MK318 Mod 0 - accuracy evaluation and chronograph data
Federal T556TNB1 (MK318 Mod 0)
A version of MK318 Mod 0 that is now available to the general public is being sold as Federal "white box" T556TNB1.The SOST projectile loaded in the MK318 Mod 0 cartridge has a nominal weight of 62 grains and a nominal length of 0.87”. It is constructed with a copper base and a small, non-bonded lead core in the ogive section of the bullet. Due to this higher copper to lead ratio, the MK318 Mod 0 projectile is longer than a traditional copper jacketed/lead core projectile of the same weight, but is still slightly shorter than an M855 projectile.
While it is often stated that the SOST projectile used in MK318 Mod 0 is similar to the Federal Trophy Bonded Bear Claw, the form of the SOST projectile more closely resembles that of Federal’s Trophy Bonded Tip projectile (without the tip of course.) As previously mentioned, the MK318 projectile does not have a bonded core.
The loaded MK318 Mod 0 cartridge has a nominal OAL of 2.20". The projectile does not have a cannelure per se, but the case mouth is crimped into the top relief band. The cartridge is sealed at the case mouth, however not with the asphalt sealant typically found on military ammunition. The primers are sealed and crimped. The casehead is stamped "FC 10". The cartridge is charged with ball powder.
A typical copper jacketed/lead core FMJ bullet will have a specific gravity of approximately 10.2. Due to its higher copper to lead ratio construction, the MK318 Mod 0 projectile has a lower specific gravity. When fired from typical AR-15 barrel lengths with a 1:9” twist rate, MK318 Mod 0 will have a gyroscopic stability factor of approximately 1.3.
When fired from typical length barrels with a 1:7" twist rate, MK318 Mod O will have a gyroscopic stability factor of approximately 2.2.
MK318 Mod 0 Chronograph Data
Chronographing of the Mk318 Mod 0 ammunition was conducted using an Oehler 35-P chronograph with “proof screen” technology. All velocities listed below are muzzle velocities as calculated from the instrumental velocities using Oehler’s Ballistic Explorer software program. All strings of fire consisted of 10 rounds each.
Each round was single-loaded and cycled into the chamber from a magazine fitted with a single-load follower. The bolt locked-back after each shot allowing the chamber to cool in between each shot. This technique was used to mitigate the possible influence of “chamber-soak” on velocity data. Each new shot was fired in a consistent manner after hitting the bolt release. Atmospheric conditions were monitored and recorded using a Kestrel 4000 Pocket Weather Tracker.
Atmospheric conditions:
Temperature: 75 degrees F.
Humidity: 47%.
Barometric pressure: 29.97 inches of Hg
Elevation: 950 feet above sea level
Two different barrel lengths were used in obtaining velocity data; a 14.5” Colt M4A1 barrel and a 20” Colt M16A2 barrel. Both barrels have NATO chambers, are chrome lined and have 1:7” twist rates. Both barrels have low round-counts on them.
M4A1 barrel.
M16A2 barrel.
For comparison, the MK318 Mod 0 ammunition was fired in sequence with two different brands of 62 grain M855 ammunition. The firing order for both barrels was as follows:
1. A 10-shot string of Winchester Ranger M855
2. A 10-shot string of MK318 Mod 0
3. A 10-shot string of IMI M855
Finally, the data:
The Crane Warfare Centers' publication, “U.S. Navy Small Arms Ammunition Advancements” reported that MK318 was “optimized” for the MK 16 with a 14 inch barrel and claimed it has a velocity of 2925 fps at 15 feet from the muzzle (presumably from said 14 inch barrel.) The lot of Federal T556TNB1 (MK318 Mod 0) that I chronographed from the Colt 14.5” M4A1 barrel would have a velocity of approximately 2889 fps at 15 feet from the muzzle (at standard atmospheric conditions.)
accuracy results in post #8 below . . .
Last edited by Molon; 10-05-10 at 18:35.
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