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...