Shimming rear base

I think it's something like .010" is worth "about" 5 moa? I'm not sure though. I had to shim my Ruger base .020" on the 416Wby before I bought the Burris rings and .040" inserts.

Each .001" tapor is worth 1 MOA (or 1" rounding it off) at one hundred yards.
Most rifles at Willimsport need around 20" to 24" of elevation at 100 yards to be on target at 1000 yards.
With his 30/06 or a 308 Win, one would need around 30" of lift (depending on the load) at 100 yards. He should start with a .025" or .030" to get on target at 1000 yards and be able to utilize his scope center setting and for further shots if he would want to.
Darryl Cassel
One thing to keep in mind is that it's better to shim a one piece base. That way the whole base is at an angle and the rings are still in alignment with eachother. When you have 2 piece bases and shim just the rear, now it is elevated above the front shim and your rings will not be aligned properly. I always lap in my rings but that only accounts for a few thousandths, putting in your shim will throw you off whatever thickness you put under there and it's too much to lap in. Now when you tighten your rings around the scope you are putting uneven tension on the scope body which can lead to trouble. I feel the one piece is the best way to go for shimming.
Having said that, I have indeed shimmed the rear base of a 2 piece system myself and it seemed to work ok. In theory though, it's not a good idea. At least that's my opinion.
remember guys that the amount of shim needed is directly proportional to the lenght of the base itself. Not all bases are the same length. Lets say your base is 7.05" front to back, now add a shim of .020". This would mean that for every 7.05" the .020" elevation is gained. Since we typicaly use 100yds as a zero and there are 3600" in 100yds, it is easy to figure the true amount of shim needed. (3600/ length of base)x(thickness of the shim) ex. (3600/7.05 = 510.63829) multiply by .020" = 10.212765". So if your base is 7.05" long and you use .020" shim you would get 10.25" elevation. Remember, the shorter the base the less shim you will need. A 6" base would give 12" elevation with .020" shim.
Txhunter, that's about all I got with a .020" shim. I think I did a little figuring like that back then to get that .020" also. Depending on where the crosshair is situated in the tube (I don't know) and where it is placed relative to the rear base being shimmed would be the exact length to figure it on , true? Or is it the occular to the crosshair distance that is correct?

I agree the two piece base is puttin the tweek on your tube as it did mine. The Leopolt 3.5-10x40 has NO evidence of the misalinement though, it looked and felt new when I took it off to put it in the Burris rings with inserts.

It shocked me too because I had the screws as tight as I could possibly get them. Almost had to drill them out.

The onepiece is the way to go or the Burris set on a two piece.
Well I'm not quite sure what you mean by the placement of crosshairs in the tube. It really shouldn't matter where they are. The placement of the scope in the rings more toward the front or the back has no effect, the distance of the crosshairs from any point should remain constant. Until you make adjustments, but then it would still remain constant from it's new position. the plane on which the crosshairs sits will be canted forward when shims are used but the position of the plane inside the scope will remain the same.

it's late and I'm tired, not sure if i'm making any sence so I'm gone.

On another note.

With the Unertl or B&L external adjustment scopes, the length of the base was/is a direct reflection to the click adjustment valuations.
A 7.25" base spacing gave 1/4 Min clicks. At 12" it gave 1/8 Min clicks evaluations. The base spacing is a direct reflection of the click valuations in these scopes.

Still use them at 1/4 min spacing.

I have also found that with a .020" tapored base or the Burrus inserts that are .020" will give me 20" height difference at 100 yards or 80 1/4 Min extra usable clicks.
.001" rear height addition has always worked out to 1" at 100 yards for me.


[ 06-20-2002: Message edited by: Darryl Cassel ]
The elevation figure for 3"ring spacing makes some sense. Say, .020 for every 3" between the scope and the target. Wouldnt the length of the scope play a part in that also. Near as I can figure, raising the rear base .020 wouldnt have as great an effect on a scope that sticks out 4" past the rings as one that sticks out 8". Doesthis make sense to you guys or are the 70's catching up with me?

It's not the length of the scope (a Unertl is a good example) it's the length of the base and where the rings attach to that base that holds the scope, that is the important factor.

Back to the original question, if you can get 120 1/4 Min clicks (30MOA) from your scope, your 30/06 should make the 1000 yard trip.
.025" to .030" tapored one piece base (higher in the back) will give you plenty of clicks to use in most scopes.

Darryl Cassel
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