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Posted: 10/10/2015 9:49:13 PM EDT
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16" carbine dpms lr308
Troy low profile gas block Silencerco Omega 165gr Federal Fusion & just about anything I put in it. More than enough lube. Went to the range to verify zero before deer season in a couple weeks. Only thing I have changed it I added a silencerco muzzle break and the silencerco omega. Didn't turn a dial, same ammo, same everything. First round in the picture below happens. And again and again until I start loading them one at a time. Same thing, the case is backasswards in the chamber. All but about 7 or 8 cases look like the picture below. I did this for 6 rounds and went to check zero. Not one was on paper. Took suppressor off and fired three rounds and not on paper. Move up to 50, still not on paper. Hold bottom of target and hit the top of target and dial to 2 inches low at 100. Move back to 100 to verify and it's all over the place. I believe mirage was playing with me here after the suppressor got hot. My previous groups were all touching with my aac cyclone k from last year. Again checked to verify no baffle strike and there was none. Over gassed is my thought. Looking at Govnah gas block.  " />
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Likely over gassed to begin with and adding the suppressor increased it even more. Heavier buffer, spring and likely an adjustable gas block to help the gassing and I would check where the bolt face orientation to the rear edge of the ejection port to see if it is indeed in front of the back edge by a good 1/8" inch or a bit more when you manually pull all the way back the charging handle. Checking the ejector for binding would also relevant.
Complete specs on your rifle could help. |
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Likely over gassed to begin with and adding the suppressor increased it even more. Heavier buffer, spring and likely an adjustable gas block to help the gassing and I would check where the bolt face orientation to the rear edge of the ejection port to see if it is indeed in front of the back edge by a good 1/8" inch or a bit more when you manually pull all the way back the charging handle. Checking the ejector for binding would also relevant. Complete specs on your rifle could help. What else do you need spec wise? |
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What else do you need spec wise? Quoted:
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Likely over gassed to begin with and adding the suppressor increased it even more. Heavier buffer, spring and likely an adjustable gas block to help the gassing and I would check where the bolt face orientation to the rear edge of the ejection port to see if it is indeed in front of the back edge by a good 1/8" inch or a bit more when you manually pull all the way back the charging handle. Checking the ejector for binding would also relevant. Complete specs on your rifle could help. What else do you need spec wise? What buffer and spring your running would certainly help. |
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What buffer and spring your running would certainly help. Quoted:
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Likely over gassed to begin with and adding the suppressor increased it even more. Heavier buffer, spring and likely an adjustable gas block to help the gassing and I would check where the bolt face orientation to the rear edge of the ejection port to see if it is indeed in front of the back edge by a good 1/8" inch or a bit more when you manually pull all the way back the charging handle. Checking the ejector for binding would also relevant. Complete specs on your rifle could help. What else do you need spec wise? What buffer and spring your running would certainly help. Standard carbine buffer and spring it came with. Up until I put this suppressor on it, I had no problems other than it ejecting at 2oclock and was kind of violent on the recoil. |
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Suppressor is going to add back pressure, so may have to limit the gas tube pulse or go to a heaver buffer to start with.
But lets back it up, Make sure that the chamber is chamber brush and CLP cleaned (read scrub the chamber, then remove all the fouled CLP). Make sure that the rest of the rifle is both CLP cleaned, and the upper receiver bearing areas CLP lubed as well. Pull the charging handle all the way back, and make sure that the face of the bolt stops 1/8" to 1/4" in front of the back edge off the ejection port window. Pull the B/C, hook a spent case on the extractor, cam the ejector in a few times, and make sure that is not binding in the bolt face channel for it. Pull the extractor and add a #60 O ring around the extractor spring and put the bolt/rifle back together. Note, the O ring is just being used as a band aid for the first few hundred rounds. After this, the chamber should polish in through live fire and cleanings, and should not be needed any more. Single loaded round in in a mag, insert the mag, charge the round while leaving the empty mag in the rig, fire the round and confirm that the bolt locks back on the bolt catch (catch in front of the bolt face and not just under the carrier only). If the bolt locks back correctly, then the rifle is correctly stroking to begin with. Granted that the bolt may be coming back on the hotter side with the suppressor in play, it's still locking back and the stroke is fine and it not a stroking problem. If the bolt does not lock back, then the rig is over gassed, or not gassed enough instead. As stated, the can will increase the gas tube pressure, so pull the can and test again. If without the can the rig runs fine, but with the can back it play the bolt will not lock back, then over gassed and you will either need to go to an adjustable gas block to lower the tube gas pressure, or will need to go a heaver buffer to slow the bolt unlock back down (both will just slow the unlock back down so the residual pressure in the bore has a change to drop down to normal levels so the spend case can be pulled cleanly instead). |
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Glitch on the gas blocks, is they can be a PITA to for a repeat setting (can verses not canned, then back to can again).
So before you just go with any gas block, see that features it has to be able to come up with some quick repeatable settings. Hence the ones with just a set screw that you have to turn out and back in to try to come up with a repeat setting can be hit or miss each time if you are counting quarter turn wrench movements, and may be end up with wasted time getting it dialed back in again between a few adjustments back and forth. Hell, truth is if all you you can find is the gas blocks with the set screw type adjusted, then any smith worth his weight can just drill and tap your gas block for a interrupter type set screw with second grub set screw. 
POF has a 9 position gas block that is easy to come back to old settings over and over again, but it's kind of spendy. 
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Glitch on the gas blocks, is they can be a PITA to for a repeat setting (can verses not canned, then back to can again). So before you just go with any gas block, see that features it has to be able to come up with some quick repeatable settings. Hence the ones with just a set screw that you have to turn out and back in to try to come up with a repeat setting can be hit or miss each time if you are counting quarter turn wrench movements, and may be end up with wasted time getting it dialed back in again between a few adjustments back and forth. Hell, truth is if all you you can find is the gas blocks with the set screw type adjusted, then any smith worth his weight can just drill and tap your gas block for a interrupter type set screw with second grub set screw. http://www.brownells.com/userdocs/products/p_100011244_2.jpg POF has a 9 position gas block that is easy to come back to old settings over and over again, but it's kind of spendy. http://cdn3.volusion.com/vrfwc.byruj/v/vspfiles/photos/00837-2.jpg I might just order a Govnah if they don't have a good gas block. The problem with the pof gas block is my rail is a 13" rail. It covers the gas block up by a good bit. The govnah is at least just a plate I got to slide vs rotating a dial. |
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Only down side is chances are you are needing to reduce the passage size from a standard size, so on that note, get an extra plate that is just pilot drilled so you can drill it out for the size passages you need in the end.
Myself with the gas block so buried in the float tube, would try to solve the problem with a heaver buffer and XP spring for can'd instead. Note, not hard to spin a carbine buffer up out of SS up to 6.5oz if needed. http://www.heavybuffers.com/hss.html Hence easy to swap out a buffer for caned to non can'd with the upper shotgunned open, then it would be to blindly pick inside the float tube instead. Also, before just calling the problem over gassed, double check with the list I gave you to make sure from the start!!!!!! |
| It is over gassed, the suppressor will exaggerate the issue with higher back pressure. Best fix is to regulate the gas to acceptable levels. Lower level fix is heavy spring/buffer. Do note, I did the buffer spring and ended up doing the adjustable gas block in the end. DPMS SASS with Gemtech Quicksand. |
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It is over gassed, the suppressor will exaggerate the issue with higher back pressure. Best fix is to regulate the gas to acceptable levels. Lower level fix is heavy spring/buffer. Do note, I did the buffer spring and ended up doing the adjustable gas block in the end. DPMS SASS with Gemtech Quicksand. What gas block did you go with? |
| That bolt face to edge of port orientation needs to be corrected. The easy way is to drop a couple of quarters down the receiver extension and then install the spring and buffer to allow the face to protrude the 1/8" to 1/4" ahead of the back edge of the port. The correct way is to face/mill the front of the receiver extension so that it is the proper length. And you may not believe the amount of issues a buried bolt face can provide. That said the rifle is still likely over gassed especially with the suppressor attached. |
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That bolt face to edge of port orientation needs to be corrected. The easy way is to drop a couple of quarters down the receiver extension and then install the spring and buffer to allow the face to protrude the 1/8" to 1/4" ahead of the back edge of the port. The correct way is to face/mill the front of the receiver extension so that it is the proper length. And you may not believe the amount of issues a buried bolt face can provide. That said the rifle is still likely over gassed especially with the suppressor attached. Yes, no, maybe, And the problem is this is a 308 rig, and he has the wrong buffer in play!!!!!!!! The top is the 308 buffer, while the bottom one is the 223 buffer instead. 
Hence with the bolt allowed to recess that far behind the back ejection port window back edge, would not be surprised if the back of the key has already cracked the lower receiver from the key slamming off the back of the receiver when the receiver extension threads into it. |
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Holy crap.... This is a factory rifle with less than 300 round on it. I ain't done shit to the buffer/spring. Only thing I've done is put a suppressor and rail on the thing.
Eta I ordered a heavy buffer from heavy buffers. The one linked above would not work per Clint and I got the correct one needed. The current buffer I have is 2.5" long. |
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Do me a favor,
Shotgun the upper open, pull the B/C back to the same distance that is with the CH pulled all the way back, and tell me how much room you have from the back of the carrier key to the back of the upper receiver. I know that DPMS has problem with the slab side receivers with the ejection port window not milled back far enough, but this is a new one with a covered ejection port instead. Hence if the back of the key still has some room before it crashed into the lower receiver, and the bolt is locking back on the catch at the last round, then a end of inside tube shim will solve the ejection problems that the rig has now with the spent case tagging the back of ejection port window to defect the case back in the action. Bluntly put, the bolt face needs to be able to retract back the bolt catch for it work/catch the bolt on the way forward with a empty mag in play, but you do not want the bolt face retracting back past the back edge of the ejection port window instead. In the case of the slab side 308 receiver, about the best you can do is limit the bolt face to the back edge of the back of window edge, since any farther forward limited will not allow the bolt catch to work correctly instead. So on that note, first lock the bolt on the catch and look to see where it stopping against the back of the ejection port window. The bolt needs to retract about 1/4" back from there to allow the catch to work correctly, and with any luck, should be able to shim the inside end of the receiver extension tube to prevent the face of the bolt from retracting back past the back edge of the ejection port window to solve the stove pipe jamming. As for the short buffer, and longer tube, they are correct, but still should not have the face of the bolt extracting back past the back edge of the ejection port window at the back of stroke, or you end up with stove pipe jams instead. 
Having said that, if you can get the face of the bolt to stop correctly in front of the ejection port window and enough room behind the catch for it to work correctly, them measure the distance of the shims needed, and add that to the 2.5" buffer for a new buffer length needed. The heaver buffer will help with the can if needed, and may as well have the correct length buffer is play from the start to prevent the stove pipe deflection jams off the back edge of the ejection port window (instead of needing to shim the back of the tube with a buffer too short instead). |
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The brake and can add back pressure to the bore, and increase the amount of gas pressure through the gas port.
Hence bolt now unlocking a touch more earlier than before, with the bolt now coming back faster. So add in that the spent case if begin deflected off the bolt at the back of stroke when the buffer is stalling out the stoke at the back, and now you have the spent case tagging the back of the ejection port harder instead. In this photo just in front of the bolt face, you can tell that the rim of the case has been tagging the inside of the receiver from the brass hitting the edge of the ejection port window to start with to wear the anodizing off, so now with the added bore back pressure from the brake and can, it just now hitting the back edge of the ejection port window even harder. 
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No worries, but with the bolt locked on the bolt catch, how far forward is the face of the bolt in front of the back edge of the ejection port window?
On the slab side 308 receivers, the best you could do to have the bolt catch still working was to shim the back of the tube so the face of the bolt is just flush with the back edge of the window instead (or just mill the window edge back father instead). 
Hence I find it strange that DPMS has made the same mistake twice now on intent designs with the bolt retracting back past the back of the back of the ejection port edge . |
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Look at the spent brass for the tell tale sign that it tagging the back edge of the ejection port window on the way out of the action.
Normal Deflector dents are up around the shoulder of the case, while ejection port window dents will be down lower towards the web of the case instead. |
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There seems to be a consistent ding about 1/3 up from the bottom of the brass. It's almost more of a scratch than ding but it's the only thing in common just about every piece of brass has. Shim the back of the buffer tube seem in order to get the bolt face in front of the ejection port window?
Yep, so long as shimming the back of the tube forward to keep the bolt from retracting back past the back edge of the ejection window does not end up with the bolt catch no longer working (bolt not able to retract back past the bolt catch). |
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There seems to be a consistent ding about 1/3 up from the bottom of the brass. The dent/scratch location is screaming that the spent case is tagging the back of the ejection port window on the way out of the action. If you can post a photo of the spent case dents, will be able to confirm such. Where are we at with shimming the tube to prevent the face of the bolt from retracting back past the back of window edge? Are you are to pull the off, and still have the bolt retracting back behind the bolt catch? |
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Works been hectic we've been trying to get ready for this rain. I'll try to get things shimmed tomorrow. Best thing to use is quarters? For now, quarters will work for shims; and if this solves the problem then a more permanent solution will be better, unlike the quarters that will fall out every time you pull the buffer and spring for cleaning instead. And again, as you are shimming the back of the tube to get the bolt to stop say 1/8" in front of the back of ejection port window, make sure that the bolt is retracting back past the bolt catch at least 3/8". Hence at back of stroke, the bolt face needs to get behind the catch far enough that it allows enough time for the empty mag to get the bolt catch all the way up before the bolt comes back forward. |
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Shimmed with quarters and started with three. Couldn't lock bolt to the rear. Did two quarters and still couldn't and this put the bolt behind ejection window. Ejection also went back to 2 o'clock on spent brass.
On top of all that, the thing wasn't grouping for shit. This rifle has turned into a headache... |
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Epic, since DPMS did not learn a lesson from the slab side receiver that caused the same problems (bolt face retracting back past the back of the ejection port window as well).
Taking a good look, I can see that the brass is chewing up the back side of the ejection port window/edge of the receiver there (which is deflecting some of the brass back into the action to cause the problem), The glitch here, unlike the slab side upper that did not have a ejection port cover, trying to mill the ejection port back is not going to work, since the ejection port cover is going to be too short instead.
So the only answer I have for the problem to band-aid a design flaw to begin with, we need to change the ejection path of the round out from a rear ejection path, to a forward ejection path instead. The easy way to solve such since it's a semi auto rig, is to prevent the buffer from having any dead blow effect so the bolt is not stalling at the back of stroke. To do this, punch the buffer bumper roll pin out, remove the buffer bumper, and using cleaning patches, you need to pack the buffer weights so they do not move. The best place it install the packing is in the front of the weight in the buffer, and use something to press the bumper bumper in with the weights to compress the packing tight to the front as you install the roll pin in. The positive here, with no dead blow effect at the back of stroke, you follow up shots will be a hell of a lot quicker. Note, first full auto AR-15's for military testing did not have a dead blow effect buffer. So without the dead blow effect of the buffer to stall the bolt out at the back of stroke to allow the case to pivot off the bolt face as the stall for the 5:00 ejection path, the case left the bolt on the way forward instead, and these rifle had a 2:00 to 3:00 ejection patch instead (and a full cycle rate fast as hell as well). I bring this up, since on a few of my running game rigs, I have the buffers weight wedged solid as well, which allows for a hell of lot faster follow up shots since there is not back of buffer stall instead. But will point out, that the mags that I run through the rigs are kept clean, and are using extra tension mag springs to make sure that they get the round back up tight against the mag lips before the bolt returns to strip the rounds out. 
Or, if you want to leave the rear dead blow effect of the buffer in place to help with the mag recovering correctly if you have some suspect mag spring on the weaker side, then you will need to start clipping ejector spring coil winds until you change the ejection path from 5:00, to about 2:30 instead. Note, don't clip a bunch of coils all at once, since the ejector spring does not have enough tension, then the ejection path becomes more to the 1:00 postion, and you chance the case mouth just being slammed into the front of the ejection port window instead. Spring #6, and more of a pain to correctly shorten with test runs, than to attack the buffer to pack the weight solid one shot isntead, 
P.S, you can pull your quarters out of the tube and put them back in your pocket, since the shims will not be needed when the rifle is forward ejection instead. |
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Just out of curiosity, would contacting dpms be an option and telling them they need to fix this? This is bullshit and an utter design flaw that it sounds like is a known issue.
Before I start doing all this, would building a new upper and just throwing this one in the parts bin be a good move? Only thing worth a damn on this thing seems to be the rail. The upper seems out of spec and the barrel doesn't shoot for shit. |
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Production barrel is luck of the draw, and even has the odds stacked against it, since the barrel has to be leade reamed for short ogive 150gr bullets ,and this causes a hell of a jump with a longer Ogive bullet when loaded to the same OAL to come out of the mag (barrel reamed to accept all the different types of ammo's that can be run through it.
The bolt face coming back behind the back of the ejection port wrongly is now a running joke/theme with a lot of manufacturers, so if one is going to rip off stoners/colts patients now that the time has run out, at least they can get the clone rigs right since there are so many mil spec drawing on the platform out there to get it right, it not funny anymore. So on that, send the upper back to DPMS, and let them know that the brass is stove piping since the bolt is retracting back past the back edge of the ejection port, and that the barrel is not grouping for shit (3moa or more), and see what their resolution is for the problems. As for building a MOA rifle, that all states with the match ammo first, and the rifle built around that ammo. If push comes to shove, you can build such a rifle up for around 2.5K, but ends up being well over $3K in the end. So here is the glitch, since everyone seems to not want to spend over $1500 on a rifle/not feed it only match ammo, Hence getting to MOA for a 20 shot string is what is going to cost the extra money in not only the rig, but the ammo as well. |
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Thanks Dano.
I was debating putting a Rainier arms ultramatch barrel in it to see what would happen. They have a 90 day 1moa or less barrel with match ammo guarantee that I might put to the test. I want to do 1moa with match ammo. (168gr/175gr) and with my hunting ammo (federal fusion 165&168gr) I'd at least like to see 1.5moa. Ive heard dpms doesn't get your product in and out very fast for warranty work and with deer season starting in 14 hours, I might wait to send it back to them. |
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All depends the amount of round in the shot strings you plan of firing.
Myself, since the rig is going to be used hot and heavy between cleanings, would favor Polygon rifling since it less susceptible to copper fouling with longer shot strings, and you gain of touch of speed as well. So barrel is side wall chambered in match from the supplied bolt, and even a shorter leade than required for 168grHPBT bullets, which allows me to increase the OAL of the round over time for plasma cutting to maintain a .003" jump as the throat erodes, and still be able to come out of the mag to the end of the life of the barrel. Hence a conventional rifle barrel may be able to print tighter for shorter shot strings between cleanings, but in an auto loader where you are running a lot more ammo through it between cleanings instead, then it a matter of the barrel being able to deal with the longer shot strings instead. Bluntly put, you have a 20 round in the mag, and you want the rifle to be able to hold MOA for all the rounds in the mag, isntead of just selective two and few group strings with cleanings in between the short strings instead. |
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All depends the amount of round in the shot strings you plan of firing. Myself, since the rig is going to be used hot and heavy between cleanings, would favor Polygon rifling since it less susceptible to copper fouling with longer shot strings, and you gain of touch of speed as well. So barrel is side wall chambered in match from the supplied bolt, and even a shorter leade than required for 168grHPBT bullets, which allows me to increase the OAL of the round over time for plasma cutting to maintain a .003" jump as the throat erodes, and still be able to come out of the mag to the end of the life of the barrel. Hence a conventional rifle barrel may be able to print tighter for shorter shot strings between cleanings, but in an auto loader where you are running a lot more ammo through it between cleanings instead, then it a matter of the barrel being able to deal with the longer shot strings instead. Bluntly put, you have a 20 round in the mag, and you want the rifle to be able to hold MOA for all the rounds in the mag, isntead of just selective two and few group strings with cleanings in between the short strings instead. So out of all that, is there a barrel recommendation in there somewhere? Lol |
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So I contacted dpms and they refer me over to bushmaster which is doing their warranty work.
Bushmaster states that the bolt face always retracts past the ejection port window and the shell should be ejected before it gets to that point. Their solution is a chamber polish. |
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https://youtu.be/cnovh3w2qRM
Watch and tell me where the spent case is not longer still on the hook of the extractor. So its the dead blow effect of the buffer that cases the rearward stall when the spent case finally leaves the extractor, and if the face of the bolt is behind the back of the ejection port window, then the spent case it catches the back window edge to cause the spent case to be defect with more chance of it ending back into the action isntead. So with the buffer creating a stall at back of stroke, then stall time at back of stroke is when the spent case will leave the extractor claw. If the buffer has no dead blow effect instead, then the spent case leaves the bolt face/extractor contact when the bolt is moving back forward again, and not such a problem with the face of the bolt retracting back past the back of the ejection port window instead. So no buffer stall, and the spent case will have a forward ejection path since it leaving the face of the bolt on the way forward. Buffer stall, and the spent case is leaving the bolt face/extractor at the back of stall, and it need to have a clear/unblocked path out of the extractor point (not a chance of catching the back edge of the ejection port window to deflect the spent case back into the action isntead. This is the problem, since the stroke of the 308 is much longer, it's why you don't try to design a 308 around a shorter 223 ejection port that is going to have a dead blow effect stall type buffer. So once the spent case leaves the extractor of the rear stall, you have not control over the spent case from that point forward, and dam sure don't want the back edge of the ejection port window angle blocking the case back into the action isntead. |
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I'm not an engineer or weapons designer so it's kind of hard to call up a small arms company and argue with them about weapons operation and design flaws and how to fix the solution.
So now dpms/bushmaster has come up with a "chamber polish solution" I'm back at square one on how to fix this. I managed to snag a VA 18" mid length barrel on sale so that is on the way and when I get that in, it's going on asap. Hopefully with changing the gas system from carbine to mid length, maybe this will elevate some of my issues. If not, looks like changing the ejection path like you stated before Dano is my next and only option. |
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Hate to say it, and thank god this a semi auto rig that you don't have to worry about bolt bonce in full auto, but with the bolt retracting back past the back of the ejection port, Packing the buffer weights tight inside the buffer to stop the dead blow effect to cause the spent cases to pivot off the extractor during the forward stroke, may be the only solution of keeping the spent brass from wedging up against the back inside edge of the ejection port window on ejection .
With the old slab side 308 upper receiver, there was deflector or ejection port door you had to worry about, so it was easy to just mill the back of the ejection port window back on those receiver instead. P.S. If I can find it, there is a video floating around on U tube of the first full auto Ar-15 testing with full auto fire with a dead blow effect buffer, before it became the M-16 with dead blow buffers to slow the firing rates down. Hence with no dead blow effect buffer, the spent case ejection path on the rig is 2:00 instead (with a much higher cycle rate as well in full auto). The funny part, the goverment wanted the cycle rate slowed down, and the dead blow effect buffer solved that. The glitch, after they went with the buffer, did not still do enough testing with the dead blow effect changing the ejection path, ending up south paw shooters getting the spent cases in there face when shooting the rifle left handed (change in the ejection path due to the dead blow effect, and a deflect was not a permanent item on the M-16 until it became the M16-a1). So until the A1 that added a deflector tab as part of the upper receiver, south paw shooter where give a plastic defector to snap on the rifle instead. 
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None of the Gen. 1 DPMS uppers I've seen will allow the bolt to stop even with or in front of the rear of the ejection port and still allow the bolt latch to work. Gen. 1 ejection port is way too small. My Gen. 1 also had cases backwards inside the upper but only on the last round and the bolt would be locked to the rear. It was cycling but all of the cases would have a mark mirroring the rear of the port just above the web. Apparently, slowing the bolt down allows the case to roll out and clear the port like it will when the bolt stops just in front of the rear of the port.
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Nope, when the buffer weights are packed tight to prevent the buffer from having a dead blow effect/stall the B/C at the back of the stroke to allow the spent case to leave the extractor then, the non dead blow effect buffer does not stall the B/C as the back of the stroke, and the spent case leaves the extractor as the bolt is moving back forward instead.
Hence without a fix tab extractor to knock the spent case off the bolt on the way back on say a 9mm blow back action, the spring loaded extractor will either release the spent case when the B/C stalls as the back of stroke (rearward ejection), and if the B/C does not stall at back of stroke, then the case it final released from the extractor as the bolt is moving forward instead. Hence with the bolt now retracting back past the ejection window, don't want the spent case to leave the extractor then, since it wall cause the spent case to tag the back inside of the ejection port window to possible deflect the case back into the action instead. So with no dead blow effect buffer, the spent case is leaving the extractor as the bolt is moving forward, and with luck, once the face of the bolt has cleared the rear ejection port window edge instead. Skip to 3:50 to show the ejection path of the E1 before it got a dead blow effect buffer to slow the cycle rate down/before it needed a defector on the upper receiver for left hand shooters instead. https://youtu.be/VntwFqcE4-g |
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