This is not true. The bullet is accelerated down the barrel and the rifle is accelerated back towards the shooter. Acceleration is a change of speed with a change of time, or a change of direction with a change of time. Since a key part of acceleration is a change of time, that tells us acceleration cannot be instantaneous
This is absolute garbage. Terrible misrepresentation of science being used to mislead here. Usually you do far better on your math.
If force is exerted on the bullet forward, the same force is exerted rearward on the rifle, and momentum is conserved. The fact the rifle is far heavier than the bullet means the rifle is accelerated much less than the bullet - if you do the free body diagram for the two, F = M1A1 = M2A2 with 1 being the bullet and 2 being the rifle. If a bullet is 150grns, the rifle 9lbs (63,000grn), then the ratio of M1/M2 is 0.00238, therefore the ratio of acceleration is then the inverse. F=M1A1=M2A2, further then M1/M2 = A2/A1. So the bullet accelerates 420 times faster than the rifle.
The acceleration of the bullet, and therefore the rifle, are not constant, however, anyone with Quickload can run the velocity curve, run the integral and get the instantaneous acceleration of the bullet, and therefore the rifle. It's remarkable how simple rocket science really is, in that way.
But at midnight with my give-a-**** pretty low, let's pretend the acceleration were constant, because even this generalization will illustrate why you think you can get away with saying it is delayed. Say you're talking about a bullet starting at zero and exiting at 2900fps from a 24" barrel. Average speed, with constant acceleration, is then 1450fps, and in a 2ft barrel, we're talking 1.38 milliseconds of normalized dwell time. So going from zero to 2900fps in 1.38 milliseconds is 2,102,973 feet per second per second in acceleration. So let's look the other way... F = MA, so F=M1A1=M2A2, meaning the force pushing the bullet from the rifle (neglecting heat, friction, sound, vibration, blah blah), so M1 = 150/7000, A1 = 2,102,973, and M2 = 9lbs, so A2 then is 5,007 feet per second per second... A bit less impressive. The resultant velocity due to acceleration, starting with zero velocity, is V=AT, so the velocity of the rifle when the bullet leaves the bore is 6.9fps. It's displacement, X = 1/2AT^2, will be 0.0048ft, or 0.057 inches... Not much displacement, considering the bullet will have traveled 24" in the same time, but displacement nonetheless - and of course, that's considering the rifle free in space, without the inertial mass of the shooter's shoulder against the stock...
It's easy to say the rifle's motion is delayed, however, it is not. The acceleration is happening in both directions exactly in congress, however, one is accelerating FAR less than the other.