One of the more common items of concern we at Shield Tactical often hear about from new builders is the issue of fitment or “spec.” When two pieces don’t go together like they seemingly should, it is easy to conclude that something in the manufacturing is amiss. But if these pieces were simply allowed to fit loosely, how would the wide tolerances affect the performance of the firearm? High performance pieces will usually have very tight tolerances and as a result will usually be stubborn during the installation. As a builder, one should consider whether or not the pieces should go together in a fashion that actually produces a technical advantage or desired characteristic. A builder must expect to encounter at least a little resistance when putting a precision instrument together.
Now granted this goes against everything we were taught about assembly. “Don’t force it.” “Pieces fit together when they should.” They still do. It takes a bit of experience to know how much force is required or how much will be too much. And of course common sense should prevail. The original rule of not forcing the square peg into the round hole still applies. When it comes to the AR-15, little more than a light tapping with a nylon/leather hammer should be required to get any two pieces to fit (with the exception of roll pins of course). Any more than that, and you’ll be using equal or greater force to get the two apart again. And just like any machinist or mechanic will tell you, if it just needs a little more help, lubricate! Yes, you can put lube on a roll pin to help drive it in. Highly recommended as a seized roll pin can be ruined if you try to force it in, and the lower parts kit only came with one! Fortunately for us gun folk, there is usually a bottle of CLP, Hoppe’s, or SLIP2000 lying around nearby.
So when is it appropriate to give it a little extra elbow grease, or when should you pack up the parts and head over to the neighborhood gunsmith?
Lewis Machine & Tool (LMT) prefers a tight fitment between the upper and lower receiver. They’re not the only ones either. Most high quality precision manufacturers have opted to fit the upper and lower together with very little space in between and eliminate as much “slop” as possible. This is usually done in consideration of performance in that the tighter fit will help reduce one variable that can affect accuracy. LMT takes things a step further on their MRP (Monolithic Rail Platform) uppers by machining in two small tabs on the underside of the upper receiver. Though the rest of the geometry remains the same, the tabs will protrude down towards the lower and act as a standoff between the two. You’ll see just a little additional airspace between the receivers but the fit will be tight. This results in a common downfall for builders using this MRP upper. If you match the MRP with an LMT lower, you’ll have the highest degree of success mainly because both pieces were built to LMT’s own specifications. Even still, the fitment will likely be tight. All the better to ensure the best accuracy possible. However, if you decide to match that MRP with a lower from a different manufacturer, you will have varying degrees of success as most manufacturers design their pieces to fit to… you guessed it… their own pieces.
We’ve had more than one customer complain about the fitment claiming that the items were “out-of-spec.” And just as many who claimed they had no choice but to file the tabs down so they could put their gun together. Was that the wrong thing to do? Not necessarily. Those tabs constitute a small degree of adjustability for the unlucky few whose receivers do not go together. If I file too much, will the resulting slop cause my rifle to shoot horrible groupings? The AR-15 by design can consistently put together 1.5 MOA groups and many quality barrels will achieve MOA accuracy regardless of that fitment (within reason of course). Now if you went through the trouble and expense of buying a high quality upper like the LMT MRP whose groups should be well below sub-MOA, I’d say it is worth equal consideration to make sure it is built in a manner that maximizes that intended performance. Is the gun ruined? Not by any means, there are ways to tighten the fit between the receivers that anyone can do. Yes there are a few, and we’ll go over those on another day.
Another common issue on some lower receivers is the tolerances of the pivot pin hole. Many builders will encounter a pivot pin hole that is too small. This can be a result of the machining or of the finishing process. Either way the solution is simple. You can try another pivot pin and hope that it was machined just a little smaller, or you can get yourself a .250” reaming bit and the handle from your tap and die set, cover it with cutting oil and SLOWLY turn out the hole until your pivot pin fits.
It is not uncommon for the pivot and takedown pins to fit so tightly that you need a punch and hammer to break them loose. Sometimes that’s even a result of the detents needing to be worn in a little. Oil up the pins and tap them into place. Don’t hammer them in or they’ll seize. If you have to do anything more than tap tap tap them into their hole, stop. If you can get the pins in place and they’re tight that’s great. Just remember that they will loosen up over time and you’ll eventually forget all about how cumbersome it was to take apart your rifle.
The moral of this particular story is simply this. If you are unsure or even don’t know whether or not something is fitting or working properly–ASK! There are plenty of experts around who know their way around these guns, especially the various iterations of the AR-15. If your local gunsmith, gun buddy, or annoying know-it-all acquaintance doesn’t know, chances are the answers are just a short visit to the Internet away. Gun forums are a particularly valuable resource for builders when vexing questions arise. And that leads to the other part of the solution. It’s important to note that no matter how easy these guns go together (typically) there is always variation from brand tobrand or even batch to batch. Rely on an experienced gunsmith to make the decisions on the unknowns. Your safety as a shooter should come first amongst all considerations when building your gun.