While at Media Day, I had the opportunity to shoot a number of .308s, including the new HK MR762 and Sig 716. The experiences were slightly different – the MR762 was off the bench with a very nice scope, and I had no problems with rapidly pinging a steel spinner at 85 or 90 yards. The Sig 716 was shot offhand, and I kept it on the steel during fairly rapid fire as well, although I shot it at a steel silhouette that was perhaps 15 or 20 yards away. I fired them back to back and can say that both had significant amounts of recoil that felt approximately equal.
I also had the opportunity to shoot some high speed video of several rifles in action. Specifically, I took high speed video of the MR762, Sig 716, and Armalite AR-10, among others. A direct recoil comparison of the three is very difficult because a different shooter was behind every rifle (Jeff at Gunblog for the Armalite, Steve from Airsoftology for the MR762, and a Sig rep for the 716), and because I do not have a large sample size to work with, the rate of fire calculations/estimations may not be representative of overall rifle performance. Another factor is ammo – although I am fairly certain that all three were using Federal’s AE308D 150gr load.
However, when I spoke to the designer of the 716 on the floor at SHOT and asked him what the cyclic rate of fire of his weapon was, I was told that it was between 650 and 700 rounds per minute. The observed speed of 645rpm would, then, seem to be fairly accurate.
Also on the topic of the 716, there is a noticeable amount of bolt bounce – that is, as the bolt carrier comes forward and impacts the barrel extension, it recoils slightly. This has the effect of unlocking the bolt to one degree or another, which will prevent the weapon from firing if the hammer falls again during this period of time.
The amount and duration of bolt bounce seen with the 716 would not prevent it from firing on full auto, in my experience. However, the designer of the 716 told me that the weapon was designed as a semi-auto rifle, and currently has a lighter buffer than the full-auto version, the final details of which have yet to be nailed down. As another point of interest, barrel details – steel and finish – have also yet to be finalized.
Armalite told me that they use high speed video to tune the performance of their rifles, but it took me a while to find an engineer-type to confirm what the rate of fire of their AR-10 platform rifle should be, which was between 650 and 750rpm, depending on ammo. Again, the observed rate of fire of 715rpm falls within this range. One (non-engineer) rep at Armalite was of the opinion that this only mattered on full auto, which is not exactly true.
The rate at which the bolt carrier assembly recoils rearward can have an effect on reliable extraction and ejection, even if extractor and ejector dimensions and springs are absolutely correct. The period of time during which the bolt is behind the stack of rounds in the magazine, neither traveling rearward or forward, has an effect on reliability in that the magazine may not have enough time to push the next round into place before the bolt comes forward again, resulting in a “bolt over base” malfunction that is most commonly seen on suppressed rifles, as they have much greater rates of fire. Also, high forward bolt carrier velocity can result in extreme bolt bounce, as noted previously, while low forward bolt carrier velocity could mean that there isn’t enough force to overcome strong magazine springs, dirt or debris in the action, etc.
As for HK, I was unable to confirm the expected cyclic rate. I was, however, told about some MR762 endurance testing that I will hopefully be able to report on at some point in the near future.
Due to a lack of control (shooter) and a small sample size, the video provided here is not intended to judge any of the weapons based on these factors, but should provide some insight as to how each weapon functions. It is also nice to know that the Armalite and Sig weapons were functioning at the rate their designers expect them to.
This is probably a dumb question (though one a google search didn’t answer), but why is it that a suppressed rifle has a higher rate of fire?
More backpressure. More gas coming back into the action, both through the bore and the gas tube if applicable. But even without a gas tube, ROF increases. For example, a suppressed 10.5″ HK416 has a ROF of nearly 1100rpm.
Did you get any time on the CM901?
Unfortunately not.
Great post. Even if the video had little comparative value, I still liked it.
I think there is much to learn in the high-speed evaluation of weapon systems. Keep up the great work.
Andrew, your high speed videos single-handedly revealed the issue of bolt bounce with ST-T2 carbine buffers.