Gunsmith question

[mattg1500]

I have a 12ga Mossberg 500 slug gun that was my first shotgun, 16yrs ago and I have killed many deer with it but every year I had issues. I put it in the safe years ago and now i took it back out. It shot lightfields great but after about 5yrs it started digging potatoes. It will not send a slug 25yds. I have tried numerous scopes and brought it to a gunsmith who said everything is fine and in line. I will max out elevation on a scope at 50yds and it will hit dirt 10' before the target. Just bought a 12ga in chamber bore sight and threw a red dot on the gun. I have the sight maxxed out and the dot from boresight is still a few inches below. Any Ideas? 

[delvalhunter]

Can you share a pic of the setup?  Something is positively not mounted correctly. 

[username]

Does the SG have iron sights? 

[Zipper]

1 hour ago, delvalhunter said:

Can you share a pic of the setup?  Something is positively not mounted correctly. 

 

[bucky]

If everything is mounted correctly and your rail is not bent, you may need to add a shim to the rear or front scope ring. I had to do it with one of my guns and it shoots perfect

Edited November 25, 2019 by bucky

[Bucndoe]

Shims cut from a beer can. It works

[MikeStaten]

39 minutes ago, bucky said:

If everything is mounted correctly and your rail is not bent, you may need to add a shim to the rear or front scope ring. I had to do it with one of my guns and it shoots perfect

Not sure what would cause the change but assuming the barrel is attached securely here are a few things you can trtry-

- Burris Signature Rings using their Pos Align plastic ring inserts.

- If you are using a one-piece scope mount like a rail, you can shim the rear of the mount up.  They sell shim material or you can cut a small piece of soda can or other really thin metal and shim with that.  Shouldn't be hard to get thickness right as you have a boresighter.  Once you have height correct, you can bed the whole base.  

If you are using two pieces bases, do not shim the rear base as you will then have the front and rear bases on different planes and all the torque will be placed on the scope itself, essentially slighty bending the scope. 

- Shop around and see if someone makes a 20 MOA scope rail for you gun.  EGW makes them for other guns but not sure if they make one for a shotgun or if they could make a custom one for you.   Here's an example- http://www.egwguns.com/scope-mounts-rings/hd-mossberg-mvp-5.56-scope-mount-20-moa-ambidextrous/

[KBfishing]

Rifled barrel or smooth?

[mattg1500]

Setup is rifled barrel, Model 500A. Originally had a B square mount on it and it was 0 @ 100 no issues. Recoil use to loosen the B square even with blue loctite so i went to weaver mounts using the drilled and tapped holes in the receiver. Like I said the gun was 0 @ 100 then also. The issue I always thought was recoil. Every year I would shoot the gun and have to make adjustments to my Leupold. I even sent the scope back and they had to replace internals on it. Eventually the scope ran out of adjustment after almost 5 seasons and its been stuck where it is now. 

I have tried other scopes also and got the same results. I tried 1pc and 2pc bases. I have tried numerous slugs, even though it zeroed with lightfields fine originally.

[mattg1500]

9 hours ago, username said:

Does the SG have iron sights? 

It does but I removed the rear to clear the scope

[mattg1500]

My whole thing is that the gun WAS 0 @ 100 with NO issues 1st year. Then every year after that it declined until it is where it is now.

[delvalhunter]

I’ll assume you’ve never dropped the gun?  Do you use this gun for anything else/remove the barrel and scope to hunt turkey or game with a traditional shotgun barrel?  Try removing the scope altogether and replace the rear iron sight. Shoot it this way and see how it works.  This would rule out any barrel issue and you can focus any repair on the receiver. I’d guess there’s something going on where your bases mount to your receiver. 

[delvalhunter]

And when you’re bench shooting this gun, how is it being held/secured?  Is it resting on sand bags?  On a sled?  Is the gun strapped down on your shooting system?

[mattg1500]

22 minutes ago, delvalhunter said:

And when you’re bench shooting this gun, how is it being held/secured?  Is it resting on sand bags?  On a sled?  Is the gun strapped down on your shooting system?

I always shoot off bags. Gun is dedicated slug gun barrel does not get removed. Chamber is where Im thinking the issue is. The shell has to be sitting offset or something. Maybe an issue where chamber is mated to the barrel, but theres no obvious wear,marks etc. And the fact that the laser boresight is a brass 12ga shell and thats way low also Im stumped.

[delvalhunter]

Your barrel may be “straight” but it may not be mated to the receiver square. The shell sits within the removable barrel once loaded and id think something is bent from where the barrel meets the receiver to where it extends into the receiver. The receiver is pointing “straight” but the barrel is not.