Even though we always must be concerned with cartridge over-all length, it is particularly important that cartridges not only fit the magazine of a gas gun, but that a little extra room between bullet tip and magazine well exist, otherwise the cartridges shift in the magazine upon firing and may not feed properly. Again, contrary to most written opinions, this rifle did not seem to care whether any crimp was applied to the bullet or not; neck tension was sufficient with all bullets tried. Many .224" inch bullets, particularly thin-jacketed varmint and match bullets, are not cannelured for crimping anyway, and anything but a very light crimp can adversely affect accuracy.
S&W M&P 15 PC with both optical devices tested by Munnell. Top is Trijicon 4x ACOG with Anti-Reflective Device attached to front; bottom is Bushnell Elite 4200 4-16 x 50 scope.
Perhaps the one peculiarity of gas guns to which we must pay the closest attention is the pressure generated by any given load and the bolt thrust delivered during firing. Not only peak pressure, but also the pressure curve and port pressure must fall within a fairly narrow range. Too little pressure and the bolt will not retract far enough or fast enough to give reliable extraction/ejection of the fired case and/or permit the stripping of the next round from the magazine. Too much bolt velocity will over-stress the action and actually may be so fast as to over-ride the next round. Although various sources list powders from VihtaVuori N-120 on the fast side through IMR 4064 on the slow side as suitable, the powders at both ends of this range can present problems. My tests indicate that the faster powders only present a very small window in which charges will reliably work the action but yet are not so ďhotĒ as to unduly stress the parts. The slowest permissible powders often fill the case too full, especially for the longer bullets, and often leave too much residue, thus creating at least the potential to jam the action. As a general rule of thumb, the faster powders are more suitable only for the lighter bullets.
Please note that velocity alone is not the determiner of the proper functioning of the gun; it is the peak pressure, port pressure and the pressure curve that make the difference between faultless function and feeding jams. For instance, using the 60 grain Hornady V-Max bullet, I achieved 100 percent function with velocities between 2452 fps and 2785 fps, depending upon the powder used, and with the 69 grain Sierra MatchKing, different loads functioned perfectly with velocities between 2503 and 2874 fps.
Powders I found suitable with one bullet or another included IMR 4198, VV N-120, AA 2460, 3031, AA 2015, Bench Mark, WW 748 and H 4895. There are several others, such as AA 2230, H 322 and H 335, that are not only suitable but practically mainstays for at least the 55 grain bullet in the AR-types of guns.
A few powders worked satisfactorily with the lighter bullets tried, but gave erratic or unpredictable results with heavier weights. VV N-120 and IMR 4198 would fall within this classification. In fact, with the 69 grain Sierra, I could not convince any load using IMR 4198 to work the action reliably.
This rifle showed no desire to require any particular primer, therefore for all testing, I relied on the CCI 400 standard force small rifle primer and encountered no problems by doing so.
Smith & Wesson M&P 15 PC with Bushnell Elite 4200 4-16 x 50 scope on authorís shooting bench. The combination provided many small groups, both with factory loads and the authorís hand loads.
Bullets tried included the 52 grain Berger Match, 55 grain Barnes Multi Purpose Green (a non-lead varmint bullet), the 60 grain Hornady V-Max, 69 grain Sierra MatchKing and 75 grain examples from both Hornady (A-Max) and Berger (Match HP). The latter two are Very Low Drag-type bullets and take about as much powder space as can be afforded. This, plus the fact that even a strong crimp probably would not touch the jacket due to the mouth of the case being well forward of the beginning of each bulletís ogive when seated to clear the magazine, means that any heavier or longer bullets would likely not function correctly.
While I shot some very good groups with all bullets tried, I still experienced several instances with the first shot being out of the group. Although the round fore-end on this rifle and the very high comb (which played havoc with my earmuffs) are not the most conducive to resting on sand bags and bench shooting, still shooting sub-one inch groups was easily accomplished, with the occasional group of around one-half inch showing up on the 100 yard target. The surprising bullet to me, considering that I donít have a lot of experience with it, was the 55 grain Barnes MPG. Even though I only fired five groups with it, utilizing three powders, still four groups were under one inch and the smallest was nine-sixteenth of an inch. This was the largest proportion of small groups fired with any one bullet. The one bullet on which I spent a lot more time and which produced a good proportion of small groups was the 69 grain Sierra MatchKing. There were several instances of one-half to three-fourth inch groups fired with this bullet and most of the larger groups had a single flier. All in all, any of the bullets I tried could be coaxed into at least a few small groups and in almost all instances, these results were repeatable. When they didnít repeat, there was more than a fair chance that the problem was mine Ė not the gun's.
With the Barnes bullet, the powder to beat seemed to be AA 2015, and with the 69 grain Sierra, it, as well as Hodgdon's Bench Mark, gave very good results. Although good accuracy with this latter bullet was also obtained with IMR 4198, the use of this combination was somewhat touchy and the rifle's action did not always completely function. 22.5 and 23 grains of Bench Mark were the most reliable loads and both gave good accuracy with the Sierra bullet. With the Barnes bullet, the same loads performed wonderfully, as did 21 and 21.5 grains of Accurate 2015. The 52 grain Berger, coupled with 23.5 grains of Bench Mark or 21.5 grains of 2015 was also very impressive and totally reliable.
There is no doubt that AR-platform guns can be used successfully for match shooting. Carl Bernosky, who uses an S&W M&P 15-based rifle, has won the NRA Match High-Power Rifle Championship, the NRA Civilian and the NRA Match Rifle Championships. In doing so, he was often competing against some darn good bolt rifles. My recent experiences with this S&W M&P 15 PC has opened my eyes to a brand new type of rifle. Just what I needed; another "hobby"!
(NOTE: Neither the writer nor the publisher makes any representation of the safety of loads quoted here in other rifles. They were safe, in that gun, on that day. Start ten percent lower with your initial powder charge and work your way up slowly, watching for signs of pressure.)
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