Removed barrel and had to re-sight, why?

[OH25shooter]

My 870 is scoped (mounted on the receiver) for deer hunting. After getting stuck out in the woods during a pouring rain I completely dried and cleaned the gun. However, I found after removing and replacing the barrel, that threw my zero off. Had to re-sight. Anyone know why?

 

[Poodleshooter]

The barrel/receiver tolerances aren't tight enough to ensure that the barrel will point in the same direction when you reattach it. Ever wondered why companies like Mossberg and Remington make those "cantilever" scope mounts in which the scope mounts to a rail welded to the barrel? That's why. If your scope is on the barrel, it won't lose zero when you remove it.

 

[OH25shooter]

That was my best guess, Poodleshooter. I'll clean it the old fashion way; with the barrel attached.

 

[Dave McCracken]

I deep clean after the season, rezero and just clean from the muzzle.

 

[Al Thompson]

Despite having to re-zero, you did the right thing with the detail cleaning. I've had the unpleasant experiance of cleaning rust from a used 870 magazine tube - better to nip it in the bud.

 

[zahc]

Another cool thing about the canteliver mounts is that you can keep your scope mounted to the barrel in the off season and switch back and forth between deer and bunnies with a simple barrel swap and not lose your zero.