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Posted: 1/22/2017 1:42:20 AM EDT
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Hello,
New member here. I've always been an AK guy, but recently dove into the AR world with a pair of LR-308s. I'm having some trouble with one that I traded for on Armslist. I've tried searching this site and others for a fix but this one has me stumped. This is what I'm working with: CMMG Mk.3 Lower with a CMC drop-in trigger https://www.cmmginc.com/shop/lower-g...a1-butt-stock/ DMPS Upper http://www.manventureoutpost.com/pro....html?google=1 I'm having an issue where the bolt does not hold open after the last round fires. It will hold open when I manually pull back the bolt BUT when I drop the magazine, the bolt slams forward. Twice I had an issue where I loaded a new magazine and went to fire. The hammer went forward but nothing happened. I pulled the bolt back, it ejected a round and there were no signs that the firing pin struck the primer. I also had one instance where the magazine did not seat all the way, but that may have been user error since I was loading while laying in prone. I also have a Oracle 308...let's call that gun B. If I put the upper from gun A (the one with issues) on the lower of gun B, everything works. If I put the upper from gun B, on the lower for gun A, I have the issues again. I did these tests live firing and manual cycling. That tells me the issue is definitely with the CMMG lower. At first I was thinking it was the bolt catch because the CMMG spring is much stiffer than the Oracle but that does not explain the two failure to fires. These are my first AR's...not sure what else to check? |
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Going to get brutal, but follow along for the ride anyways.
DPMS chambers their 308 chambers on the tight as fuck side to promote accuracy , and this causes problems with reliability until the chamber has had a chance to self polish out through live fires and cleanings. So from the start, I will strip the barrel off the upper, clean the ever loving piss out of the chamber with a chamber brush with CLP by hand, get the chamber bone dry, then go in and polish the chamber by hand via a lathe. This loosens up the chamber side walls about .001" without increasing head space, and gets the chamber to a mirror shine throughout to solve this initial reliability problems (So I don't have to struggle the first few hundred rounds of life fire and cleanings to allow the rifle to do this itself). From there, the upper is rebuilt, making sure to align and that there are not leaks in the system, and then the upper is ready for about 200 rounds of live fire to unify the production rifling, and allow the production barrel to stress relieve as it cleaned more often alone the way. If this is all beyond you, then breaking out a chamber brush with CLP to clean the barrel, and making sure to use only Sweets copper solvent to clean the bore, CLP to clean the chamber and rest of the rifle, should allow the chamber to self polish in in a few hundred rounds instead. As for the gas block, gas tube, and carrier key, these are items that you can check yourself. Note, do not use Hoppes solvent and hoppes gun oil on the rig. The solvent leaves behind a gooey coating that chokes the chamber out quickly, and the gun oil has not cleaning agent in it to disolve fouling before it has a chance to build up and choke the gun out as well. CLP around the gas block to barrel, gas block to gas tube, use a piece of tubing on the end of the gas tube with about 100lbs of air pressure, and make sure that you don't have any leaks. On the B/C, shove the bolt all the way in, put CLP around the back of the key to carrier and 100lbs of air through the front of the key. The key to carrier surface should have Zero leaks. Pull the Bolt off the key, and via the carrier with key, dry fit it into the upper receiver to check the alignment of the end of the gas tube to the carrier key. If the gas tube needs to be indexed, then the tube should be tweaked over the middle of the barrel. One last thing on the bolt and barrel, you really need to check the heads space of the rig. I bring this up, since there are two different 308 systems, being a stoner/DPMS system, and an Armalite system. If you are using a DPMS/stoner chambered barrel, it will not head space correctly with a Armalite bolt (or the other way around). If you are using a Armalite carrier (narrow u slot), then it will not work with the wider Stoner/DPMS hammers. As for once all the above has been checked/resolved, then single round in a mag, it loaded with the empty mag still in the well, and round fired for effect. The correct effect will be the bolt locking back on bolt catch and the spent case ejected about 10' to 15'. If the bolt does not lock back, then find the spent case and take a very good look at the spent case. Scrap marks on the sides of the spent cases scream that either the chamber has not been scrubbed clean correctly, or the chamber is reamer chatter central. The spent case rim bend to hell by the extractor screams that the chamber is either the above, or that the bolt is unlocking too soon. Shiny case, and the rim not bent, screams that either you have a leak in the gas system, way too much recoil tension or to much buffer mass to allow for a clean full stroke. Lastly, when you pull the charging handle all the way back, the face of the bolt should stop just about even with the back of the ejection port window (slab side DPMS thing) and the last inches of the charging handle pull should not get harder to pull back (spring coil binding up the last of the pull, or the hammer not able to lower down far enough, and it binding between the bottom of the carrier, and top of the trigger). P.S. Welcome to the site, and it would help if you listed a location in your sig line. The reason for this location, might be someone local to you that knows the systems really well, and can give you a hand in person to get the rig up and running with very little head ache along the way instead. |
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I was looking at it again after reading your post and the problem is definitely that the bolt is not going back far enough. It is about a half inch short of even engaging the catch. The magazine follower is actually what is holding the bolt open. When I pull it back all the way, it is a hard stop so something solid is interfering. The only modification I have done to the rifle is a CMC drop in trigger...don't know if the previous owner did anything but it was basically new when I got it from him.
Here are some pics: http://i259.photobucket.com/albums/hh309/itsinthecards07/7FBDD360-A015-4AA2-8361-7A00E5C5A010_zpsqke9bauf.jpg http://i259.photobucket.com/albums/hh309/itsinthecards07/75FEEBEA-8D2B-412C-AFBF-B3CC03B60E77_zpsfhs3k9jv.jpg |
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I took the lower off and used a dowel to push the buffer back and measure the length of travel. It will depress about 3 5/8 inches before stopping....maybe that means something?
My Oracle 308 buffer has about 4.25" of movement so it definitely appears to be an issue with the buffer system. Is it just the wrong part? That seems kind of ridiculous on CMMG's part... |
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Here's a little more info as I keep digging.
The buffer is just under 6" long and the spring is 12.75". My Oracle buffer is just under 2" long and the spring is 10.75". If I put the 6" buffer with the 10.75" spring, the problem still exists. If I put the 2" buffer with the 12.75" spring, it functions perfectly (cycling by hand). So...the problem is definitely the buffer. Is the 2" buffer actually the correct length or is there something in between that and the longer 6" buffer...4" difference seems like a lot. |
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Without seeing the back of the rifle to know what stock is in play, would dare to guess that the wrong buffer lenght (and maybe even spring) is in play for the receiver extension in play.
On the slab side 308 receiver, the correct lenght buffer will have the face of the bolt even with the back of the ejection port window edge with a full charging handle pull, and the correct spring for that buffer lenght will allow you to compress the spring on the buffer itself, and it will not coil bind out before the end of the spring coil clears the end of the buffer bumper. 308 buffer rifle extension lenght on the bottom, 223 rifle extension length buffer on top. 
Now having said all this, pull the buffer and spring, and make sure that someone did not install a stack of quarters or installed a spacer on the bottom of the tube under the spring and buffer. Also, if a fixed stock, back out the butt stock screw about 3 winds, and see if this makes a radiance on the amount that the buffer can be pushed into the tube. Lastly, ideally you want the face of the bolt to stop 1/4" to 1/8" in front of the back edge of the ejection port window, but there is a glitch in the 308 slab sides with the back of the ejection port window not milled far enough back for this. So it's normal for the bolt face to stop even'ish, or slightly behind the back edge when the correct lenght buffer and spring are in play. As for why you want the bolt face to stop short of the back of window, is so the spent cases don't tag the back edge of the ejection port window as they are pivoting off the bolt face at back of stroke, and possibly cause stove pipe jams. So the solution to the problem if you do start to get stove pipe jams, a quarter or two behind the buffer and spring in the tube to limit the bolt face to flush with the back of window, or just mill the back window edge to gain the needed clearance isntead. |
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Quoted:
Fixed stock: https://308ar.com/wp-content/uploads/2015/08/ar15-ar10-rifle-buffer-1.jpg Carbine receiver extension: http://i.imgur.com/EPYhwpe.jpg Yes, no, maybe, Since we don't have a clue what receiver extension is in play in the lower receiver. God only knows, and could even have some air-soft extension that the void in it, was milled too short to start with isntead. Myself, I have a lathe and mill and not afraid to use them. So would take the long buffer in play, figure out what it needs to be shortened, and then just have it instead. From here, quick check of the spring in play, and make sure that it was not going to coil bind out before the buffer bumper reached the back of the tube. |
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Since you do know the needed lenght of the buffer needed, mark that distance on the longer buffer, and double check the recoil spring you have in play to make sure it not too long (will coil bind up before reaching that distance mark on the longer buffer you have now).
Simply, the buffer has to retract all the way, and if the spring is too long, its going to cause spring bind before the buffer can make it all the way back instead. |
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Quoted:
Since you do know the needed lenght of the buffer needed, mark that distance on the longer buffer, and double check the recoil spring you have in play to make sure it not too long (will coil bind up before reaching that distance mark on the longer buffer you have now). Simply, the buffer has to retract all the way, and if the spring is too long, its going to cause spring bind before the buffer can make it all the way back instead. I see what you are saying. Well the buffer is certainly too long so I had to get a shorter one no matter what. When that gets in, I will evaluate the spring situation since I have both sizes. |
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