"Morgans" writes:

Whether that's "quick" or "a myth" depends on your point of view (and
on whether I did my estimations correctly).

What you failed to take into account, is that the engines keep
applying thrust, and will partially negate that issue

Indeed, according to Wikipedia, the engine throttles are firewalled
automatically when the gun fires. What matters is the net force,
which despite the incremental oomph of the engines, appears to be able
to cause considerable braking.

and that the gun is nearly always fired while the airplane is in a
rather steep descent (to get guns on target), so there is more force
to keep the airplane from slowing down. [...]

Some people confuse descent with acceleration. A mere constant-rate
descent (apprx. zero net force) does nothing to change the analysis.

- FChE