I have an 870 with a rifled slug barrel with cantilever scope mount, weaver steel rings and a Leupold VX-II 3-9x40 scope. I've been trying to get it zeroed in with no success. Once I get the slug hitting close (3 or 4 shots) then my next shots start hitting all over the place, sometimes 8 - 10 inches away. I rezero and get close and then the scope moves again. I've had this happen with several slugs. I have tightened everything on the scope and nothing is loose. I even filed some material off the forend to eliminate the forend/barrel contact I was having. Still, at some point zeroing in, my scope moves off by several inches and sometimes off the paper completely. Does anyone have any ideas or suggestions?

It has to be something in the scope or the mounts. It could be the scope itself, slug guns are rough on scopes.
-Jake

Your scope is good quality, but you never know, it could be broken.

I would first suspect the mounts/rings. Swap them out and start over. Clean/degrease the new mounts/rings and the screws, and use lok-tite (the kind that allows removal later).

I know a guy that had to really tighten his 870 barrel onto the receiver to get accuracy. He used some locking pliers to get it really tight. I'm not recommending that, but do make sure your barrel is tight.

Thanks for the replies. The scope is fine on anothe gun, 30-06. The mount is the fixed cantilever mount on the barrel (the screws are tight). My only option is to try new rings I suppose. Otherwise, I'll be looking for an 870 with the receiver drilled and tapped for a scope and remove the cantilever mount. I know if if I can't get it ironed out, this gun doesn't see the deer woods shooting like it is.

12 guage slug guns kick like a mule.if your rings are suspect i would get some basic good steel rings. something like weaver grand slams . if this is a new barrel make sure you clean it good. if you put 2 or 3 boxes downrange without cleaning you could have some nasty build up. i like jb bore paste to smooth out any burrs in the rifling. i had a mossberg 20 guage slug barrel that shot great after trying that stuff.it shot all over the place before that. finally make sure you shoot off a solid rest. because of the kick on a set up like that you can get lazy and develope a little flinch. i love to use the led sled rest without the weight bags to let it recoil. . it allows you to dial in the correct height and left-right and keeps everything steady as can be.also tapping into the receiver isint gonna make it more accurate since the barrel is removable. the cantelever is attached directly to the barrel making it a better choice. i thinks its rare to get a bad barrel but it does happen. i would send it back if your not happy.

I say scope or mount. Most likely scope.

Just cuz it worked on another rifle, does not mean that the scope didn't get damaged during the process of switching it. Or it was close to damaged already and the recoil knocked the crosshairs off their mounts.

Put the scope back on another rifle and see if you can get it to hold zero. If it does, then your mounting system is F'd. Rings don't tend to move once mounted. You usually can see any damage that would cause that kind of movement. The mount could easily seem tight, but with the recoil of the shotgun, move just enough. Don't take much movement in a mount to throw rounds that far off.

Try the scope first though. Let us know, I'm interested to see how this comes out. Good news is that it's a Leupold. Lifetime warrenty baby! Those guys are legendary for their excellent customer service.

When you are sighting in, how are you resting the gun? Im not an expert but Ived owned and sighted in a few slug guns .The biggest mistake I see is guys make, is resting the pump handle on the front bag, this is not a stable rest, especially on the more wiggley pumps like 870s and mossbergs, you will get flyers. Im also amazed at the amount of people I see at the range, that dont even use a rear bag when sighing in The best way Ive found to sightin a pump slug gun, is to rest the receiver(not the pump) on the front bag and pull down hard on the receiver w/ your left hand ,and back into your shoulder hard w/ your right hand(this is if you shoot righty) It might sound trivial, but I definatly found shooting form is critical when sjghting in pumps.

X2 on keeping the barrel nut tight, and mabye think about "pinning" (bolting) the barrel to the reciever if this is a dedicated slug gun, smiths supposedly charge around 50$

Ive also found that sabot slugs w/ thin walled sabots like lightfield, brennekee,hastings etc. give the least flyers. Good luck.

If your bases and your rings are tight, then you can only have one of three problems....

1. Frustration.

2. Bad rounds (probably not this one but you never know)

3. Bad scope.

Give it one more try and make sure it wasn't frustration. Check everything before you leave the house. Make sure your bases and rings are tight and you are bore sighted good. I had a problem recently with one of my rifles and it was frustration. Good luck and let us know.

I've never had much faith in those cantilever mounts with the scope's rear end hanging out in space like that. Sounds like your scope may be slipping in the rings due to recoil. A dab of rubber cement on the inside of the rings may stop that. Worth a try anyway.