Sig Sauer MCX Rattler .300 Blackout
Model: RMCX-300B-5B-TAP-SBR
Sig Sauer MCX Rattler .300 Blackout
Model: RMCX-300B-5B-TAP-SBR
Full Specifications
Threaded 5/8x24 barrel: compare .30 caliber suppressor prices.
About This Firearm
The Sig Sauer MCX Rattler is a short-barreled semi-automatic .300 Blackout built to pack down small. Sig no longer makes it, so it is a used buy. Its barrel is 5.5 inches and the whole rifle is 23.5 inches with the stock out, on a PDW upper with a thin folding stock Sig built for concealment and stowage.
Sig names no sights for this configuration and fits none. The handguards free-float the barrel and carry M-LOK slots, one 30-round polymer magazine comes with it and the well is AR-15 pattern. A gas piston drives the action and the gas system adjusts, so a barrel this short will run subsonic loads behind a suppressor. The muzzle is threaded 5/8x24 with no device fitted. That barrel length puts it under the NFA, so what changes hands is a registered short-barreled rifle.
If you're cross-shopping, the Sig Sauer MCX Spear LT has an MSRP of $2,179, is in production, and adds 3.5 inches of barrel to reach 27.75 inches overall at 6.40 lbs, which is 0.70 lbs more rifle for more velocity. The Q Honey Badger is the other compact .300 Blackout of this kind, lighter again at 4.80 lbs with a 7-inch barrel and 24.75 inches overall. Sig has replaced the Rattler with the MCX-Rattler LT, so a used rifle is the route to this exact configuration.
Best For
Strengths & Limitations
- The handguards are free-floating, so a light or a laser clamped to the M-LOK does not move the point of impact.
- The receiver is hard coat anodized, which holds up on an aluminum gun that gets handled and stowed a lot.
- Sig gives the 9-inch Spear LT barrel the same 1:5 twist, so a subsonic load worked up for one short Sig carries over to the other.
- There are no sights and no muzzle device, so an optic and a can or a brake are both purchases.
- A 5.5-inch barrel loses velocity to every longer .300 Blackout here, and it is built for subsonic work.
- The MCX is Sig's own system, so the upper, the carrier and the stock are not AR-15 parts even though the magazines are.
Compatible Ammunition
Find the best prices on compatible .300 Blackout ammunition.
Shop .300 Blackout Ammo →Ballistics Calculator
Calculate trajectory, drop, and energy for .300 Blackout ammunition.
.300 Blackout Ballistics →Where to Buy
No prices available at this time.
Frequently Asked Questions
Is the trigger the same as the Spear LT's?
No. Sig fits the Matchlite Duo here and a Flat Blade Match trigger to the Spear LT.
What does the adjustable gas system adjust?
How much gas the piston sees, so the carrier is driven hard enough for supersonic loads and softened for subsonic ones behind a can.
Does a used one still need a tax stamp?
Yes. It is a registered short-barreled rifle, so a transfer goes through the same NFA paperwork as any other registered item.
