This article was originally posted on Thefirearmblog.com
Next up for the Big Freakin’ Cartridge Test is Korean manufacturer PMC’s clone of M855, called X-TAC M855 LAP. This ammunition features very uniform-looking external dimensions, attractively finished brass cases and projectile jackets, and a reasonable price. I formerly used M855 as a stockpiling round, although I have since switched to other rounds like Federal’s T556TNB1. The test procedure was as follows:
- Condition ammunition to 70 °F +/- 5 degrees for at least 1 hr (in practice ammunition was always conditioned overnight).
- Mount chronograph to barrel or rail.
- Record the temperature in the conditioned container before each string.
- Withdraw one round of ammunition from the cooler.
- Load and immediately fire the round.
- Cool chamber back to ambient conditions for 30 seconds
- Repeat steps 3 through 6 nine more times.
This procedure was followed for 14.5″, 16.1″, and 20″ barrel length velocity tests. To measure velocity, a Magnetospeed V3 chronograph was used attached to the barrels of the 16.1″ and 20″ uppers, and the rail of the 14.5″ upper. In addition, three 10 shot groups were shot for each round through my Criterion 14.5″ chrome-lined 1:8 twist hybrid contour midlength barrel in .223 Wylde, to determine accuracy. These targets were then analyzed using OnTarget analysis software.
The chronograph results for the 14.5″ barrel are as follows (Shot #, followed by muzzle velocity in ft/s):
Which gave us the following figures:
Standard Deviation: 22.4
Extreme Spread (highest minus lowest): 83
Compared to PMC’s M193 clone, XP193, the PMC M855 was significantly less consistent. Although the SD of the ammunition remained low enough, the ES jumped to over 80 – more than double that of the XP193. Velocity was also a bit lower than what we’d expect for this barrel length – although not too bad. In previous articles I have used a nominal muzzle velocity of 2,920 ft/s for M855 from an M4 barrel. The PMC M855 is kissing this value at its upper end, with the max being just 10 ft/s slower, and the average a little over 40 ft/s slower.
Now for accuracy:
For the PMC M855, surprisingly, the average extreme spread was significantly lower than that of the XP193. Mean radius, too, was a bit better at an average across three groups of ~0.95 MOA, vs. ~0.97 MOA for the M193. These results are curious, as we’d normally expect the ammunition with more consistent velocity to be more accurate, but that does not seem to be the case here.