Tightening the Fitment between the Upper and the Lower
As an AR-15 builder and especially as a builder in the retail market, Shield Tactical has seen just about every mix and match of upper and lower receiver you can imagine. The fit is never the same even when the fit is good. Because of the nature of how the pieces go together and because of the acceptable variances in manufacturing, you will almost never be able to replicate the fitment of a receiver pair.
For example, a customer of ours was so pleased with the way his Spike’s Tactical lower receiver fit on his Bravo Company Manufacturing upper that he wanted to build another gun with the same pair. When the receivers came in, the pieces fit together as well as could be expected from unmatched pair, but they were not as tightly fitting as his original gun.
It is no slight on Spike’s Tactical or BCM. Both are high quality manufacturers that build excellent parts and well within tolerance. In fact, most manufacturers will bias the fit slightly towards loose to ensure fit regardless of the mix. As a military armorer, to be able to swap and mix parts from different manufacturers of the same weapon system is an invaluable characteristic that saves on cost, training time, and reduces the logistical footprint of having to manage and maintain these guns. A Colt lower fitted to a Sabre upper with an FN barrel and CMT parts should have the same relative fit and performance as an FN lower with a Sabre barrel and a Colt upper. So generally speaking, because these parts must fit each other and must do so every time, the manufacturer will be sure there is enough room for pieces to reliably fit together.
Now that isn’t always the case with manufacturers of complete guns, especially in the consumer (non-military, non-LEO) market. LaRue Tactical goes the extra mile and makes sure their guns are built to such exacting tolerances that a new gun will almost certainly need a tap or two from a small hammer and nylon punch to break the takedown pin free. It is that kind of attention to detail that will result in the reputable performance that Mark LaRue’s guns always seem to deliver. Complete manufactured guns will tend to have a more reliably tolerable fit as well as matching finishes and most manufacturers understand that blasting and anodizing of receivers will often need to be done in the same batch in order to have exactly matching finishes. Of course some finishers are so dialed in they can achieve very close finishes in even separate lots. Does that mean your “Frankengun” will not achieve the same performance as an out-of-the-box rifle? Your home built rifle has the same potential for accuracy and consistency that any store bought gun does. The real question is how to harness that potential.
Today’s article will focus on three known methods for shoring up the fit between the upper and lower receivers to reduce the “slop” and maximize the receiver set’s potential for solid accuracy. Though the barrel is really the component that will determine 80% of a gun’s accuracy, the little details will make the ounce of difference necessary when you as a shooter need just a little more. Most average shooters will find that these methods provide little more than an anti-rattle solution. For more accomplished shooters, the difference is still yet hard to measure and can easily be eclipsed by a strong cup of coffee. Nonetheless, when it comes to shooting competitively, hunting for food, or defending life, it is better to have it than to not.
METHOD A: THE ACCU-WEDGE
DPMS Panther Arms sells a “slop solution” in a low cost, easy to install, drop-in device called an accurizing wedge or what most of us have come to know as the “Accu-Wedge.” This small, flexible, and always-colored-red wedge mechanically adds upward and forward tension to the takedown lug. It does not remove the slop, but it certainly dampens it. Installed properly, it dampens quite a bit.
With the Accu-Wedge installed, you can grab the receivers by hand and wiggle the two pieces slowly canting them side to side but shaking them will not yield a rattle like a pair of loose receivers will. Assuming it is installed properly, the upper receiver should not have any more room to move up and down away from the lower, and thus the axis differential between the bore and the stock should remain more consistent during firing resulting in more consistent accuracy. At least, thusly goes the theory, but we have yet to see anything that would prove otherwise.
Whichever benefit you are aiming to achieve, the Accu-Wedge is a good solution and literally takes seconds to install. To moderate any common concerns about its design, the Accu-Wedge does tend to stay in place and rarely falls out of the gun. But, in the case that it does not fit snugly inside the lower receiver, you may want to opt for a different solution. Otherwise, the Accu-Wedge can be quickly modified with a sharp knife to fit receivers where the device does not allow enough room to close the gun. The longer you use the Accu-Wedge the more “nested” it will become as it conforms to the shape of both the trigger control pocket of the lower receiver and the takedown lug. In the case that it does wear out, replacing it is as easy as pulling out the old one and dropping in a new.
METHOD B: TENSIONING SCREW
Mega Arms LLC introduced their tensioning screw on their GATOR lower receiver and it is now featured on everything from their GTR series billet lowers to their MA-TEN series .308 lowers. It is an integrated feature that allows the AR user to introduce vertical tension between the lower receiver and the upper receiver’s takedown lug, just like the Accu-Wedge. As an included feature, Mega does not charge any perceived extras for this proprietary screw. The aqua-blue-green nylon-tip is designed to be quiet and to provide variable tension on the takedown lug to reduce slop.
Because of the location of the feature, the only way to access the hex-driven end of the screw is to remove the pistol grip and drive the screw from underneath. This is best done while building the gun prior to installing the pistol grip. It is a good idea however to adjust the screw with a fully built upper as the additional weight will better simulate the required tension. Another issue is that in certain cases the screw may walk just a little and change the amount of tension that was originally set. This is easily solved with a small dab of low strength threadlocker or clear nail polish.
Despite the potential inconveniences the design of this device may pose, the tensioning screw is far more discrete than the Accu-Wedge and is much less likely to get lost. That in itself may be considered a greater convenience to the end user.
METHOD C: JB WELD
For those of you who don’t know, JB Weld is a two-part epoxy resin and steel adhesive/patch/filler etc. It works exactly the same way as any two-part epoxy cement except that it has steel in it. Incidentally, JB Weld also originates here in Texas so despite the number of existing alternatives, we’re proud to promote the use of this product. There is another version of JB Weld which looks like a stick of putty that comes in two colors. Sorry but this version just won’t work so be sure to buy the JB Weld that comes in twin paste tubes.
Firstly, you’ll need to prep the surface of the receivers to accept the JB Weld. Clean the lower on the top of the receiver with a residue free cleaner like rubbing alcohol or the like. You can clean the whole thing but you really don’t need to. Just clean the curved shelf on top of the receiver just forward of the receiver extension (or buffer tube). You’ll do the same on the upper receiver in the same place. Once both surfaces are clean, take a light grease/oil/wax and wipe a little with your finger on the surface you just cleaned on the UPPER receiver only. The lower you’ll leave cleaned and un-oiled because that is the surface your JB Weld will adhere to.
Next, pin your upper and lower together via the Pivot Pin but don’t close the two halves. Mix up your JB Weld really well and with the end of a toothpick put a small line of JB Weld along the aforementioned and cleaned curve on the top of the receiver in front of the receiver extension. You’ll want to do this while it’s fresh. You really don’t need much, just a tiny bit. Once you’ve got it on there, just let it sit for a few minutes while the top hardens a little.
Finally, after 15 minutes or so, close the two halves and engage the takedown pin. Now you’ll need to wait for the JB Weld to harden. Since hardening increases over time, you’ll want to leave it in there for at least a few hours. While it is hardening, set up the rifle such that the lower is fixed in a vice and that the weight of the barrel forces the receivers apart as much as possible. After a few hours have passed, undo the takedown pin and pivot the gun open. You should see two flat surfaces, gray in color, that run along the curves of the lower receiver only. They should be flat along the top. It takes 4 to 6 hours for JB Weld to set and it should be left to sit overnight before using it. Once it is cured, you can color it with an oxyphosphorus bluing solution to match the black of your lower.
The JB Weld method fills in some of the space in between your two receivers and should mitigate most or all of the slop. You’ll have varying degrees of success based on how you time the drying of the adhesive. The difference between this method and the first two is that the additional pressure comes from the sides and not the middle thereby making the receivers less likely to pivot from side to side. The reason why we run the JB weld along the curve of the receiver is to take up any slack not just from up and down but from back and forth as well. This 10 cent method (or probably less) is great if you don’t mind the unsightly line of JB weld on the INSIDE of your gun and you want your receivers to remain reasonably easier to open in the field.
You can get creative with solving the slop problem or even use a combination of methods to fix the ungainly rattle in your AR. I’ve seen folks use everything from lead tape to set screws to big rubber bands. Which leads to one last discussion: do you need to remove the slop at all?
The slop exists because it is inherent in the design of the gun. I doubt that it was intentional but whether it was or not it’s still there. The real question becomes simply this: Is it really a problem at all? Most shooters hardly notice it at all. In this case, ignorance is bliss. If you’re normally shooting your AR from an offhand position and not from a rest, you probably won’t notice any significant gains if at all. If you’re normally running and gunning or using a particular rig for MOUT operations, you definitely won’t notice. Also remember that the additional space in tolerance offers some convenience in that your rifle will often be easier to strip. But if you’ve built a precision AR for competition use or just for a personal challenge, you may look to one or more of these methods to maximize your accuracy. Either way, have fun and happy shooting!