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Thread: Upper wear at cam pin pocket

  1. #1
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    Upper wear at cam pin pocket

    I recently put together a new SBR upper, using a blemished BCM upper, 11.5 inch BCM BFH barrel, and one of the Rubber City Armory coated BCGs. Vltor low pro gas block and BCM charging handle are also installed. This is also set up for a suppressor, and I am using an OPS Inc M4M and M4S , but primarily the M4M.

    So I take it out the other day for the first time to get it zeroed. Once irons and RDS were gtg, I attached the M4M and ran some rounds through, same with the M4S. Fired a total of about 140 rounds, pretty even between Fed. XM193 and Hornady steel training rounds (limited range time this day). Probably about 80 of them were suppressed. All functioned fine, one fail to lock back on a 20 rnd Pmag I was using on the bench while zeroing.

    So here is my concern. When cleaning, I noted some wear/gouging at the rear part of the cam pin pocket on the upper. There is also a bit at the front of the pocket, but this looks like finish wear, no material removed (sorry, no pics). I also noted some slight wear in the finish on side of the cam pin "cap" (for lack of a better term). I searched and found varying opinions on how normal this is. Only one of my other rifles shows this type of wear, a fairly beat Colt carbine.

    This is my first SBR, and my first time with suppressors. I was using a H2 buffer with a blue Sprinco recoil spring. I am not sure if this is a timing issue with the BCG, or was an indicator of another problem. The cam pin cap measured within a few thousandths of others I have in other rifles. Could overgassing be causing this wear, and would using a H3 buffer to slow the BCG help?

    Thanks for any help you can provide.

  2. #2
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    Normal, and self-limiting. Honestly you probably should notice it on your other guns as well(ETA:Just read that you did on one.) Some seem to do it more than others(and earlier or later in their service life,) but they all do it to a degree. If you think about the way the cam pin gets cammed...it makes sense.

    Then again, without pictures...it's just a stab based on your description.
    Last edited by thopkins22; 03-14-13 at 13:45.

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    not to beat a dead horse but if you had typed in your thread title at the orange search button above, you would have gotten more information and saved yourself all the time/trouble of typing in the rest...
    never push a wrench...

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    Quote Originally Posted by ra2bach View Post
    not to beat a dead horse but if you had typed in your thread title at the orange search button above, you would have gotten more information and saved yourself all the time/trouble of typing in the rest...
    Actually, the one thread that came up for me more or less states that while some find it common, the system was not designed for some hitting there, and that any hitting, no matter how common, is still outside spec.

    My question, which was not addressed (again in the one thread that came up for me) was would overgassing potentially cause this type of wear, and would a heavier buffer slowing things down help. I included the other info as a means of providing information regarding parts and quality.

    Thanks

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    Can you link the thread you are talking about? Unless a harder material is used or the wear area was machined with a smooth radius, I don't see how that wear can be avoided regardless the specs. The reason for that wear is that when a round is stripped and chambered, it puts a force on the bolt that tries to push the bolt into the carrier. Since the bolt has to rotate to be pushed into the carrier, that means the cam pin is bearing on the side wall of the upper receiver anytime there is an inward force on the bolt. As soon as the cam pin gets to the pocket, it rounds over that sharp corner.

    As to your question, any over-gassing will exacerbate wear. How much, however, is anyone's best guess. A heavier buffer will help correct the weapon's function but I don't think it will have any effect, noticeable, perceived, or otherwise on the wear in the cam pin pocket.
    Last edited by Eric D.; 03-15-13 at 00:06.
    B.A.S. Mechanical Engineering Technology

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    Here is the thread I referenced earlier.

    https://www.m4carbine.net/showthread.php?t=85555

    I am of of the opinion that if the cam pin was supposed to hit this spot, there would be an allowance for it. I also realize that as long as it does not impede function, it is not a huge deal, and once it wears in, it will probably become a non-issue.

    I will probably try a H3 buffer to see if it mitigates this at all. As this is my first suppressed SBR, I was hoping someone with more experience may have recognized this as a symptom of something specific, and would be able to help.

    Thanks again.

  7. #7
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    Quote Originally Posted by PD Sgt. View Post
    Actually, the one thread that came up for me more or less states that while some find it common, the system was not designed for some hitting there, and that any hitting, no matter how common, is still outside spec.

    My question, which was not addressed (again in the one thread that came up for me) was would overgassing potentially cause this type of wear, and would a heavier buffer slowing things down help. I included the other info as a means of providing information regarding parts and quality.

    Thanks
    no. gassing would not have anything to do with it. it's a clearance issue.

    without a picture I can't see the type of wear you are talking about so I can't say if it's self-limiting or not. I will say that I have not witnessed any to very little wear in the area you are talking about in my guns so if you are getting shavings, it may be cause for concern. can you post a pic?
    never push a wrench...

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    Take that line of reasoning that we saw in the linked thread just a hair further and we should believe that any metal touching other metal is bad engineering and that nothing should ever wear to any degree in a properly designed weapon.

    My carrier shouldn't have rails to ride in the upper receiver...it should just float.

    Seriously, if you're going to worry about it, I'll snap pictures of upper receivers from four of the most respected manufacturers and show you exactly what you describe.

    I don't believe that the person who said that in the thread is the end all and be all authority with regards to the platform. I would disregard that completely. And as ra2bach said, slowing the speed of the carrier won't address it even if it were a problem...it has nothing to do with the speed of the carrier, but the position of the cam pin in the cam pin slot on the carrier at a given point in the operation of the rifle.
    Last edited by thopkins22; 03-14-13 at 23:57.

  9. #9
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    http://s783.beta.photobucket.com/use...tml?sort=3&o=0

    http://s783.beta.photobucket.com/use...tml?sort=3&o=1

    Here are links to the photos of the wear. I hope they work, not my strong point.

    I have no problem with wear, and I know it will occur on rails, etc., but this looks a little more like gouging (to the rear part of the pocket especially), and seemed outside the norm to me, though I could not get a good angle to show the extent of it in the pics. This is not the norm on my other carbines, but is present on one Colt. Again, if it is not a big deal, no worries, but I wanted to put it out there for a more experienced eye.

    Again, thanks for any help provided.
    Last edited by PD Sgt.; 03-15-13 at 00:53. Reason: Additional info

  10. #10
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    Completely normal. It'll peen that area to a certain amount and stop.
    "You people have too much time on your hands." - scottryan

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