Update, October 9, 2026: ATF no longer requires a Form 4, tax stamp, fingerprints or registration for suppressors, short-barreled rifles, short-barreled shotguns or AOWs. They now transfer on a standard Form 4473 with a background check. Anything below that mentions a Form 4, a wait or a tax stamp was written before this change. What changed
The Sig Sauer MCX SPEAR SBR 6.8×51 13″ Barrel is a short-barreled rifle chambered in the 6.8x51mm cartridge, finished in coyote anodized and carrying the factory model code RSPEAR-68-13B-NG-SBR.
This is an NFA-regulated firearm. An approved ATF Form 4 is required before you take possession.
The 6.8x51mm cartridge delivers rifle-level performance in a compact SBR package. The 13-inch barrel keeps overall length short while maintaining the terminal effectiveness of this cartridge.
Key Facts
Model: RSPEAR-68-13B-NG-SBR
Caliber: 6.8x51mm
Barrel Length: 13 inches
Finish: Coyote Anodized
NFA Status: Short-Barreled Rifle, requires ATF Form 4
Understanding the 6.8x51mm SBR Configuration
The MCX SPEAR platform was designed around the 6.8x51mm cartridge from the ground up. This chambering offers ballistic advantages over traditional intermediate cartridges, particularly in a short-barreled configuration where maintaining energy matters.
A 13-inch barrel in 6.8x51mm keeps the rifle maneuverable while retaining more velocity than comparable SBR builds in other calibers. The trade-off is increased muzzle blast and weight compared to 5.56 or 7.62×39 SBRs.
What to Know Before You Buy This 6.8×51 SBR
Ammunition for 6.8x51mm is less common than 5.56 or 7.62 NATO. Plan accordingly for training and range work. The cartridge generates higher chamber pressure than most AR-pattern rifles, so the MCX SPEAR uses a beefed-up action.
Because this is an SBR, you’ll file an ATF Form 4 through Western Sport as the transferring dealer. An approved ATF Form 4 is required before you take possession; see our NFA wait times guide for current approval times.
If you’re also considering a 16-inch variant or a different caliber in the MCX SPEAR line, send us a message to compare configurations and barrel options.
Ordering from Western Sport
Order online and we ship it to the licensed FFL dealer you choose near you — see how buying a gun online works. Western Sport has been a licensed FFL/SOT dealer since 2007, and everything we sell carries the full manufacturer warranty. Questions before you order? Send us a message.
Frequently Asked Questions
Q: What is the difference between the MCX SPEAR SBR 6.8×51 and the 16-inch model?
A: The SBR has a 13-inch barrel and is classified as a short-barreled rifle under the National Firearms Act, requiring ATF Form 4 approval. The 16-inch model is a standard rifle and does not require NFA paperwork. The SBR is more compact but generates greater muzzle blast.
Q: How long does the ATF Form 4 approval take for this 6.8×51 SBR?
A: An approved ATF Form 4 is required before you take possession; see our NFA wait times guide for current approval times. Western Sport handles the transfer paperwork as your FFL/SOT dealer.
Q: Is the RSPEAR-68-13B-NG-SBR the correct model code for the coyote finish?
A: Yes, RSPEAR-68-13B-NG-SBR is the Sig Sauer factory model number for the MCX SPEAR SBR with a 13-inch barrel in 6.8x51mm, finished in coyote anodized. Send us a message to verify this is the exact variant you want.
Q: Can I use standard AR-15 magazines or barrels with the MCX SPEAR in 6.8×51?
A: No. The 6.8x51mm cartridge requires its own magazine design and the MCX SPEAR uses a proprietary action that does not accept standard AR components. If you need advice on compatible parts, send us a message.
Q: Who should consider buying the MCX SPEAR 6.8×51 SBR instead of a 5.56 or 7.62 SBR?
A: The 6.8×51 SBR suits shooters who want terminal performance beyond 5.56 NATO in a compact package and are willing to manage less common ammunition and higher recoil. If your priority is lightweight handling or cheap range time, a 5.56 SBR may be a better fit.
