Recently there was a long thread on TOS about BCM uppers being looser/out of spec. Lots of different opinions. I didn’t get involved but did follow it a bit. I am planning to pick up 3 complete BCM mid-length uppers so I thought I would ask over here and hopefully get some honest feedback w/o the drama. So here is the question.
How does your BCM upper fit in general? For those who use Colt/LMt lower(all I have or will use), how do yours fit? Is the play within acceptable limits?
I do realize that there is supposed to be some play, I just would like to know if it is typical or not.
My advice to you would be to consider the source… TOS is full of “cheap gun snobs” who will do just about anything to downplay the quality of any AR that meets or exceeds the TDP. BCM is highly regarded by people who know more about the AR platform than I could ever hope to learn.
That said - my sample of one fits great. Not too tight, not too loose…
I have one BCM upper. It has been on a DSA lower and now it resides on a BCM complete lower. To be honest, I’ve never bothered to wobble it on the lowers so I have no idea how it fits.
Upper/lower slop, upper/lower color match, coloring in logos with crayons on lowers, front sight base obscures view through Aimpoint, 30 rounds in a 30 round magazine, if the gun is too heavy you just need to work out, etc. tend to betray a certain kind of gun owner. I actually find these clues very helpful in my dealings with people. They tend to be the result of bored people sitting at home mucking around with things rather than going out and shooting the gun to see if it matters.
If these blueprints are authentic (http://www.scribd.com/doc/14939212/A...ver-blueprints), the hole in the rear lug of the upper is designed to be sloppy (.024" of horizontal slop in the spec) so that all uppers will fit all lowers without trouble. My new BCM upper is made exactly this way, confirming in my mind that these blueprints are legit.
Perhaps other companies have made a simple .250 hole instead of the kind of hole shown in the blueprint: a “racetrack” shaped cross section with .024 straightaways. If all the holes line up just right, this would give less slop, but it would reduce interchangability in the case of a replacement receiver (upper or lower) slightly off spec. In other words, it appears that .024 of slop is actually in the spec to make sure there’s no trouble interchanging parts, and that companies ignoring this spec and making .250 holes provide a fit actually tighter than spec at the risk of limiting interchangability.
I love these questions and I did read the thread on BARFCOM.
So I always like to ask “what is acceptable.” My definition of acceptable is that I cannot fit a match head between the upper and lower. Other people want the fit so tight that they cannot push the pins in and have to use a hammer and punch to break the weapon down. This is a HUGE FAIL in my book!
I have now purchased a feeler gauge and have started to capture tech data on the “fit” between uppers and lowers. If you want to be able to push the pins on the upper and lower, you would not want any tighter fit than .005 with an acceptable range of .005-.009. This is of course just my opinion as the TDP does NOT give specs on how much play between the upper and lower is allowed.
My sample of one 11.5" upper shows a nice snug fit. The lower I have it on, an Aero Precision, was finished in a protective powder coat that makes pushing the pins a bit of a chore. If it does not start to loosen up, I am going dremelling with a buffer!
There is a lot of pointless griping running rampant on TOS so I rarely ever go there anymore either. Seriously, why does the upper/lower fit matter so much to some people? Does it still group 1 MOA or better with match ammo? Yeah? Then what are you bitching about?
My sample of 7 is just right, with a minimal amount of play, and pins that are removable by hand, with the exception of one LMT lower that I need to tap the pins with the plastic side of a gunsmith hammer to get the pins started coming out. That being said, this is coming from TOS, and who cares? I’m just happy I started coming here before I ever looked at TOS.
I’m just adding to this earlier post to emphasize that the slop designed into the upper is horizontal slop, not vertical. The racetrack-shaped hole in the upper’s rear lug is .250 top to bottom and .274 front to back (that’s the spec on the blueprint posted above) and doesn’t affect wobble at all as far as I can see. That would come from tolerance stacking of the pin sizes, hole diameters, and distance of hole centers from receiver edges.
The oval-ness of the rear hole seems a good way of engineering for mass-produced tolerance stacking in order to be dead sure of interchangability.
Just for curiosity’s sake, y’all ought to get out a piece of precision .250 drill rod, your small hole go/no go gages, your depth gages, your micrometers, and your dial caliper–and measure your receivers and pins for yourselves. From a machinist’s perspective it’s fascinating to think through just what you’d want to do to get the parts tight enough but not too tight. For example, if the upper hole is .251, the lower hole is .251, the pin is .248, and the distance from hole centers to edges of the receivers is .249, you could easily end up with (horrors! ) .006 of slop. If each of these numbers rose another .0005 you’d be out around .009 for your total slop. On the other hand, allow just a couple of number to go the other way, and you’d be hunting for your rubber hammer.
Mine fit well on either my Colt or Territorial lowers. Not much experience personally with any others, but I have seen them on Spikes, LMT, Stag, Noveske, their own BCM’s and even a Bushy and I saw no problems of excessive wobble when I picked them up and whatnot.
I look at it kind of like the 1911 tightness issue and what the actual purpose of the rifle is for. Now I’m not a gunsmith or expert with some of the details of the AR platform by any stretch, but I am a pretty good shot and can hold my own running and gunning I feel and I don’t think a little wobble is anything to worry about.
If it doesn’t affect the purpose of me defending my or my families’ life then I don’t worry about it. My rifles and training are geared for one mindset solely. It is a hobby in a way, but with a purpose indeed.
This is actually addressed on page 215 of the current 23&P. If you can insert a 0.020 thickness gage between the pivot pin lugs and upper and lower receiver and the gage penetrates at all accessible locations then repair is made by replacement of the upper receiver or rifle is required.
As has been said already,the wobble/fit is just a variance of tolerances in manufacturing.Grant has probably eye balled more uppers and lowers recently than most due to the fit concerns of his customers.So I think what he has said on the subject pretty much says it all.
Personally Ive seen differences in fit of the receiver mating even from the same manufacturer.Then again Ive got uppers from one manufacturer that fit somewhat tight on a different manufacturers lowers.
I worried about this very subject myself back in the late 90’s during my HP days.Soon learned there is no such thing as a custom or match fit.Even if you do have receivers matched(which I did with Armalite on a NM)after a couple thousand rounds of beating and banging the parts burnish and you get some wobble in the end.
I’ll further add in my days of marijuana eradication as a LEO in the early 90’s I worked with a couple local NG units that carried some well worn A1’s that actually rattled when handled they were so loose.Could easily see daylight through the gap of the receivers.Never saw one fired but was told by the NCO’s that all the weapons functioned 100%.
Other than my NM A2(rear lug is bedded) all my AR’s have some slop and wobble no matter the manufacturer.Particularly the carbines and they all run so I dont worry about how they feel.
Bro inlaw has a very tight fitting RRA that has no gap or wobble.“Feels good” but the downside is not all uppers will fit his lower without some effort.
So Ive come to the conclusion that worrying about this very subject is a total waste of time in any regard.As long as I have quality components and the weapons function 100% thats all I want to worry about other than ammo and quality mags…
