for Visit, Board, Search, and Seizure (VBSS) missions and, as of 2006, for NCIS agents deploying to active combat zones. The Mk 18 is also used by the Coast Guard's Tactical Law Enforcement Teams and Maritime Security Response Teams and the United States Navy's Explosive Ordnance Disposal (EOD) Operators. It is also used by both Marine Force Recon Teams and Critical Skills operators from MARSOC, and is in most cases the standard weapons of choice for said operators where the range of an M4 is not necessary. The short 10.3 in (262mm) barrel length requires special modifications to function reliably. The gas port is opened from 0.062 to 0.070 in (1.6 to 1.8mm). A one-piece McFarland gas ring replaces the three-piece gas ring set. The standard four-coil extractor spring is replaced with a commercial off-the-shelf (COTS) five-coil spring. An O-ring surrounds the extractor spring. The standard M4 flash hider has been replaced with the M4QD flash hider for suppressor compatibility. Ballistic performance Shortening the barrel by 50% from the original pattern has a dramatic effect on the velocity of 5.56x45mm NATO ammunition. With a 20" barrel, the bullet is moving at , but this drops to at 10". This has led some to question the effectiveness of the Mk 18 compared to other cartridges like 7.62×39mm or .300 AAC Blackout which are not as affected by barrel length reduction. Evolution CQBR type 1: Made from M4A1 uppers by shortening the barrels and opening gas ports, no longer produced, STD carbine buffer in use in host lowers at time of fielding. CQBR type 2: NSWC assembled with contracted and /or overhauled parts complete with all accessories to current standards at time of build, will work with H, H2 or H3 balanced to ammo requirements: H works with M193 and M855. H2 needed for MK 262, MK 255 R2LP, or MK 311 R2LP; will still reliably feed and fire M193/M855. H3 may be needed for an extreme case, or when the gas port is eroded but the barrel still groups. CQBR type 2A: NSWC assembled with contracted and/or overhauled parts, naked without any accessories or buffer. The buffer in the host carbine would be used or upgraded as necessary via the supply system same as type 2. CQBR type 3: Colt-contracted upper, complete with all accessories. NO buffer supplied. The buffer in the host carbine would be used or upgraded as necessary