Thanks for the question! Keep in mind that a huge explosion (with 40,000 PSI) is happening inside the chamber of the gun. Some of that energy pushes the bullet forward and the rest pushes the gun backward. You're right though, the gases escaping the barrel also contribute to this, similar to a rocket engine!

Based on my understanding of the physics involved, it's entirely possible that the rifle has accelerated to the recoil velocity by the time the bullet leaves the barrel, yet not actually moved measurably yet.

Well, but he says that he's measuring acceleration, not velocity. The bullet's in the barrel for just ~2 msec, and there's appreciable acceleration -- not just movement -- for >50 msec. It's true that his custom accelerometer only records data at ~6 msec intervals, but I don't see how that could smear ~2 msec out to >50 msec. Unless maybe there's some design limitations of the sensors that he used.

The PRB covers the role of propellant gases in recoil, and measures effectiveness of muzzle brakes.[0] He talks about 20%-50% reduction in recoil with muzzle brakes. And that implies that gas expansion from the muzzle accounts for at least 50% of recoil.

Even so, some additional measurements might be useful. It'd be interesting to know, at sub-msec sampling rates, the velocity of gas leaving the muzzle. And if there's still appreciable unexplained recoil: 1) the length of the barrel; 2) the displacement of the barrel from the stock; and 3) the diameter of the barrel, measured at maybe 4-5 points from the chamber to the muzzle.

Because steel is, after all, notoriously elastic. And the barrel may be attached to the stock in a way that smears out recoil. Also, the rifle may have other recoil-management features that reduce recoil, and smear it out. It's bolt-action, so there's no blowback, and I don't see a muzzle brake, but ???

0) http://precisionrifleblog.com/2015/07/01/muzzle-brakes-recoi...

The barrel and the entire gun must stretch and flex while firing. That flex will store some acceleration.

Also consider that multiple bits of the gun are mechanically attached to each other. It looks like the accelerometer is attached to the barrel, whereas the majority of the rearward forces before bullet leaves the barrel is acting on the bolt. Even as a bolt action, there's plently of interesting mechanical interfaces to absorb energy and movement before transferring to the barrel.

Thanks. In the PRB post that I cited, it's noted that recoil force can be "in excess of 1,000 pounds", but that "the recoil event is very short, lasting only about 10 milliseconds". So yes, I suspect that the gun is absorbing most of that elastically, and then relaxing on a longer timescale.

0) http://precisionrifleblog.com/2015/07/01/muzzle-brakes-recoi...