Many speeds, many guns, many angles, many different cameras.
Notice the difference between that first, current production Mini-14 and the second, old production Mini-14 spaghetti barrel.
This next one is an air rifle…pretty flexy, as it doesn't need to contain the pressures a firearm does.
This one is good, as there is straigh stuff in the background.
AK:
And this last dude puts straight lines on some of the film clips for reference, last rifle is labelled as an "AR-10 .308"
Somewhere (can't find it now) there used to be military or ammo/firearms company footage, of rifles mounted rigidly on a fixed surface, fired and then filmed in slow motion against a background of a grid (1" squares I beleive). The barrel flex was astounding.