Accurize an AR-15 with X-Shims for the AR-15 and M16 | The Social Regressive

Accurize an AR-15 with X-Shims for the AR-15 and M16 | The Social Regressive

Does a loose connection between the upper and lower receivers of an AR-15 affect the rifle’s precision? Can tightening up the rifle decrease group sizes? These are controversial questions, and I am going to test the premise.

My Rock River Arms LAR-15 is no spring chicken. The once-tight connection between the upper and lower receivers now audibly rattles and wobbles. I can see light through the gap.

There are two schools of thought on this matter: one school says that consistent accuracy is reliant upon all elements being tightly fitted; the other says that there is no significant movement of the upper receiver before the bullet exits the muzzle. I suspect that the “tight” school is right. When I put together a long-range precision bolt-action rifle, I take great care to ensure that the action is perfectly mated to the furniture. I want no movement between any of the rifle’s components.

The X-Shim is a set of a four color-coded plates that fit over the lugs on an AR-15 or AR-10. They bridge the gap between the receivers, reducing or removing wobble. I will be using shim #4 at the back, and shims #3 and #4 at the front.

This cursory test compares two 5-round groups without the X-Shims and two 5-round groups with the shims. Without the shims in place, my group sizes were 1.700″ and 1.925″. The X-Shims reduced group sizes to 1.575″ and 1.250″. The average group size reduction was a whopping 0.400″. Not too shabby. This doesn’t PROVE the concept; these were only four groups fired by one guy. But it is some interesting evidence. I plan to continue testing the X-Shim to see how the rifle performs.

Here is a link to the product:

A competing product is the Accu-Wedge. It performs a similar function, but only affects the rear lug.

– The Social Regressive


  1. kentlyone on October 28, 2021 at 8:24 pm

    A thin o-ring will do the same thing

  2. Harrison McCullough on October 28, 2021 at 8:27 pm

    I use rubber O rings. Cheap and available at any auto parts store.

  3. Sunshine_Shooter on October 28, 2021 at 8:29 pm

    4 groups, 2 with and 2 without, doesn’t tell us a whole lot. It could just be the shooter (you) warming up that day. It could have been the wind. Luckily, this is an easy problem to fix, just shoot a lot more groups 🙂 Statistically, over the course of 5 or 10 groups with and without, you will be able to say with great certainty one way or the other.

    Still interested in seeing what you plan on doing with that budget build!

  4. Jonathan Wilfred Michelin on October 28, 2021 at 8:30 pm

    I live the video’s, I have a Winchester XPR rifle I’m trying to make into a hunting aswell as a marksman rifle if I can , just not many stocks aroused d for it lol

  5. Un Garza on October 28, 2021 at 8:30 pm

    I put a big glob of KY on the upper receiver tabs and insert them slooooowly into the lower receiver, and I gently wipe up the excess with a very soft tissue. Then I massage the charging handle back-and-forth several times to purge any air bubbles from the takedown & pivot pin holes. Then I am ready to shoot…Buahahahaha!!!

  6. Jen Psaki’s Cousin on October 28, 2021 at 8:30 pm

    Play in the upper receiver and barrel extension is far more important to eliminate than upper lower fit. I have done extensive testing on 4 different rifles. Only 1 of the four showed improvement after eliminating upper lower slop, and this was the 1 of the 4 that had the least wobble, I still find the slop annoying tho. Slop in barrel extension/upper, and bolt carrier tilt are the 2 killers of accuracy in the AR-15. read Joe Carlos articles on American gunsmith for techniques.

  7. Je Gr on October 28, 2021 at 8:31 pm

    Unnecessary. No the wiggle doesn’t matter on an AR platform. It’s a different beast than a traditional walnut and blue steel rifle. Putting rubber shims, accu-wedges, o-rings, aluminum foil in your gun to remove the slop, all bullshit. Unnecessary and stupid. There are only 3 necessary steps: Put ammo in your mag, put your mag into your AR, shoot. That’s it.

  8. RyeOnHam on October 28, 2021 at 8:35 pm

    Yes, I bought a couple dozen Chinese imitation acu-wedges and have used them for about 20 years now. I don’t do a whole lot of benchrest and was mainly looking for the peace-of-mind feeling of a tight gun. I do think I shoot better with them, but the biggest change I made was when I started doing free-float builds rather than just regular handguards. Yes, I know that’s almost mandatory these days, but when I started building there were all of about two choices: DPMS and YHM. My national match rifle has the free-float handguard under modified rifle-length handguards I built it in about ’95 I think for High-Power shooting.. Still has the same wedge in it, so the Chinese rubber is still going strong.

    I have thought about doing an M-14 / M1 Garand style bedding job on the AR-15 by using devcon. Would have been in the same general area as the X-Shim. Had a system worked out for it in my head, but never got around to it.

  9. Dean Grifin on October 28, 2021 at 8:36 pm

    Better machining would fix that but I guess that would be to expensive to do that

  10. The Wondering Angler on October 28, 2021 at 8:39 pm

    Electrical tape works wonders

  11. Nebula Ultra on October 28, 2021 at 8:43 pm


  12. Voo Doo1 on October 28, 2021 at 8:44 pm

    Put a small oring over the front of the lug for the take down pin and put back together…NO MORE WIGGLE.

  13. JB on October 28, 2021 at 8:46 pm

    the problem is out of spec holes. JP Microfit pins are made bigger and bigger by .001" use the pins that are snuggest fit and the problem is permanently solved for a $28 set of pins.

  14. E2qNX8btraQ3zRD6J7fc on October 28, 2021 at 8:48 pm

    I picked up an Accuwedge for my 20" A2. I’ve had it in my rifle for 13 years. It’s starting to break down from the oil; time to replace it. I never expected any improvement in accuracy but it did tighten up the rifle nicely. It took out the play between the upper and lower and prevented wear from the two halves hitting each other and kept the pin holes in the lugs from wallowing out.

  15. Nathan Calige on October 28, 2021 at 8:48 pm

    You can accomplish the same thing with aluminum foil.
    Also you should have tested this blindly to remove any bias. If you think the shims increase accuracy you might do better and so its not the Shims but you shooting better.

  16. rob nunziata on October 28, 2021 at 8:53 pm

    it may not help looks like it did though but it cant hut

  17. Beardoe on October 28, 2021 at 8:55 pm

    On my LR308 I used oversized pins and an adjustable reamer to get a perfect fit. Now it locks up tight and the only rattle is the BCG because there is no way to remove that slop.

  18. jesse mccoy on October 28, 2021 at 8:56 pm

    You need more than 20 shots to tell you anything.

  19. Dougboffl on October 28, 2021 at 8:57 pm

    Would you please shoot a ten round group without shims, one group with shims and a group with the wedge only. Then repeat the set to see if/where there is improvement. Total 60 rounds. Display but discount each groups single furthest flyer.

  20. Blazkowicz on October 28, 2021 at 8:59 pm

    You are suppose to be able to see light through the upper and lower, rifles held together by two pins are not suppose to have a welded fit.

  21. 50 Stitches Steel on October 28, 2021 at 8:59 pm

    I have never used any shims on my AR but I know that when doing my CQB training, while using a forward grip, having a sloppy interface, and pulling the rifle solid into my shoulder will most definitley create side to side torque on both halves and throw my shot off quite a bit if im not pulling perfectly straight back into my shoulder. Its unacceptable when talking about shot placement for quick takedowns…I might give this a try

  22. HermCorps on October 28, 2021 at 9:02 pm

    I, personally, don’t think a lil bit of slop between the receivers effexts accuracy on an M4/combat rifle, but could wear out the takedown/pivot pin holes

  23. Importworx on October 28, 2021 at 9:04 pm

    I cerakote both receivers and sand down for a tight but doable removal of the take down pins. Wobble is zero and gap is not existant

  24. Stickfighter on October 28, 2021 at 9:04 pm

    Great video. I tried the accu wedge years ago but did not notice any accuracy improvement. My rifle at the time had a 20" stainless steel match barrel and free floating handguard and 9 power scope so accuracy was quite acceptable starting out even with a bit of receiver gap. I wish that I had recorded the group sizes now (refreshed all my AR rifles over the last couple of years so currently don’t have significant receiver gap). I think that the shim system might be more successful because it supports both the front and rear of the receivers.

  25. Effrey Jeppstein on October 28, 2021 at 9:04 pm

    I HATE camo! To death!

    But yours is so pretty! 😍 nice camo job, it looks phenomenal.

  26. 99cobra2881 on October 28, 2021 at 9:07 pm

    Pieces of crap

    Accuwedge type device does it better

  27. Tyrone Kim on October 28, 2021 at 9:09 pm

    Very good video. Thanks for the post. It looks like shimming does make a difference. I ‘m wondering if you have re-shot after cleaning the shimmed rifle and if you did… did the cleaning make any difference?

  28. Isaac Rascon on October 28, 2021 at 9:12 pm

    I used accu wedge on a BCM upper and spikes lower build and yes there was significant difference in accuracy,but its been 6 months now and accu wedge is loosing its size i guess u could say ,its easier to push down and put upper to lower,when before it was really tight. I am going to try the new product in your video,thanks.

  29. Richelle Peralta on October 28, 2021 at 9:13 pm

    man if I ever come to see you one of this day you need to show me how to shoot that rifle lol!🤗🤗🤗

  30. Russell Cowher on October 28, 2021 at 9:17 pm

    I have also found side to side play in my A2 rifle I compete with, and to eliminate that I did shim one side of it to eliminate play as well as using an accu wedge, that was years ago so I don’t remember how thick the shim was, but I have a selection of shims and I’ll look to shimming the upper and lower as you have done here also. I can use every 14" I can get. Thanks!

  31. Greg Keen on October 28, 2021 at 9:18 pm

    This might work kinda, but if you’re serious about accuracy then you might want to use .001" oversized pins. Or drill and tap your lower to accept a nylon tipped set screw. One in the front and one in the rear. Of course this helps a little more than shimming. I typically see 1/2 MOA or better improvement using the set screw method. This doesn’t take into account the other inherent problems with the design of the AR platform, but every little bit helps. Shimming the barrel into the upper receiver with some green loctite will tighten up your groups as well. So will addressing Bolt carrier tilt. It’s definitely possible to take a $500 rifle and make it shoot as well as a $2000 rifle. It just takes a little knowledge.

  32. Raymond Kuk on October 28, 2021 at 9:22 pm

    Does the x shim helps close the gap between receivers?