Range Report: PMC 62 Grain X-TAC
PMC 5.56mm 62 Grain X-TAC Ammunition
PMC’s 62 grain X-TAC ammunition is loaded in brass cases that have the annealing iris still visible. The 62 grain projectile has a copper jacket construction with a lead core and a steel insert in the ogive. The tip of the bullet is painted green. The case mouth is taper crimped into the cannelure of the bullet and the case-mouth is sealed with asphalt sealant.
The boxer primers are sealed and crimped and the load is charged with “ball” powder.
(The individual squares in the red grid below are 1/10th of an inch.)
After reading the above description of this PMC ammunition, some of you might be thinking, “I wonder how this ammunition compares to M855?” So, let’s compare!
The US mil-spec for M855 (MIL-C-63989C [Amendment 4]) states that the average velocity of the cartridges “shall be 3,020 feet per second (fps) plus or minus 40 fps at 78 feet from the muzzle of the weapon. The standard deviation of the velocity shall not exceed 40 fps.” This specification is for a 20” barrel and depending upon variables equates to a muzzle velocity of approximately 3105 FPS (plus or minus 40 FPS.)
As an aside, after reading the above specification, some of you may be wondering, “Why 78 feet from the muzzle?” The answer to that question is that this specification is simply an historical hold-over from the days when “circuit” chronographs (e.g. Le Boulenge Chronograph and the Aberdeen Chronograph) were used at Aberdeen Proving Ground, Frankford Arsenal and Springfield Armory. These types of chronographs required a significant distance between their first and second screens to produce accurate results.
As an example, when using the Boulenge Chronograph, the first screen of the chronograph was placed 3 feet in front of the muzzle and the second screen was placed 150 feet beyond the first screen. For those of you who might not be aware of the following fact; chronographs determine the velocity of the bullet at a point that is midway between the first and the second screen (i.e. not at the location of the first screen). Therefore, with the above spacing, the velocity of the bullet is determined for a point that is 75 feet from the first screen. So, add the three feet (from the muzzle to the first screen) to the 75 feet (the midway point of the screens) to obtain the “78 feet from the muzzle” distance.
I chronographed the PMC 62 grain X-TAC ammunition from a semi-automatic AR-15 with a chrome-lined, NATO chambered 20” Colt M16A2 barrel.
Chronographing was conducted using an Oehler 35-P chronograph with “proof screen” technology. The Oehler 35P chronograph is actually two chronographs in one package that takes two separate chronograph readings for each shot and then has its onboard computer analyze the data to determine if there is any statistically significant difference between the two readings. If there is a difference, the chronograph “flags” the shot to let you know that the data is invalid. There was no invalid data flagged during this testing.
The velocity stated below is the muzzle velocity as calculated from the instrumental velocity using Oehler’s Ballistic Explorer software program. The string of fire consisted of 10 rounds over the chronograph.
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: 78 degrees F
Humidity: 54%
Barometric pressure: 29.99 inches of Hg
Elevation: 950 feet above sea level
The muzzle velocity for the 10-shot string of the PMC 62 grain X-TAC ammunition fired from the 20” Colt barrel was 3073 FPS with a standard deviation of 14 FPS and a coefficient of variation of 0.46%. For comparison, IMI M855 chronographed from the same 20” Colt barrel had a muzzle velocity of 3110 FPS with a standard deviation of 21 FPS and a coefficient of variation of 0.68%.
For those of you who might not be familiar with the coefficient of variation (CV), it is the standard deviation, divided by the mean (average) muzzle velocity and then multiplied by 100 and expressed as a percentage. It allows for the comparison of the uniformity of velocity between loads in different velocity spectrums; e.g. 77 grain loads running around 2,650 fps compared to 55 grain loads running around 3,250 fps.
For comparison, the US mil-spec for M855 allows for a coefficient of variation of up to approximately 1.3%, while one of my best 77 grain OTM hand-loads, with a muzzle velocity of 2639 PFS and a standard deviation of 4 FPS, has a coefficient of variation of 0.15%.
….
The accuracy specification from the US mil-spec for M855 (MIL-C-63989C) states that the average vertical standard deviation and the average horizontal standard deviation shall be “no greater than 1.8 inches at 200 yards using an indoor range.” * The accuracy testing is conducted using machine rested, bolt-action, heavy test barrels. All other things being equal (which of course they seldom are) this accuracy specification equates to an average vertical standard deviation and an average horizontal standard deviation of 0.9 inches at 100 yards (the distance at which I evaluated the accuracy of the PMC 62 grain X-TAC ammunition.)
I conducted an accuracy (technically, precision) evaluation of the PMC 62 grain X-TAC ammunition following my usual protocol. This accuracy evaluation used statistically significant shot-group sizes and every single shot in a fired group was included in the measurements. There was absolutely no use of any Group Reduction Techniques (e.g. fliers, target movement, Butterfly Shots).
The shooting set-up will be described in detail below. As many of the significant variables as was practicable were controlled for. Also, a control group was fired from the test-rifle used in the evaluation using match-grade, hand-loaded ammunition; in order to demonstrate the capability of the barrel. Pictures of shot-groups are posted for documentation.
All shooting was conducted from a concrete bench-rest from a distance of 100 yards (confirmed with a laser rangefinder.) The barrel used in the evaluation was free-floated. The free-float handguards of the rifle rested in a Sinclair Windage Benchrest, while the stock of the rifle rested in a Protektor bunny-ear rear bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was attached to the objective-bell of the scope. Wind conditions on the shooting range were continuously monitored using a Wind Probe. The set-up was very similar to that pictured below.
The Wind Probe.
The test vehicle for this evaluation was one of my semi-automatic precision AR-15s with a 20” stainless-steel Lothar Walther barrel. The barrel has a 223 Wylde chamber with a 1:8” twist. Prior to firing the 62 grain X-TAC ammunition, I fired a 10-shot control group using hand-loads topped with a 62 grain OTM bullet. That group had an extreme spread of 0.83”.
Next, three 10-shot groups of the 62 grain X-TAC load were fired in a row with the resulting extreme spreads:
2.76”
2.24”
3.07”
for a 10-shot group average extreme spread of 2.69”. The average horizontal standard deviation was 0.66” and the average vertical standard deviation was 0.76”. The three 10-shot groups were over-layed on each other using RSI Shooting Lab to form a 30-shot composite group. The mean radius for the 30-shot composite group was 0.88”.
The smallest 10-shot group . . .
The 30-shot composite group . . .
….
* There is also a 600 yard accuracy specification for M855, that is greater than the mathematical equivalent of the 200 yard specification.
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Last edited by Molon; 06-07-15 at 09:50.
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