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Posted: 10/2/2010 8:29:49 AM EDT
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Disclaimer - this is my first M4 and though I grew up shooting, I have never run a gun this hard.
M4 in question is a CMMG MOD4SA chambered in 5.56, with a moderate round count and lots of use. After choking a couple times during break-in with the first magazine or two (steel-case), it has always gone bang and worked properly when I used good ammo and good magazines. In other words, it's been as reliable as I could ask for. Recently I replaced the FCG with Colt parts because the rifle had started to double. All other parts are what came on the gun. Lately I've noticed the gun being a little harder on my brass. Seems to chew up all brass (.223 and 5.56) that I put through it and mark up steel, too. What's going on? Is my rifle telling me to put a new action spring and H2 buffer on it? Just trying to pinpoint my problem since it seems like empties shouldn't have rough edges on it when they come out of the gun. I'm guessing that the bolt is unlocking too fast and that a heavier buffer plus a stronger spring might slow that down a bit. I wonder also, if the ammo might have been loaded a little warm or the brass might have been too soft? Who really knows? Before I move onto photos, let me ask for opinions. I want to avoid doing anything unnecessary but I would like to correct any problems that may exist. Here's a handful of brass from yesterday. This was '94 vintage Winchester Q3131. Never had a popped primer before until now... 
Standard Carbine buffer and action spring... 
Groove worn into the upper receiver by the cam pin - about 1/8" deep now, normal? 
"This house is protected by the Good Lord and a gun. Come here unwelcome, and you might just meet both." 
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| I don't think a spring change will make much difference. I would buy an H3 buffer, if the H3 is a little too heavy you can pull the weights out and swap one with your standard buffer, this will leave you with an H1 and H2 buffer so you can find the one that works best. |
| Have you looked closely at your bolt? Check to make sure your ejector spring/pin compresses flush with the back of the bolt. I had one where dirt or crap wouldn't let the ejector pin seat completely and it started marking cases. Also look at your extractor too. while you're at it, research how to check the gas rings in your carrier to see if they are holding well. There is a test out there, remove firing pin and cam pin, extend, see if it holds the weight of the carrier or something like that. |
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Chamber may just be a tad dirty since the extractor marks on the spent case rim are happening on loading, not unloading.
The bolt rotates counter clockwise to unlock, and the spent cause gouging is clockwise (extractor on loading). Best guess, out of spec extractor canted to the one side (right hand side) and why is is digging into the cases, instead of just gliding around on it. Plus the ammo is on the hotter side, so the heaver buffer should help out with the blown primer problem. |
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Chamber shouldn't be that dirty - I cleaned the rifle less than 200 rounds ago and when picking up my brass noticed that it was relatively clean and free of carbon. I use a boresnake and CLP after range sessions but every few hundred rounds I'll give the rifle a good cleaning with Hoppe's 9 and a rod/jag and a USGI chamber brush. After a short range session I usually just wipe down and relube the rifle with CLP, making sure everything is the way it ought to be inside. Didn't mean to digress, but I make a habit of keeping the chamber nice and clean.
If the extractor was out of spec, would it make sense that the rifle just started chewing up cases in the last 350 rounds or so? Not being facetious, just asking for an answer to something I don't know. |
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Before I went to Chrome-Silicon springs, I replaced buffer springs every 5,000 rounds in my rifles, more frequently in my carbines.
Also the 'H' buffer is the lightest buffer I use. In a well worn carbine, the H3 will be a better choice. Inspect your extractor and ensure your chamber/throat is clean. Like everything mechanical, items wear. Springs are cheap. and buffers are not that much money (H2 is under $30, then H3 is under $40) either. |
| I have to agree with Dano ... looks like the extractor is cutting into the brass on the (clockwise) unlock ... maybe it was bent at some point on a bad/deformed casing around 350 rounds ago? I would replace the extractor or maybe the whole bolt depending on what it looks like. The wear mark is normal to a curtain extent but could be made worse by the the extractor digging into the cases like that ... it's probably making it very hard for the cam to rotate the bolt which is making it dig into the side of the receiver. |
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I've been gathering a small list of spare parts (good to have anyway) and this is good motivation to go ahead and pick up a spare bolt. I've got a new FN bolt headed my way from someone on the EE this week and will try an H2 or H3 buffer and a new action spring to see if it clears up the problem. Depending on how things go with the new bolt, I may grab an extractor upgrade kit and keep the bolt currently in the rifle as a spare after confirming it runs well with a new extractor. Thanks for the suggestions, all. This is a great website for questions like this..might have to pull the trigger on a membership after all.
I'll post back after I get all the parts installed and get a little range time. |
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This kind of primer issue happens. It can be tough to figure out exactly why sometimes.
The guys at ADCO can check your chamber to make sure it is 5.56 and ream it if it is not. In addition to what the guys already said that would be a good check if you are worried. adcofirearms.com I have a 16 inch AR ,5.56 ADCO barrel, A2 stock. Using winchester Q3131A1 I also had some "swipes" (ejector marks) on the brass but not as deep as what you are showing in your pics. This same rifle had some primers pop with XM193...but the chamber is cut properly so what can you do.......the ammo just a bit too hot or otherwise out of spec. Ned C. has a chamber reamer he sells but based on my reading it really cuts the chamber to 5.56 ++ size. (http://www.m-guns.com/tools.php) look for the reamer Does your problem really happen with .223 as well? Those rounds should not be as hot as the 5.56. If you are having .223 issues ask ADCO guys to look at it. |
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Hey Tommy,
Without being able to rule out any problems just yet (haven't gotten the new parts), I wanted to ask if anyone knew what was going on based on the marks left on my empties. I have noticed that this rifle has always marked 5.56 case heads. I try to police my brass and so I've been able to observe this getting progressively worse with a variety of ammo recently. To answer your question, yes - it actually does mark some .223 cases as well, just to a lesser extent. I have some Hornady .223 Vmax that I recently confirmed zero with for the next time my grandpa wants me to pop coyotes with him, and those cases had the same sort of marks showing through the nickel-plating. Other, underpowered .223 rounds do not mark, though (walmart Federal bulk and PMC). Will update again after I swap some parts and get back out to shoot. Thanks for the suggestion and lead on the chamber-reaming service at ADCO, but I'd like to avoid having that done so I can keep the chrome in the chamber. I might decide to send it off to them, though, to ensure that everything is within normal parameters before I proceed to buy some nice extras for this rifle since there are no real good AR smiths in my area that I know about. |
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Hmm,
That's a tough one. I saw similar issues before with 5.56 ammo showing brass with the ejector markings (although not as bad as the pics you are showing) and not marking up the .223 PMC, federal stuff. If you look at the http://www.m-guns.com/tools.php web site you will see he sells a 5.56 chamber gage. If you are worried about the chamber dimensions and the manufacturer of the rifle will not help you out then you could get a gage to check your chamber. Or you could send it to ADCO...they have a gage as well (they told me they have one). Once you figure out what's up then they can fix it. I'm pretty sure it won't cost much money to get it done. Maybe it's simply the ammo you have. Reports of blown primers are common enough and happens with the XM193 and Q3131 types. Rule of thumb seems to be that marks on the case head are not too big a deal unless you start to loose primers. Once you start to loose primers look into why. Start with the rifle. It's easier than shooting a ton of ammo and trying to read into how ejector marks relate to pressures. PS....I wouldn't worry to much about the chrome in the throat area of the barrel if it needs reaming. Quoted:
Hey Tommy, Without being able to rule out any problems just yet (haven't gotten the new parts), I wanted to ask if anyone knew what was going on based on the marks left on my empties. I have noticed that this rifle has always marked 5.56 case heads. I try to police my brass and so I've been able to observe this getting progressively worse with a variety of ammo recently. To answer your question, yes - it actually does mark some .223 cases as well, just to a lesser extent. I have some Hornady .223 Vmax that I recently confirmed zero with for the next time my grandpa wants me to pop coyotes with him, and those cases had the same sort of marks showing through the nickel-plating. Other, underpowered .223 rounds do not mark, though (walmart Federal bulk and PMC). Will update again after I swap some parts and get back out to shoot. Thanks for the suggestion and lead on the chamber-reaming service at ADCO, but I'd like to avoid having that done so I can keep the chrome in the chamber. I might decide to send it off to them, though, to ensure that everything is within normal parameters before I proceed to buy some nice extras for this rifle since there are no real good AR smiths in my area that I know about. |
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