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Posted: 3/9/2012 6:24:52 PM EDT
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I got my first BCM upper today, and I fitted it to my spikes tactical lower, but i noticed that the fit was kinda off - in the first picture, you can see that the "rib of the magwell doesn't line up to the rib of the upper. There is also a considerable gap between the rear of the upper and the lower. I noticed that if I link the upper with the rear takedown pin and remove the front one, there is lots of play in that pin hole
Is this normal for an AR? my last build (del-ton) was VERY tight, and fit without any play. I'd hate to think that BCM builds out of spec AR uppers... http://img26.imageshack.us/img26/4333/snv30700.jpg http://img820.imageshack.us/img820/6896/snv30705.jpg |
thats a really bug gap..
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Not so much of an up and down play (that the accuwedge fixes) rather than the fact that it seems to be too forward Regarding the magwell piece lining up on the upper and lower - Not out of spec. That's out common, even with uppers/lowers from the same manufacturer. I've seen it with PSA, and I've even seen it with colt. Weird that there's play between the receivers, that's weird. But sadly, it happens. If it really bothers you grab some o-rings. |
 This is a joke thread right? There are different forge houses (Cardinal, Anchor Harvey, etc...) If your upper and lower come from two different forges they often will not line up right there. Has absolutely nothing to do with being "out of spec" And play is normal. Unless you can stick a matchstick in the gap its in spec... ETA: Its not uncommon for some makers to source receivers from more than one forge, so its possible to see this even on "factory complete" rifles. |
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This is a joke thread right? There are different forge houses (Cardinal, Anchor Harvey, etc...) If your upper and lower come from two different forges they often will not line up right there. Has absolutely nothing to do with being "out of spec" And play is normal. Unless you can stick a matchstick in the gap its in spec... Preach it - 
Unless you're buying a complete rifle. Quite often fit and finish won't match up completely. I'm sure that BCM/Spike build will still run like a raped ape. |
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Just got it today, so I have no idea how it shoots. I don't build ARs often (as in other than once before), they're too expensive so I don't know this stuff (not a joke thread, just new to stuff)
Edit: The funny thing is, it only got be worrysome when I remembered reading that Iraq sand is as fine as flour, and got to wondering how much would seep into a gap like that =P |
| Normal. My BCM upper and lower don't match up. Neither does my Colt. If I mixed and matched all my lowers and uppers, I might get most of them to line up. But, why bother? It doesn't affect anything. They still shoot fine. It's all a matter of machining tolerances. |
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I guess bcm and spikes don't go together 
I have spikes upper and lower and they match perfect so it must be the bcm Please learn a bit more about tolerances before you make a statement like this. Brands have nothing to do with it, neither the mixing of them, nor having the same. OP you have just encountered a tolerance stacking issue. It appears to be normal, nothing out of spec, you just have one part on one end of the tolerance spec one part on another end. Shoot the shit out of it, it poses no functional issue. With that said, if you arent happy with it then call up one of the companies and explain your trouble. |
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Quoted: Just got it today, so I have no idea how it shoots. I don't build ARs often (as in other than once before), they're too expensive so I don't know this stuff (not a joke thread, just new to stuff) Do you understand what I mean by forge houses? Most makers like BCM, Spikes, etc... get their recievers forged by a handful of large forges. Cardinal, Anchor Harvey, Cerro, etc.. The makers then go on to machine and finish the receiver. These big forges will have slight differences, like exactly how that fence lines up. It has absolutely nothing to do with whether the rifle is in spec. ETA: The square next to the BCM indicates that upper was forged by Brass Aluminum. Also as mentioned, tolerance stacking is also an issue. Also, that gap as shown in the second pic is not big at all. |
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Just got it today, so I have no idea how it shoots. I don't build ARs often (as in other than once before), they're too expensive so I don't know this stuff (not a joke thread, just new to stuff) Edit: The funny thing is, it only got be worrysome when I remembered reading that Iraq sand is as fine as flour, and got to wondering how much would seep into a gap like that =P It's okay, I'm exactly the same as you are. I look over my rifles with a fine tooth comb, at the end of the day if you're shooting well you have nothing to worry about. Enjoy it! 
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Several of my "mixed and matched" ARs look like that, and even some of my complete factory rifles look like that too. It's purely cosmetic, and nothing to be concerned about 
I realize of course that these rifles are expensive, no matter whose name is on them. For this reason, anything less than 100% cosmetic perfection is sometimes hard to accept. Just take solace in the fact that these rifles are based on military weapons, and form follows function. How well it performs is the underlying criteria for judging quality, not how it looks. Trust me Posted Via AR15.Com Mobile |
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Magwell rib - already covered. Normal. Cosmetic only.
Popping the front pivot pin = fore & aft play with the rear pin secured = Normal. All rear lugs on AR15 uppers have .250" holes that are so slightly oblong it's hardly noticeable. The upper's position on the lower is indexed off the front pin. |
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Quoted:
This is a joke thread right? There are different forge houses (Cardinal, Anchor Harvey, etc...) If your upper and lower come from two different forges they often will not line up right there. Has absolutely nothing to do with being "out of spec" And play is normal. Unless you can stick a matchstick in the gap its in spec... ETA: Its not uncommon for some makers to source receivers from more than one forge, so its possible to see this even on "factory complete" rifles. Completely normal... function check it, shoot it and if that is all ok, forget the cosmetics of "offness." That's the way the rifle was designed.. to be loose between lower and upper... just like a hammer has a big head to hit a small nail, the lower/upper assembly was designed to have and still work with some "play" in the two... |
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Totally normal. I've seen many ARs that have the channel on the mag-well misaligned, including the one in my closet as my gtg. Play between the receivers is nothing special, and doesn't need to be addressed unless it bothers you, but it's easily fixed with a wedge or o-rings.
As for the concerns about sand and the such entering the gun, your dust cover, mag-well and muzzle will all let more garbage into the gun than that gap will, so I wouldn't really be concerned with it. Shoot it and enjoy. |
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Not so much of an up and down play (that the accuwedge fixes) rather than the fact that it seems to be too forward Regarding the magwell piece lining up on the upper and lower - Not out of spec. That's out common, even with uppers/lowers from the same manufacturer. I've seen it with PSA, and I've even seen it with colt. Weird that there's play between the receivers, that's weird. But sadly, it happens. If it really bothers you grab some o-rings. +1 seen this a lot. Pretty common when miss-matching uppers/lowers, just like color of finishes not matching. Seen this with RRA lower/YHM upper, YHM lower/ Bushy upper, Spikes with Colt and so on and so on.........not really a big deal. The play in the upper/lower fit on the other hand.... that may be an issue if it's as serious as described. |
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I guess bcm and spikes don't go together
I have spikes upper and lower and they match perfect so it must be the bcm Please learn a bit more about tolerances before you make a statement like this. Brands have nothing to do with it, neither the mixing of them, nor having the same. OP you have just encountered a tolerance stacking issue. It appears to be normal, nothing out of spec, you just have one part on one end of the tolerance spec one part on another end. Shoot the shit out of it, it poses no functional issue. With that said, if you arent happy with it then call up one of the companies and explain your trouble. Hey man I'm just saying if u wanna make sure it matches then get the same brand. (not saying that'll be perfect either, but there's a better chance). Thanks |
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I guess bcm and spikes don't go together
I have spikes upper and lower and they match perfect so it must be the bcm Please learn a bit more about tolerances before you make a statement like this. Brands have nothing to do with it, neither the mixing of them, nor having the same. OP you have just encountered a tolerance stacking issue. It appears to be normal, nothing out of spec, you just have one part on one end of the tolerance spec one part on another end. Shoot the shit out of it, it poses no functional issue. With that said, if you arent happy with it then call up one of the companies and explain your trouble. Hey man I'm just saying if u wanna make sure it matches then get the same brand. (not saying that'll be perfect either, but there's a better chance). Thanks What Durabo stated is correct. Even buying from the same manufacturer is no guarantee of a "perfect" fit since many companies out source to multiple forges. These multiple sources make it entirely possible to receive parts that are within the specification range, ( + or -). Looks like what you have here is possibly a lower at the far end of the "minus" range and an upper at the far end of the (plus) range, or vice-a-versa. Hence the issue of tolerence stacking. |
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The mag pieces not lining up is not a problem.
The gap at the back could be. If the gap is so large that the BCG doesn't relieve the pressure of the buffer on the retaining pin when closed, his retaining pin will chew up his buffer around the edge. I mean it will still work, but it doesn't mean it's normal. |
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