Need help shimming scope bases

[Oliver Whipple]

Just got back from the range with my new rig. It's a 300 Winmag set up as a light weight walking rig with long range capabilities. Optics are a Leupold 3.5-10 VXIII w/turrets in Talley LW's. Problem is this: sighted in for a 200 yard zero, I've only got 4/5's of a complete turn in elevation left...so I know I've got to shim the rear base. I've never done it before, though have some inkling how to do it. Questions are this:

1) I've got two complete turns of down left in the turret. Approx what thickness do I need to shim the rear base to recover those turns?

2) Is it necessary to lap the rings after shimming? Wondering if the resultant offset will induce unkind stresses into the scope tube?

Thanks in advance for help from those that have BT/DT.

 

[zuba]

I'd buy a base with some moa built in, I got one from egw and really like it. in my opinion shims=bad

 

[CS T]

I do not recommend shimming it can add problems. I also recommend get a base that has MOA.

Mike @ CSGW

 

[Oliver Whipple]

Forgive my ignorance, but where is the best place to get a lightweight set of rings w/moa built in? Also, since I'm losing approx 24 minutes in revolutions, I'm guessing that I'll be looking for a 20 moa base? Do they do this w/two piece bases or is this one piece only? This is all new to me, so thanks for the help.

 

[Buffalobob]

From one of my old posts but I do not remember if it was a short action of long action being discussed:

You will need 0.00138 inches of shim thickness for each MOA .

So for 20 MOA of cant you will need 0.028 inches of shim.

Soda cans are thinner than beer cans. Cheap beer like Keystone and Bud has thinner cans than better tasting beer. Think about it. Kinda makes sense doesn't it.

You will have to cut and measure with your calipers to get the right number. To make the shim you can just use scissors. The aluminum will cut just fine. Cut out small "H" looking things that fit in between the screws and don't fall out while you are trying to tighten the screws down. If you put too much shim in you will need longer screws. Be careful here and don't tear out the thread in your action by trying to tighten in too short screws without good thread engagement. If the screws are too long they will bind the action. (Does it sound like I have been down this road myself?)

Sometimes it is good to glue everything in but you really need to be sure that is what you want before you do it.

If you are using a one piece base you will be OK. If you are using a two piece base you will need to lap the rings.

If you wish you can get Burris Signature rings with inserts and use them to get 20 MOA without changing bases or lapping.

 

[Oliver Whipple]

BB,
Thanks. 'This is exactly the option I was looking for. I had heard of them but wasn't aware you could get 20moa with them. When using these offset inserts, do you have to use offsets in both rings, or can you use the .020 in one and the regular insert in the other?

 

[Buffalobob]

You may have to buy the insert kit separate from the rings. Check carefully what comes with the rings in terms of inserts. As we learned earlier in life when it comes to skinning a goat there are a number of ways to do it. My preference is +10 and -10 but my memory fails me on whether the kits come with those inserts. Lots of times I have old partially used kits around and just start grabbing what I have on hand.

 

[J E Custom]

BB,
Thanks. 'This is exactly the option I was looking for. I had heard of them but wasn't aware you could get 20moa with them. When using these offset inserts, do you have to use offsets in both rings, or can you use the .020 in one and the regular insert in the other?

Not to butt in but you have to offset both rings 10 MOA opposite of each other for a total of 20
MOA,

You can also get different offsets in the burris rings if you don't want 20 MOA.

With the offset bases available nowadays you have plenty of options without shimming.

J E CUSTOM

 

[Buffalobob]

You type too slow.

 

[Oliver Whipple]

Thanks again Bob. The +10 and -10 makes sense. I've placed an order with Midway and should be GTG by next weekend.

 

[J E Custom]

You type too slow.

Tell me about it !!

J E CUSTOM

 

[noone]

The shims I got from Leupold were .010 inch thick. As I have one piece bases, I didn't need to re-lap the rings as both changed angle together. However, my guess is that because the back ring is changing angle while the front ring isn't that there would be some mis-alignment of the rings.

But, as I haven't ever used a two piece base set up, I don't know if the change in angle is enough to bugger up the scope. I imagine at the very least, it will show marks on the scope from where the top front and rear lower bite into the scope a bit harder.

 

[ss7mm]

Sometimes a picture is worth a thousand words. You can see how to accomplish things using shims or custom bases. You can also accomplish the same thing with the Burris ring inserts. Just thought I'd put these up in case anyone was interested. Please note that the picture examples are very exagerated to make it easier to see.

 

[noone]

For SS7

Absolutely outstanding graphics and explanation. I have rifles with the MOA machined into the base, and have used the shim/bed method. I found out about the screw issue the hard way, but was fortunate to be able to locate proper screws. The shim wasn't high enough to throw off the base/receiver holes-I didn't even have difficulty getting the threads to catch.

Although both methods worked out fine for me, I think that having the base with the angle machined into it is the best option, and certainly the least work.

But for those folks lacking access to a base with the angle machined into it, doing the shim/bed (with a one piece base) might be the only option until the proper base can be located. I sure wouldn't recommend shimming a two piece base setup (unless you don't care what happens to your scope, and performance isn't an issue).