Gunsmithing Shim for classic scope mount

[BasraBoy]

First...apologies if this is in the wrong slot but it seems more of a smithing issue than optics.

I've got a pre-war Zeiss Zielvier attached to a Mauser 98k on a Short Side Rail mount (which is screwed into the receiver wall).

I have developed a load that works extremely well with the original rear sight ramp on the iron sights.

Problem comes when the scope is zeroed at 100m (first mark on the Zielvier elevation scale). The 100m scope zero is right at the upper end of the adjustment on the elevation turret.

As I understand it there is no way to adjust the scope internally to get more elevation so as I see it there are only two solutions:

1) Remove and refix the mount.

Am I right in saying that this is not possible if the receiver has already been drilled?

I imagine the difference in height necessary to get the elevation back would be tiny.

Can you advise?

2) Shim the mount

The mount (which is a single piece) attaches by sliding on to a dovetail (this is the part screwed to the sidewall of the receiver) and secured by screwing it down.

Tolerances allowing, I could try to insert a shim (plastic or metal) between the upper and lower halves of the dovetail.

Should I shim the whole length or just the front or rear part?

I am worried that only shiming the front or rear part could lead to the securing screw crossthreading.

Also I have no real idea how much thick the shim neads to be to raise the scope sufficiently. Am I right in saying that it it would be thousands of an inch rather than anything bigger.= (I've read an avereage of 3/4 MOA is equivalent to 0.001 at the mount?)

I really appreciate any help you guys can offer and am happy to answer any questions if the explantion I've tried to give is not clear.

Thanks in advance.

 

[Greg Langelius *]

Re: Shim for classic scope mount

I've got to believe that this was always a possibility with this mount, and that some means has already been devised to deal with it.

Moreover, there may already be such means already built into it, perhaps in the form of some rotational play provided by mounting holes that are slightly larger than the screws which secure them.

If so, loosening the screws, then twisting the mount to raise the rear and dip the front while retightening might add the needed slope.

Not having pictures makes this diagnosis harder. For example, I can't recall whether the receiver side is flat or cylindrical.

Greg

 

[former naval person]

Re: Shim for classic scope mount

Are there separate rings on the scope? By that I mean, are there front and rear rings? Or is it just a solid piece? Are the rings split horizontally? If front and rear rings, the bottom of the rear ring could be shimmed...I'd use strips of soft drink can aluminum...and build it up enough to gain sufficient elevation, then bed both the front and rear in an epoxy mix before torquing the securing ring screws down, avoiding scarring the scope body. If the rings are split vertically, then that option is off the table. Just a suggestion.

 

[BasraBoy]

Re: Shim for classic scope mount

Thanks for the replies guys.

Greg, this link shows the Mk3 SSR identical to that on my rifle:

http://www.k98k.com/ssr.html

From what I can see the received wall is cylindrical. The outside part of the mount is flat, the inside appears machined to match the curve of the receiver.

formernavalperson, the rings are split horizontally but are tight on the scope tube - so much so that I have considered hand lapping them.

The rear ring is dovetailed into the upper part of the mount and provides windage adjustment via two counter rotation screws.

The front ring is secured by a single screw (loosened to provide play when adjusting windage).

SO far I have tried the following:

1) Shimming the main dovetail where the upper part of the mount slides on to the protion that is screwed to the receiver. The tolerances on this are pretty fine and it is hard to even get a single sheet of paper between them. I don't think there is enough space to insert anything that would make any significant difference.

2) As elevation is now at the extreme upper end of it's adjustment I have inserted a shim on the bottom half of the rear scope ring . This is plastic card (about 20 thousandths).

I'm concerned that the thickness of the card may be way to much and I now won't be able to get low enough even with the elevation at minimum.

It'll be a while before I can test this...will post again when I've tried.

 

[Greg Langelius *]

Re: Shim for classic scope mount

Shimming inside rings can put odd stresses on a scope tube.

If there is play between the mounting screws and mounting holes, placing shim washers onto the screws between the round receiver and matching mount will space the two apart, allowing the rotation of which I spoke. If this provides enough slope, it can be taken down again, and reassembled with bedding compound and release agent, to reconfigure the mount to the new angular attitude.

If there is no such play, the mounting holes can be enlarged (slightly) to provide some.

I am very loathe to do these kinds of mods to a historic firearm. By confining these mods to areas that are less visible, my intent is to preserve at least the appearance, although not the reality, of originality.

Greg

 

[BasraBoy]

Re: Shim for classic scope mount

Greg, thanks for your further comments.

I understand what you mean about not wanting to mess with rifles such as these - that is one reason why I don't want to take of the receiver mount and start "filling and drilling"

I had a good look at the mount and it's attachment to the receiver wall. It is VERY solidly done - no play and no movement on the screws. I think they have been thread-locked.

As above, there was no room to place a shim on the dovetail between the two parts of the mount.

At the moment I have opted for a 20 thou plastic shim described above.

It has certainly added a significant amount more elevation when I boresighted it this morning.

I understand about stressing the tube, and this does concern me....stuffing bits of plastic (or brass, or aluminimum) between it and the mount would not be my first choice.

I'll shoot the rifle again next week and then, if I have found the elevation I need, take the gun to a smith and see if there is any other more elegant solution they could suggest.

Thanks again

Andy

 

[Greg Langelius *]

Re: Shim for classic scope mount

Sounds good. If the plastic shim is too thick, get an emory board and sand it thinner.

 

[BasraBoy]

Re: Shim for classic scope mount

I've just taken a couple of minutes to check it over - since this morning the focus knob (on top of the elevation ring) has pretty much frozen up.

Any movement I was able to make with it was twitching the reticle up and down.

With the rifle clamped I checked the elevation with reference to a fixed point. The elevation ring was also not returning to zero.

Guess this is "stress related",

Removed the scope again and everything frees up.

Think this is definitley a job for a smith.........