Assuming we are talking about 2Fort5 or similar maps, Sentry gun killing
depends on the positioning.  In general, Mephiston's method is going to
be used more often than Stipple's, simply because Stipple's method
requires a LOT of room.  To clear up what these guys are talking about,
Mephiston recommends finding a space where you can hide, relatively
close to the sentry, and simply move back and forth, shooting at the gun
in short bursts.  This prevents the gun from getting a sure fire lock on
you.  The problem with this is, if the gun is a level 3, or the distance
between you and the gun is to short, you stand a good chance of getting
killed.  His method is best for sentries around corners, in passage
ways, or in the same horizontal plane as yourself.  Stipple's method
works much better when you can get a clear sight on the gun, and the gun
is at a higher altitude.  Specifically, when you're entering the Ramp
Room via the right room full of shadows in 2Fort, and the gun is perched
on the ledge approaching the elevator.  From this angle, you can usually
creep up far enough to get a view of the gun's base without getting in
the gun's sights.  From here, you can let loose without fear of the gun
hitting you.  The downside is, obviously, this is a rare situation.

Here's my general guide to dealing with guns:
A) Try to hit it with a long distance burst from the nailgun without
getting in range.  Requires a lot of distance, and no enemies.
B) Concuss it and try to run by.  Yes, concuss grens do help, and may
make the trip survivable.  Remember, in tight games, you're just trying
to move the flag, even if it's just an inch.
C) Strafe it, using cover.  In a heavy populated base, you can't do this
without someone jumping to attack you.  I play with some engineers that
know how to defend their guns well, and getting lured away from it for
some half dead scout is not likely.  Remember, they're looking to lure
you INTO the gun's range.
D) Call in reserves and wait in the wings.
E) Get it using another class.  I only do this after dying three or four
times at the gun's hands.  By that time, the offense is rocket jumping,
and no one has the time to bail you out.  I recommend the pyro's napalm
grenades.  Keep in mind, they explode on three, and no matter where the
grenade goes after that, the explosion stays wherever the gren was at
"3."  I am fairly good at tossing a green, having it start exploding
next to the gun, and watching the glowing grenade roll across the room.
Doesn't matter, the explosion is next to the gun.

The last possible tactic is extremely unreliable.  Oz has a demo of me
doing this on a gun at the bottom of an elevator shaft.  I killed the
engineer, jumped down, and proceeded to move back and forth, strafing
the gun and ducking for cover before it could track me.  On my second
pass, the gun got enough of a lock to get off a rocket.  The gun had,
however, tracked me behind a wall.  The gun shot the rocket into the
wall next to it, blowing itself up.  This is rare, and requires
impeccable timing.  I just got lucky.

Octagon[qVa]