The Accu-Wedge

I have mentioned many times in the pages of this magazine that when Stoner designed the AR platform he probably never envisioned it becoming a target gun. There were design elements that made it reliable on the battlefield and easy to work on, but not as accurate as some of the demands put on it today. One of the most glaring accuracy detractors is the method of attaching the barrel to the upper receiver.

I offered some simple solutions to curing the problem in "The Relationship Of Barrel Extension Diameter To Accuracy In The AR-15" in the March and April 2013 issues. The other accuracy design shortfall is the method of attaching the upper receiver to the lower employing two loose fitting captive pins. Custom pistolsmiths spend time getting the slide-to-frame fit of 1911 semi autos just right. Precision rifle builders glass bed actions.

Yet, a fair number Below center: Thinning of the base of an Accu-Wedge is sometimes necessary to fit certain lower receivers. of AR-15 builders overlook all the play between the receivers of the guns they build despite the fact that tightening the receiver fit of the AR takes less time than properly lapping a slide. Takedown Pin Solutions I have told two-thirds of the following story several times but I have been holding the final third for inclusion in this article.

When I first came on as the Armorer for the Army Reserve Service Rifle Team I assessed the team’s 135 National Match uppers, first asking the team members to thoroughly clean them. Machine rest testing with quality factory match ammo revealed horrible accuracy! In my testing I found many of the uppers arrived quite fouled, so I cleaned the several dozen that I had already tested properly and retested them.

This yielded noticeable improvement, but initially failed to keep records on the degree of improvement, a mistake I learned to not make again. However, even with proper cleaning accuracy still wasn’t acceptable. The final part of that story pertains to this article. Prior to trying Accu- Wedges, I would have rated the fit of those uppers as average.

If you saw my machine rest you would swear that, even without an Accu-Wedge, there couldn’t possibly be any movement between the upper and lower when everything is clamped in place for testing. The stock and lower is clamped with twelve locking points and the upper is clamped by seven. Despite the machine rest making accuracy-robbing movement nearly impossible, I found groups were tighter in the third tests. I learned to keep detailed records and used the same ammunition lots.

The Accu-Wedge groups tightened an average of 26%! I can only imagine how much potential these Wedges offer when shoulder fired without the machine rest bracing. Since then, I have never tested an AR without a tight, properly-fitted Accu-Wedge in place. While Accu-Wedges are meant to be a dropped in, some upper/lower combinations may need fitting. The Wedge’s bottom sits on a ledge in the lower receiver near the take down pin, a dimension varying among manufacturers.

If the ledge is cut deep, the Accu-Wedge may not sit high enough. I have a Colt receiver with a thick metal washer under the Wedge to get enough support. Other brands of lower receivers require thinning the Accu-Wedge’s base. Proper fit needs to be tight so as to require a lot of muscle to close up the firearm. If you are fitting a Wedge for a smaller statured or female shooter, have them present to make sure they can close up their gun.

While fitting a Wedge, look at what I call the "tail." Think of the Wedge as a bar stool shape. The circular part on bottom would be the seat and the piece that sticks up behind it would be the back of the stool. If that section sticks up too high it can rub against the bottom of the bolt carrier during function. Don’t hesitate to cut off 1/8" or so from the top to prevent this. An Accu-Wedge will not last forever. Eventually it will compress and will not be as effective.

If kept under compression during storage the Wedge will tend to develop a set sooner. Cure this by shotgunning the AR (release the rear takedown pin) and store it with no pressure on the Wedge. Even at that you may want to replace the Wedge every six months or so. Watch your groups and replace if they start to open up with no other obvious reason. Always confirm zeroes after rotating in anew fresh Accu-Wedge.

It isn’t unusual for between 1/3 to a full minute change in point of impact after replacing a worn out Wedge. Because the Accu-Wedge is an internal modification, it is legal to use in most competition events. A device similar to the Accu- Wedge is the Accu-Screw, a nylontipped Allen set screw that is precision located in the lower receiver and hidden by the pistol grip. It presses up on the underside of the bottom of the lug in the upper receiver where the takedown pin resides when the rifle is fully assembled.

Sun Devil (SunDevilmfg.com, and Mega Arms (MegaArms.com, offer an Accu-Screw in their lower receivers. Accu-Screws are installed by removing the pistol grip and drilling a hole under the center of the upper receiver rear lug. The hole is tapped and an appropriately-sized nylon-tipped set screw is adjusted to bear upward pressure on the bottom of the upper receiver so as to take as much slop out as limited by the physical strength of the owner to close up the weapon.

Once the tightest setting that the customer can physically endure is arrived at, the pistol grip and accompanying parts is replaced. There is nothing to keep you gunsmiths from performing this modification on your customer’s rifles. According to Sun Devil’s website, they have applied for a patent on the idea but until that patent is finally granted (which may take years) it is my understanding that there is no infringement nor is it unethical for other gunsmiths to offer the service.

If you want to see this job properly done before diving into it yourself, send a receiver to Turners Tactical Arms, LLC (TurnersTactical.com, They also sell lowers with Accu-Screws installed. As with the Accu-Wedge, the Accu-Screw is an internal modification that is invisible and likely fully legal in most competitions. Another option for tightening is the JP Enterprises (JPrifles. com, Tension Pin. Replacing the rear takedown pin, it’s placed through the existing upper and lower receiver holes.

A supplied Allen wrench expands the pin inside the holes to tighten them. JP cautions against getting too hoggish on the amount of pressure used as it’s possible to damage the innards of the pin. I would also be a bit afraid of causing damage to the receivers.

Point of impact shifts are possible when changing internal pressure. This Tension Pin will be going out and in during every cleaning. Using just feel on the Allen screw, most shooters will likely apply inconsistent torque to the pin and that may lead to POI changes, though I haven’t yet experimented enough with these to say for certain. If it does, the cure is a torque wrench calibrated in inchpounds used to get the same torque each time.

Since this pin performs its tightening function without the need for a lot of physical strength it might be a good option for some smaller statured shooters. Varmint Lights (Varmint Lights. com, manufactures their X-Shim Kits for AR-15 and AR- 10 rifles. Incremental shims for both front and rear takedown pins fill any gaps between the upper and lower at each of these two locations. For only $9.99, the company says one kit is good for two or three rifles.

Arma Lite has made National Match Oversized Pivot and Takedown Pins, advertised as being 0.001" fatter than stock pins normally 0.250" in diameter. However, pins available from other common sources sometimes exhibit poor quality control and can run on the skinny side of published specs. The corresponding receiver holes also run various sizes. Measure the dimensions of your pins and receiver holes to see if such oversized pins will actually make a difference for you.

As I was researching for this article, I found Arma Lite’s website (ArmaLite.com) listed the NM Oversized Pivot Pin (part #ENM001) for the front but not the rear takedown version. I may have simply overlooked it and Far Use of an inch-pound torque wrench is not mandatory but may be helpful. Match pins are an effective and invisible way to tighten worn receivers. hope it’s not discontinued as I’ve found these worthwhile, especially in tightening worn receivers whose pin holes had stretched with age.

My rule of thumb is if either pin can be pushed in with just my thumb, I replace with a bigger pin. If more resistance is needed I leave them alone. I feel tapping the pins in and out just hastens wear. I’ve found these Arma Lite NM pins to be a very effective tightening device where warranted and completely invisible, so they are likely legal for nearly every competition. To minimize their tendency to permanently enlarge the corresponding holes in both receivers, lube them frequently and generously.

I recommend using a Pivot Pin Detent Installation Tool when installing or swapping pins. The spring and detent holding the pin captive are tricky to hold while working into place and I’ve sent bunches of these into orbit before the Brownells tool came on the market. The tool won’t idiot proof you so I still recommend having a few spare springs and detents on hand.

Other armorers place the lower half of the gun in a large cardboard box while working on the pivot pin or use a clear plastic bag over the front of the lower to help catch flying parts. Pivot Pin Solutions Let’s look at how we can improve the front pivot pin. Power Custom (PowerCustom.com, offers Receiver Shims, available from Brownells in ten packs for $15.99 (713-000-108).

These little washers go between the "ears" at the front of the lower receiver and front lug of the upper with the pivot pin pushed in normally. This fills a gap between the

lower and the sides of the pivot pin lug in the upper, a gap that contributes a lot of horizontal movement, rotational torque, and slop in the final fit. The washers I’ve measured mostly go about 0.004" thick, with a few a little thinner but none under 0.003". Use your feeler gauges to get an overall gap size, which may need more than one shim. Getting just one of these washers to hold alignment well enough to push the pivot pin through is a little bit of a challenge and two increases the difficulty.

I suppose with practice one may gain a certain finesse in performing this juggling act, though There are three of them shown. I’m certain that "inventive language skills" are also helpful from a morale standpoint. I have tried placing one shim on each side of the "ears" as well as stacking both on the same side. Neither was easy and I have ruined a number of them in my admittedly "all thumbs" learning curve.

If you are willing to compromise and accept more slop, the job becomes "easier" but that is defeating the purpose of using the shims in the first place. Michigan Center Outdoors (MichiganCenterOutdoors.com, also offers shim packs made up of three assorted thicknesses (0.002", 0.003", and 0.005"). This along with the 0.004" Power Custom shims offer a fair amount of flexibility.

Because I often end up needed spacers thicker than any of these, I also keep shims from MSC Industrial Supply (MSCdirect.com, sized at 0.373" on the outside with a 0.258" hole size. The slightly fat hole size which gives some leeway in clearing 0.250" pivot pins. Stock #76424308 is 0.007" thick and #76424266 is 0.010" thick. MSC also has shims in 0.006" and 0.008" with skinnier 0.250" holes that will just clear the pivot pin but alignment becomes really critical.

I like working with the thicker spacers as they don’t tend to kink and bend as severely when getting them aligned, and it’s far easier to use a single, thicker washer than to combine two thinner ones. Don’t worry about overstocking these shims because they also can be used to take windage slop out of rear sights for the A2 rifles and the Crosman MAR air rifle upper for the AR-15. For more information see my article, "AR-15 Sub-Caliber Training" in the October 2015 issue.

Decades ago, Colt replaced the user friendly, one-piece pivot pin with a two-piece creation having a slotted screw head on the left side. Shooters hated it with many feeling it was a solution to a nonexistent problem and replaced the two-piece pin with a conventional one-piece option. Turns out we may have been short sighted. People who kept their two-piece pivot pins discovered that a little file or grinder work can modify the long section to tighten the upperto-lower fit at the pivot pin.

The long section normally exceeds the outside distance between the two "ears" of the lower receiver. Screwing the left side slotted section in place, the pivot pin can’t back out but the head of the screw never pressures the left "ear" of the lower receiver. However, filing a short bit off the end of the long section of a Colt pivot pin and dressing it down so it’s shorter than the distance between the outsides of the "ears" allows it to tighten.

Thus modified, applying a bit of muscle when screwing the pin together pinches the

"ears" closer together, taking up some of the gap between them and the pivot pin lug in the upper receiver. Since we are dealing with aluminum receivers, that made me a bit nervous and I was afraid of damaging a valuable lower, however, I’ve never found an instance where it happened. When AR spacer shims became popular, I started using them with the modified two-piece pin, taking up as much gap as possible with the washers and leaving only a couple thousandths to be pinched by the modified pivot pin.

I put blue Loctite on the threads and tighten to my desired level. I recommend using Kroil in the tight areas, applying it frequently as you will be "shotgunning" the firearm after each range session for cleaning. Two-piece pins can be had from Specialized Armament (SpecializedArmament.com, as part number is SA08001, or from KNS Precision, Inc. (KNSprecisionInc.com, Double Up! There are two pins holding Stoner rifles together.

To achieve true match grade tightness in any AR, apply one of the tightening methods to each of the two pins. Far too many folks think they can do it by just tightening at one pin location and that ain’t so! Virtually all of the methods I have described are cheap, fast, and simple.

Just as the best way to cure a cold is not catching one in the first place, the best way to cure movement between the upper and lower receivers is to start with a matched receiver pair, such as the set offered by Rock River Arms (RockRiverArms.com, Because ARs are made with aluminum receivers, eventually even a matched receiver pair will need to be tightened back up using the hints in this article but most shooters will likely get years of accurate performance before that happens.

Conclusions As I’ve discussed, I was disappointed with the accuracy of the USAR Service Rifle uppers when I first came on as their Armorer. Having a negligible budget for parts, I instituted an "AR-15 Upper Quick Fix Program" as a low cost but fast accuracy enhancing effort. Many of the fixes can be applied cheaply by shooters, don’t always require a gunsmith, and can be done without a bunch of expensive tools.

In the decade plus that I was the USAR Armorer I effectively cut the group sizes of the Team’s uppers in half and my Upper Quick Fix Program was a huge contributor to that effort. This is the fourth installment in my Upper Quick Fix series, with the previous three being "Match Bolt Carriers", "Magazine Checks", and "Electropolishing And Fire Lapping." Generally, firearms have more similarities than differences.

There are folks who will try to make you believe that tight slide-to-frame fit in a National Match pistol and a double pillar bedded stock in a rifle aids accuracy, but for some mystical reason upper-to-lower fit Stoner-type rifles can be completely ignored. These folks fumble for answers when challenged or come up with reasons that don’t compute. As a competitor with the Army Reserve Shooting Team I could see a difference in groups and scores when I replaced a worn Accu- Wedge with anew, tight, fitted one.

It’s why I saw a 26% average group reduction when machine rest testing. If you aren’t convinced, try any of these tightening methods, with the cheapest and easiest being a five dollar Accu-Wedge. If you don’t see an improvement, you don’t need to bother any more. Regardless, that piece of bubble gum in your receiver or other improvement won’t make up for the need to ruthlessly apply the fundamentals of marksmanship. AG