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Thread: First Look at BCM's NEW M4 Upper Receiver

  1. #91
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    Positive. Its a BCM BFH 11.5 barrel. I wiped off the shoulder of the barrel extension before inserting it and after tightening up the barrel nut it was not pulled in any further.

    Quote Originally Posted by C4IGrant View Post
    Perfect fit from the barrels I have tested. Assuming your are using a BCM barrel, are you sure you inserted it all the way??



    C4
    Last edited by Eric D.; 02-08-14 at 11:50.
    B.A.S. Mechanical Engineering Technology

  2. #92
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    Quote Originally Posted by Eric D. View Post
    Positive. Its a BCM BFH 11.5 barrel. I wiped off the shoulder of the barrel extension before inserting it and after tightening up the barrel nut it was not pulled in any further.
    Strange. I would take a pic of it and send it to BCM and see what they say.


    C4

  3. #93
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    Quote Originally Posted by Eric D. View Post
    How do the feed ramps line up with your barrel extension? I swapped a barrel over to my new BCM4 upper last night and I concur that the barrel fit is phenomenal. There is also minimal play between the upper and lower. My only 'complaint' is that the M4 feed ramp cuts in the upper aren't quite flush with the ramps in the barrel extension. It doesn't look like they're not cut deep enough into the upper; rather it's almost like the barrel doesn't insert far enough into the upper. There's maybe 0.030" of 'shelf' in the transition from receiver ramps to barrel extension ramps. I'm sure it won't cause any issues - certainly better than having the barrel go in too far and having a lip where the bullet tip would get caught.
    Same thing here. Barrel fit going inside the receiver was excellent, but I'm seeing a 'shelf' that is created from the M4 cutouts on the receiver to the barrel extension. This is with a DD 10.3" barrel.



    Barrel is fit flush to the receiver, with the barrel nut torqued to spec.

    As stated above, I doubt it will cause any issues.
    Last edited by thecolter; 02-08-14 at 12:51.

  4. #94
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    Here's mine for comparison.
    *Pic sent to BCM

    B.A.S. Mechanical Engineering Technology

  5. #95
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    Quote Originally Posted by Eric D. View Post
    How do the feed ramps line up with your barrel extension? I swapped a barrel over to my new BCM4 upper last night and I concur that the barrel fit is phenomenal. There is also minimal play between the upper and lower. My only 'complaint' is that the M4 feedramp cuts in the upper aren't quite flush with the ramps in the barrel extension. It doesn't look like they're not cut deep enough into the upper; rather it's almost like the barrel doesn't insert far enough into the upper. There's maybe 0.030" of 'shelf' in the transition from receiver ramps to barrel extension ramps. I'm sure it won't cause any issues - certainly better than having the barrel go in too far and having a lip where the bullet tip would get caught.
    They looked good from what I recall, but I just glanced at them.

  6. #96
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    Quote Originally Posted by Eric D. View Post
    Here's mine for comparison.
    *Pic sent to BCM

    I checked with BCM and all is normal and within spec. Once you tighten the barrel nut to at least 40lbs, the the spacing is virtually non-existent (at least with BCM barrel extensions). Also, if the barrel extension sits flush with the receiver ramps with no tightening of a barrel nut, there is a chance that you can get the barrel extension to be higher than the receiver ramps. This is the kiss of death and I have seen this from time to time on cheaper AR's.

    So everything is good.



    C4

  7. #97
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    Quote Originally Posted by C4IGrant View Post
    I checked with BCM and all is normal and within spec. Once you tighten the barrel nut to at least 40lbs, the the spacing is virtually non-existent (at least with BCM barrel extensions). Also, if the barrel extension sits flush with the receiver ramps with no tightening of a barrel nut, there is a chance that you can get the barrel extension to be higher than the receiver ramps. This is the kiss of death and I have seen this from time to time on cheaper AR's.

    So everything is good.



    C4
    Yeah, I've seen this with several quality uppers and I'd call it normal...both pre-assembled (Colt, DD, LMT, and BCM) and also uppers I've built.

    As Grant said, the game ender is when you have a 'ledge' (a reverse of what is seen in the pix). That's when you either replace the upper, barrel , or both....or do some SERIOUS grinding on the feed ramps. Which is bad, because it is extremely difficult to maintain the same geometry (basically impossible by hand).
    "That thing looks about as enjoyable as a bowl of exploding dicks." - Magic_Salad0892

    "The body cannot go where the mind has not already been."

  8. #98
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    Roger that. Thanks for taking the initiative.

    Quote Originally Posted by C4IGrant View Post
    I checked with BCM and all is normal and within spec. Once you tighten the barrel nut to at least 40lbs, the the spacing is virtually non-existent (at least with BCM barrel extensions). Also, if the barrel extension sits flush with the receiver ramps with no tightening of a barrel nut, there is a chance that you can get the barrel extension to be higher than the receiver ramps. This is the kiss of death and I have seen this from time to time on cheaper AR's.

    So everything is good.



    C4
    B.A.S. Mechanical Engineering Technology

  9. #99
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    Quote Originally Posted by C4IGrant View Post
    I checked with BCM and all is normal and within spec. Once you tighten the barrel nut to at least 40lbs, the the spacing is virtually non-existent (at least with BCM barrel extensions). Also, if the barrel extension sits flush with the receiver ramps with no tightening of a barrel nut, there is a chance that you can get the barrel extension to be higher than the receiver ramps. This is the kiss of death and I have seen this from time to time on cheaper AR's.

    So everything is good.



    C4
    Thanks. I figured it would be fine and really didn't think anything of it until I saw Eric D's post.

  10. #100
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    Quote Originally Posted by carolvs View Post
    Looks like they are already using these on new uppers. I purchased this new 20" upper receiver assembly a week or so ago from BCM, and it is marked BCM4:



    I have the mark on the side from an upper I ordered in mid December. I have the C4 rail installed so I can't see if it has the BCM4 on the front.


    Sent from my iPad using Tapatalk HD

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