Released at SHOT Show in 2019, the SBM4 is one of the original AR pistol braces offered by SB Tactical, and among many along its AR, AK, and shotgun models. The M4 variant features a fixed length pistol brace on a mil-spec buffer tube, and soft rubberized support wings that provide for improved comfort and function for shooters over the first SB15 brace.

As an improvement in design over the first SB15 brace, the SBM4 was intended to be a more comfortable variant, with improved ergonomic forearm support wings and a 1” hook-and-loop strap to secure the brace to the arm. The M4 brace is mounted and fixed on any AR pistol buffer tube (not included with brace).

Fully seated, the M4 has an overall length of 7.2”. Understanding these measurements can become impetrative when determining the overall length of an AR build to meet ATF regulations on AR pistols.

With a wide 2.1” rubberized body, the spine of the M4 also enables an improved cheek weld for the user. This is an improvement over the SB15, and for comparison an improvement over the smaller, thinner, and lighter Magpul CTR buttstock.

The SBM4 is only available in Black (featured) and FDE.

Product Evaluation Scores:

• Cost – Good (4/5): As one of the more inexpensive braces offered by SB Tactical, the SBM4 has a sticker price of just $99.99. The M4 is also available at secondary sites for approximately $79, with some retailer coupons or sales during holiday’s providing the brace for even a lower cost. Given that SB Tactical is the market leader for firearm braces, there is no direct competitor for comparison to the M4 other than the original SB15 or perhaps the first Sig Brace (neither of which are currently offered). Nearly all other pistol braces within the SB Tactical line (and other vendors) cost more, with a variety of additional features. But for its bare basics approach and simple functional design, the M4 is among the best balance of these with regards to inexpensive cost.
• Comfort – Average (3/5): The M4 brace is a duality of comfort—one with pliable rubberized support wings that gave the wearer a good base of support to the forearm. This prevented the AR pistol from canting while in use, and the support strap kept the overall brace secured to the arm even with the weight of the firearm held out at full extension to the body. The downside being that the brace itself had a fixed length of pull on the AR pistol buffer tube, and was un-adjustable for either the length of the forearm or against the cheek and thus made it difficult for those with thicker forearms. Elsewhere, the nylon support strap was slightly elastic that made getting a positive/tight bond somewhat difficult, but not unattainable. Internet research showed some aftermarket replacement straps to most SB Tactical braces and the M4 that prevents overlapping or permanent warping of the rubberized plastic while in storage, and the recommendation to the manufacturer would perhaps examine improving or offering alternate straps for improved comfort and function.
• Durability – Good (4/5): The SBM4 was made with an ABS rubberized polymer body that gave it a similar profile to a A2-style stock or the newer SBA4, but was less rigid and provided a very stable platform for a positive cheek weld due to the lock with the buffer tube. The forearm support sides also had sufficient flex to contour around the arm without strain or cracking. Because of the polymer material, perhaps the only risk to the durability of the brace itself would be from extended exposure to UV (i.e. sun) light that has been known to make ABS plastics brittle over time.
• Functionality – Good (4/5): The SBM4’s function as an AR pistol brace was pretty straightforward, however installation on a pistol buffer tube did prove difficult (let alone removal) given the tight tolerances and the friction resistance of the ABS material. The upside is that when on, the M4 was very solid. Fit and function did as intended, with the brace securely attaching to the forearm and providing some measurable distance (similar to a collapsed A2 stock) for a cheek weld. THose with thicker forearms may experience some difficulty in getting a full fit of the M4’s side wings, but not to the point of being detrimental to a secure fit. The only notable negative aspect from a functional point (aside from the lack of adjustability as discussed in Comfort) was an ensuing gap at the back end of the M4 brace where the AR pistol buffer tube did not fully extend. A suggestion here to SB Tactical would be to consider developing some type of endcap or utilize the space as an impromptu storage space like the Magpul grip modules.
• Weight – Good (4/5): Weighing in at 8.04 ounces (w/o buffer tube) the SBM4 is still one of the mid-weight braces offered by SB Tactical, and most likely attributed to the sheer volume of ABS polymer. In comparison, the heaviest AR pistol brace in the SB line is the SBPDW (18.14 ounces) while others, such as the SBM47 (15.5 ounces), SBA3 (6.75 ounces), and SBA4 (10 ounces) demonstrate the weight of the SBM4 is directly in the middle of other AR pistol braces.

Overall Rating – Above Average (19/25)

Product Link: https://www.sb-tactical.com/product/sbm4/

***Editor’s Note: The history and ruling of AR/AK pistol braces is a sordid one. From the initial ATF ruling in 2014, the “clarification” letter by Max Kingery (then-acting Chief of Firearms Technology Criminal Branch) in 2015, and a second ATF ruling in 2017, and the most recent ATF clarification in 2019 the regulation agency has struggled to provide a clear ruling on the application of such devices. Specifically shouldering, the topic of proper use of braces has caused more internet arguments among “internet lawyers”. Currently ATF guidance as of 2019 states as follows:

“To the extent the January 2015 Open Letter implied or has been construed to hold; that incidental, sporadic, or situational ‘use’ of an arm-brace (in its original approved configuration) equipped firearm from a firing position at or near the shoulder was sufficient to constitute a ‘redesign,’ such interpretations are incorrect and not consistent with ATF’s interpretation of the statute or the manner in which it has historically been enforced.”

As such, Per the ATF the use of an AR/AK pistol brace comes down to intent. Thus, accidental or “sporadic” shouldering of an AR pistol brace is not illegal. When consistently shouldered however, it demonstrates the intent to subvert regulations on SBRs by utilizing the AR pistol and brace as an impromptu work-around (and thus illegal). The same is said by adding accessories intended for precise accuracy as found with an SBR, such as scopes or utilizing irons. During the course of evaluations, any shouldering of the SBA3 was purely accidental, as efforts are made to test the brace within various range iterations to the extent of design, while still adhering to ATF regulations.

I am reviewing this product as a courtesy to the manufacturer and via STL Shooting Enthusiasts, so that I can evaluate it and provide my honest feedback. I am not bound by any written, verbal, or implied contract to give positive reviews. All views are my own, and based off my personal experience with the product.

The views and opinions expressed on this website are solely those of the author. The views and opinions do not necessarily represent those of the administrative staff, and/or any/all contributors to this site.