For purposes of this analysis, the muzzle velocities from the 24" barreled (suppressed) XM-2010 will be considered most relevant, and therefore used for all calculations. The A191 (MK248 Mod0) and Mod 1 rounds are fairly well known and have often been compared against each other. For purposes of the present analysis, an additional ammo type is proposed for comparison, namely 230 OTM. This proposed round will be considered in relation to the other familiar ammo types in several environments to see how the hit percentages compare.
It's clear from the dimensioned drawings that the 190 and 220 grain SMK's are very similar bullets, with the only major difference being the length of the bearing surface. The relatively blunt nose results in G7 form factors of around 1.06 for both of the SMK's. By stark contrast, the Berger 230 grain OTM has a much longer nose with a tighter meplat diameter and a optimal 7 degree boat tail. The G7 form factor of the Berger Hybrid OTM is 0.91, which indicates 15% less drag than the SMK design. This 15% drag reduction, in addition to being heavier than both of the SMK's results in the Berger Hybrid OTM having a dramatically higher Ballistic Coefficient (BC) and better ballistic performance. All of the bullets under consideration are fully stable when fired from 1:10" twist barrels.
In order to conduct a complete WEZ analysis, the 3 bullet options will each be evaluated for hit percentage in various uncertainty environments. The intent of the different uncertainty environments is to determine how hit percentage is affected in relation to the variables commonly encountered in real world shooting. Atmospheric conditions will be modeled as ICAO standard sea level values1. Other uncertainties will be modeled in 3 confidence sets; high, medium and low.
The high confidence uncertainty set is intended to model a highly trained shooter with a laser rangefinder, 1/2 MOA rifle, and ammunition loaded with 10 fps Standard Deviation (SD) in muzzle velocity. The medium confidence uncertainty set models a nominal shooter with an average ability to estimate wind speed, compromised use of laser rangefinder, average 1 MOA rifle with typical ammunition having 15 fps SD. Finally the low confidence uncertainty set represents poor wind estimation, MIL'ing targets for range, and below average rifle and ammunition. Detailed plots and tables of the hit percentage results for each uncertainty environment can be found in the Appendix.
Of course any calculated hit percentage depends greatly on the size and shape of the target. The target that's modeled for this analysis is the standard IPSC silhouette target shown in Figure 2. Trajectories that intersect the target area are considered hits, those that don't are misses. There is no consideration made for where the bullet strikes the target. The benchmark Kinetic Energy (KE) indicated on the plots is 1000 Ft-lb. The importance of this metric is debatable, but it's shown on the plots as a reference for comparison. The benchmark Transonic (TS) speed is Mach 1.2, which equates to 1339 fps in ICAO standard conditions. The transonic speed is indicated on the WEZ plots with a TS, and all values in the hit percentage tables that correspond to velocities lower than transonic are printed in gray.
The first element of the results analysis will be retained velocity for the 3 ammo types. Figure 3 shows a plot of velocity traces from muzzle to 1500 meters with numeric table values at; muzzle, 500 meters, 1000 meters, and 1500 meters. Since A191 sniper rifle is the lightest bullet, it starts out with the highest velocity. However the heavier 220 and 230 grain bullets quickly catch and surpass the 190 grain bullet in retained velocity. It only takes 162 meters for the 230 bullet to match the retained velocity (2585 fps) of the 220 grain bullet. The 220 grain bullet meets the 190 at 372 meters with a retained velocity of 2260 fps. At 262 meters, the 230 surpasses the 190 grain bullet in retained velocity (2456 fps). In summary, beyond 262 meters, the 230 grain OTM retains more velocity than both the A191 Rifle and MK248 Mod1 Rifle.
Retained velocity is an indicator of how flat a trajectory is. Flatness of a trajectory is related to danger space. Therefore we can say the 230 OTM has the greatest danger space on any sized target beyond 262 meters if all rounds are zeroed at the same range. In addition to relative retained velocity, it's also interesting to look at where the transonic zone falls for the three ammo types. The WEZ analysis uses Mach 1.2 (1339 fps) as a common transonic speed.
However, it can be difficult to predict the exact velocity at which transonic stability effects begin. Figure 3 indicates the zone between Mach 1.2 and Mach 1.0. This is the zone in which one can reasonably expect the onset of transonic instability effects in a sea level environment. Figure 4 shows the ranges that correspond to the upper transonic speed zone (1339 fps to 1116 fps) for each ammo type. It's clear that the 230 OTM pushes the transonic zone much farther than either A191 or MK248 Mod1.
Note that the specific levels of retained velocity are highly subject to atmospheric conditions. The above results are all for ICAO standard sea level conditions. In environments higher above sea level, the bullets will retain velocity much better and the ranges corresponding to transonic zones can be dramatically pushed out. For example, in a density altitude (DA) of 5000 feet, the range at which the 230 OTM encounters Mach 1.2 is extended from 1285 meters to beyond 1500 meters. In other words, the 230 OTM can remain comfortably supersonic to beyond 1500 meters at DA's of 5000 feet and higher. Ranges corresponding to Mach 1.2 for A191 is increased from 982 to 1150 meters, and for MK248 Mod1, Mach 1.2 is extended from 1079 meters to 1260 meters.
If the differences in retained velocity appear dramatic, one only needs to refer to Figure 1 which shows the 3 different bullets under consideration. The dramatic difference in bullet design is directly responsible for the dramatic difference in BC and retained velocity.
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