case to shrink away from the chamber walls. It still may not be enough, you won't know until you try. Even if it does, it is likely that your margin for reliability when using Mk262 will still be small. As is the often the case, failures are not caused by a single culprit but a combination of circumstances. My hunch is that in this case it is Mk262+rough/otherwise bad chamber+gas piston system. However, if the chamber is bad enough it won't work with direct gas impingement either. BTW, you don't need a mallet to tap the charging handle. Collapse the buttstock, grab the rifle by the charging handle with your dominant hand and slam the butt into the ground. Make sure the rifle impacts straight into the ground, not at an angle. In other words your barrel should be pointed straight up. Also make sure you pull straight back on the CH so you don't bend or break it. To keep everything going straight, it helps to guide the rifle with your non-dominant hand on the handguards. Don't be shy, it can take it (and will ask for seconds  ).Obviously if it can't be extracted by using the CH, the extractor does not have enough tension to hold onto the case rim or the extractor has pulled through the case rim destroying it. Either way you are going to need to tap it out with a cleaning rod. I would suggest testing the ammunition in another known-good rifle to verify that it functions properly. It is unlikely that it is out of spec as BH is a pinnacle of excellence in the QC department which all ammunition manufacturers strive to achieve. Still, we are all humans, mistakes can happen and you should verify that the ammunition functions normally in other rifles if possible. Second, I would suggest converting your rifle back to direct gas impingement even if only to try it out and see if it works or doesn't. Either way you will have a lot more information to work with and a better idea of what is going on. |
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wyv3rn, I follow what you're saying as far as the residual pressure of Mk262 taking longer to subside. One question though: even if the extractor jumps the rim due to the case still being expanded, once the pressures dropped wouldn't the case then shrink back and be easily extractable manually? In other words it is still stuck after the firing process and requires force to remove it. It's NOT unsticking, even after pressures drop. BTW, the rims are intact (not ripped through). I'm even using the D-Fender ring as an enhancement to extractor force. For some reason it's jumping the rim instead of ripping through it. I measured the case rim thickness for Mk262 and M193 and M855---all the same. Maybe a faulty extractor but again, wouldn't it show itself on the other 2 rounds? Can a chrome lined chamber be polished? |
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