Machining and fitting taper shims for Schultz & Larsen Apel mounts. (Part one)

Remember me saying we have a Schultz & Larsen for Jagtfeltskydning?

Well the S&L needs a few minutes of angle on the ‘scope rails to bring the Kahles to something close to useful mechanical zero, by this I mean when you wind the elevation drum down it bottoms out at around the chosen zero for the rifle. OK, why should I want this? Well a ‘known’ mechanical zero helps to prevent the shooter becoming confused in moments of stress, the last thing I need is to get lost on the number of turns the drum has been wound, and have to wind it back to the very bottom and then forward say a turn and a bit to get to my chosen zero. With me so far? If not humour me here as it is just the way I prefer to have all my rifles set up and it is so much easier to wind the drum down, feel resistance as it bottoms out and immediately know I am at 100m.

The ‘normal’ way to set a mechanical zero is to shim the rear mount which places undue stress on the tube, or leave it alone and just have a zero somewhere in the middle of the elevation travel. Neither are acceptable so I am off to machine some taper inserts.

I will take pictures along the way. Usually people only publish successes with such things but in the interest of science I thought I would publish the whole thing.

I know how much elevation I need on the rear mount, it is 1.31mm, this was determined by some math a while ago and got me to exactly where I wanted to go so it is important to replicate this rear elevation shim.

The plan is to machine a length of aluminium at the correct taper and for the same distance as the rings are set apart (130mm) then drill for the rings, cut a piece off each end and fit the shims under the rings, so in theory I have the correct taper and the ‘scope is held in a stress free method.

After this I can bed the ‘scope to the bottom section of the rings to complete the job.

Whilst I am at I will be swapping out the bottom mounts as the existing ones shift under recoil and I have a set of Apel mounts with a recoil lug built in.

You can see the difference here between the front mount with the recoil lug (right) which engages into a hole in the front mount.

First thing is find a suitable piece of aluminium for the rail and machine it to the correct length and width for the job, 130mm x 21.5mm then turn it on it’s side and machine a relief on either side, I will need to cut the shims out at some point so the relief is the start of this.

Now I know the elevation needed is 1.35mm so I place the work piece on parallels and then add a 1.30mm slip at one end, as the slip is just under the edge of the work piece this is not strictly correct but I can at least tighten things up and then use a clock to confirm the difference over the 130mm.

Crikey! It is 1.35mm! Life is not always so kind in the workshop so I run a 20mm cutter across the top of the rail until it has a cut across the full length. In theory this now gives me a taper with a difference of 1.35mm and checking with a micrometer confirms I am correct. For those of you that work in imperial the taper is within a few tenths of a thou across the distance.

The taper is drilled at this stage for the mounts and two passes made across the work piece to give me two pads.

Here you can see the rings on the pads which are still too thick so some more cuts along the top will be needed. 

I am starting to wonder if it is really worth all the hassle doing this but  I am not shooting today and it is fun finding out.

Part two to follow…..