Shooting the Short-Barreled XCR

Robinson Armament XCR-L 11-inch with HTG Eureka-4 suppressor, EOTech 512, AR-Tripler in LaRue 649-S mount

I spent some time at the range with a chronograph, my short-barreled XCR, and my newest 5.56mm suppressor: HTG’s Eureka-4. The Eureka is 4.6″ long and weighs only 15 ounces. It extends the gun’s overall length only 2.8″ more than the standard flash suppressor it replaces, making it ideal for a compact, CQB-style carbine like this.

George Vais, who patented and developed the HTG suppressors, assured me that the Eureka is designed to absorb the higher muzzle pressures of shortened 5.56mm barrels, which some people fear will erode the blast baffles of cans not designed for that abuse. Granted, my 11″ barrel isn’t remarkably short given that some people run these rounds through 7.5″ barrels. At 11″ even the standard flash suppressor does a decent job moderating the muzzle blast, so shooting this XCR is no harsher than any other carbine I have shot in this caliber.

With the silencer attached, the gas piston valve can be turned to its lowest setting (“S”) and the gun still cycles reliably. This is thanks to the longer backpressure curve that a silencer maintains. However, a consequence of putting rounds designed for 20″ barrels through a system this short is that a significant amount of propellant gas blows back through the ejection port, which is not the most pleasant thing to have venting in front of your face. If I had to do this all day I’d probably wear goggles to keep it from irritating my eyes.

Without the suppressor mounted I turn the gas valve to its highest setting (“4”), as is suggested during the first few-hundred round “break-in” period. Factory 55gr .223 loads that chronograph 3150fps out of my 24″ bolt gun leave the unsuppressed 11″ barrel at just over 2600fps. I was surprised to discover that the gun’s action can tap a lot of power out of the shots, even though with this barrel the propellant doesn’t reach the gas hole until 1.5″ from the muzzle: Turning the gas system from “4” to off — essentially making this a bolt-action gun — the bullets gain an extra 250fps! (Of course, this extra velocity is accompanied by a more violent muzzle blast.)

Screwing the silencer on increases muzzle velocity even further (thanks to an effect known as “freebore boost“): With the gas system off and the silencer on this gun shoots factory 55gr bullets over 2930fps, which is not bad for a barrel assembly measuring just 14” long overall. Turning the gas system back on to “S” drops velocity just below 2900fps. Out of curiosity I kept the silencer on and stepped the gas valve up a notch to “1,” which pounded the action harder, ejected the case further, and dropped muzzle velocity to 2830fps. Always one to test the limits, I then cranked the gas back up to “4” and fired one round, at which point the action crushed the neck of the spent case and somehow produced a triple-feed malfunction!

Robinson Armament XCR-L 11-inch with EOTech 512, AR-Tripler in LaRue 649-S mount

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