Altering scope mountings to accomodate more height adjustment

[Againstthegrains]

I have a std Howa action with Warne 2 piece base set and rings on which I mounted a Swarovski z3 4-12x50.

The scope has one of those ballistic turrets, which allows you to make rapid adjustments to your point of aim at set clicks, from 100m zero to about 450m. basically a full circle of turns.

When I set up my adjustments for 200, 300, 400m, the adjustment turret got stuck just past 300m. i.e. it could not complete a full turn. I read the manual, and figured that there are not enough clicks on the scope, and I would have to raise the height of the barrel relative to the scope on zero, to be able to get enough clicks.

I guess the expensive solution is to just buy new bases with a 20MOA adjustment.(more money )

There are 2 other options:

1) put a  shim under the rear base to raise it about 0.5mm
2) grind/mill/sand 0.5mm off  the front base with a rounded surface

Any idea's advice from those who have done it?

I'm also worried that a 0.5mm adjustment might change the position of the bases relative to each other so that the scope rings will require some lapping. I'm not sure that this is a real concern?

[Tripodmvr]

I think that Burris have an insert that fits inside the rings. With that you can raise the scope at the back to get more vertical travel.

[janfred]

Just be carefull about raising too much; specially with wide scope rings. Enough scopes have been damaged like that.

[223]

The Howa rifles occasionally have rear action rings that seem a little to low for normal mounting (no extra cant).

The usual solution is to shim the rear base.  0.5mm sounds about right.  I found some 7 thou (0.007" = 0.18mm) brass shim stock at a local car parts shop.  You would need about 3 layers of that.  The advantage of thin shim stock is that it readily takes the shape of the action top.  A 0.5mm thick shim might not bend so well.  This method may require some ring lapping to get perfect alignment.

Another option would be to place a small piece of 0,5mm material between the 2 screw holes, and "cast" a shim of epoxy around it.  Remember to use release agent on the action.   If you do this with the ring and base already attached to the scope, or an alignment rod, the angle of the base will automatically be correct.  You then only need to lap the front ring.

The angled (canted) bases are normally of the 1-piece type, which limit your access to the top of the mag for loading.  They usually work better with detachable mags.

Lynx also make rings with inserts. The inserts are available with various offsets.  You can even offset the rear up and the front down, to maximize your cant. They also help with alignment, so no lapping is required.  So far I have only seen them in Stud-type mounts.  Not sure if they can be had for weaver/picatinny style bases.

[Againstthegrains]

Just a follow up:

So I worked out I needed to take off exactly 0.4mm off the front.

I figured, to keep the mounting surface of the front and rear scope mounts parallel to the scope  so that the rings don't buckle the scope tube , I need to angle them slightly, i.e. take a bit less 0.1mm of the back of the front mount and a touch 0.1mm off the front of the rear mount.(as per JanFreds advice)

I then took an accurate set of digital verniers and mapped out the thickness of the metal bases, as they are not flat, and profiled to the rifle. I can then use these measurements to map the sanding process.

With different grades of sand paper wrapped over the action, and rubbing the underside of the bases on the abrasive  to maintain the profile, I was able to trim the desired amount of metal off. I used slightly more pressure on the front than back to get the angle I wanted.

They refitted like a glove, and worked like a bomb. I'm not sure if the 3.5 hours of demounting measuring, sanding and remounting was worth the trouble, but it is a cheaper option if you have the time, and don't want a 20MOA picitinny rail limiting the size of the loading bay.

[BBCT]

I had a similar problem on an air rifle scope. We put shims between the scope and the rings.

[big5ifty]

... and don't want a 20MOA picitinny rail limiting the size of the loading bay.

There are 2 solutions with the rail.

1. The port facing side of the rail can be milled away, so that the rail keeps only half or one third of it's width over the port.

2. You could cut the middle of the rail completely out, and have the front and back as separate mounts.

In the case of both options, it's advisable to bed the rail before cutting.