I spent a decade designing and testing small arms ammunition in the defence sector, writes Hexagon Ammunition’s Paul Bradley. During that time we faced many confounding variables. With no apparent change to build standard or materials we would find performance parameters shifting. We discovered many variables after hours of head scratching which we had not previously accounted for. One of those was the humble muzzle device and surprisingly it had little to do with barrel harmonics.
Most people will use a muzzle device of one sort or another on one of their rifles. It could be a sound suppressor, a flash hider, a muzzle brake or one of the many derivatives/combinations. The common theory when installing such a device is that the barrel harmonic will change thus effecting the precision of the round. Reloaders will spend several hours fine tuning their charge weight to try and mitigate this. As I discussed in the June issue of GTA the effect of harmonics when utilising optimal propellant/case volume is minimal at best. It certainly didn’t explain why we were seeing drastic changes in precision. Even different brands of suppressor or brake could be the difference between meeting a required precision standard (usually sub 1MOA) or not.
There is of course the obvious answer – the projectile contacting the muzzle device. We only saw this once in a rifle borrowed from a rather elite unit. The expanding projectiles in this case were a little too keen and decided to expand a bit at their first opportunity. This resulted in several ragged holes appearing in the paper target and a catastrophically damaged suppressor. That was an embarrassing hand over at the end of the loan period. Of course, there is also a chance that the muzzle device is faulty or that the threading has been improperly done. But this can be quickly verified by checking for copper deposits on the internals of the muzzle device.
Our first step was to use a Doppler radar to gain accurate reading of the projectile’s ballistic coefficient. The equipment we used would also produce a graph detailing changes in stability. It was obvious from the data that different muzzle devices altered the projectile stability which was clearly visible from the muzzle after which the yaw would slowly decay over time and distance. Of course, any instability will effect the ballistic coefficient which is ultimately a value defining drag. A projectile increased with yaw exhibits higher levels of drag as it travels through the air. A lower BC also equates to a lower velocity at greater distances.
The next question was how a different muzzle device effected stability given that there were no signs of contact between the device and the projectile. This is where the second piece of useful technology came in handy – Slow motion video cameras. We were able to see distinct changes in gas exiting the muzzle around the projectile. The swirling gases contain debris travelling far faster than the projectile itself. This could be alleviated by a reduction in charge weight but one would then sacrifice velocity and move away from the optimal case volume which was by far the biggest factor regards charge weight/precision.
Now I should note that there were other variables at play here. The projectile type and barrel played a part. Tolerances exist in everything engineering, when a projectile on the low side of tolerance meets a bore on the high side the effect is magnified. This is due to increased gas flow around the projectile. The gases move out of the bore and immediately strike the muzzle device. The way in which the gasses flow post contact with the muzzle device is important. Chaotic swirling is certainly the least desirable outcome. The effect is less visible when it comes to sound suppressors. This is because the gases have been substantially slowed, however the same performance outcome can be seen. The destabilizing effect likely occurs while the projectile is travelling through the suppressor.
So, what can be done? Well forward-thinking designers are already employing gas flow software to model this situation and achieve consistent output at the muzzle. The phenomenon is not really that new as the importance of a consistent crown at the muzzle has long been tied to precision potential. For the end user it is simply another consideration. If you are happy with your rifles performance and are only tempted by a device because it looks cool I would urge you to rethink. If you really need the recoil mitigation or sound suppression then I suggest trying a couple of different options before you make up your mind. A friendly shop or fellow at the local gun club might be able to aid with this. Anytime ballistics are involved you can ensure there is a trade off somewhere. You can rarely increase one performance metric without paying with another. It comes down to a happy middle ground.
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