Shimming a Scope - I get Confused !

[DoctorBill]

I have a sporterized 6.5mm Arisaka converted to .257 Roberts which I recently bought used.
http://castboolits.gunloads.com/showpost.php?p=1100722&postcount=40
http://www.mynetimages.com/e4b0a88438.jpg

I bought some new Winchester .257 Roberts Brass and reloaded them
with the following for reduced kick loads:

** 138 grain 6.5mm Cruise Missile (milled down to 138 gr)
http://castboolits.gunloads.com/showthread.php?t=103567
http://www.mynetimages.com/7e9716be7f.jpg

** Filled with 12 gr of TRAIL BOSS (80%) of full.

It shoots so very nicely - I love this rifle !
The cut down Cruise Missile Bullets worked great - no GC was used.

However - a problem arose that I need to attend to.....

It shoots high at 80 meters by about 8 - 10 inches.

My scope is at it's vertical adjustment limits - won't adjust any further - needs to be shimmed.

I know this is a DUMB QUESTION....but do I shim the front or the back of the scope mount ?

And by how much ? I want to shim it to where the adjustment in in the middle
of the range of adjustment.

I intend to use pop can aluminum which mics out to 0.004 inch thick.

Yes I know - I am a dunderhead about scope adjustment....I should know this stuff !
I can never get it straight - raise the back or the front of a scope...?

I believe that I should raise the front of the scope - but then start to doubt myself.
I don't want to go thru all that hassle and find out I did the opposite of what should
have been done - at the range....!

Shouldn't I raise the scope to where the bullet is hitting ?

Can someone help an old twit ! ?

DoctorBill

[btroj]

I always just remember FORS. Front opposite, rear same. Works for me.

I have a Marlin 45-70 that had the same problem. I decided that rather than shim the bases I went to the Burris Signature rings with the plastic inserts. You can get a set of offbeat inserts that allow you to basically get the same effect as shimming the scope but it is much easier. Works like a charm too.

Brad

[Johnch]

Yes you would want to shim the bottom of the front ring

I would try 2 layers of pop can

I saw a graph someplace ......Maybe Accurate reloaders forum as to how much you need to shim for a given distance

I do it with the guess and by golly method

John

[DoctorBill]

I'll try the Pop Can sheets - three of them (I always push my luck).

New rings cost money - which I ain't got much of.

Gotta save my money for this Fall's BIG Gun Show....

May be there is a treasure with my name on it just sitting around waiting
for me to buy it !

I will let y'all know how the shims worked when I can go shooting again.
.....i.e. when it quits freaking raining around these parts.

http://www.mynetimages.com/5f810985e9.jpg

DoctorBill

[Uncle R.]

To remember which direction to shim, think of the rifle as being fixed solidly in a machine rest. Right now the rig shoots ABOVE where the crosshairs point - and you want to make those crosshairs align with the bullet hole in the target. To do that you must make the scope aim higher - so you shim the front end of the scope UP.
<
As for how much to shim - it depends on the distance between the rings. Once you measure the ring spacing the math is fairly straightforward. Assuming that the rings are 6" apart, you'd have to shim the front ring about .038" (38 thousandths) to move those crosshairs 20 inches at 80 meters. You want to move more than enough to allow for the 10" difference you have now at 80 meters - because you want to have some adjustment left in the scope too.
<
If the rings are closer than 6" apart you'll need slightly less shim - if they're farther apart you'll need slightly more.
<
Good luck - it looks like a fun project.
Uncle R.

[Rick459]

DoctorBill,
for more elevation you would shim the rear of the scope mount. Assuming you have about 3 1/2" between the rings, placing a .001 shim under the rear will raise your impact point by 1 MOA. IOW if at 100yd, and your adjustments centered, your impact point is 6" low, than place a .006 shim under the rear of the scope (either under the base itself, or in the lower half of the ring). long range scope mounts say like a 20 MOA are thicker at the rear of the mount . so to raise your elevation you would shim the rear. HTH
http://www.abousainc.com/SightIn.htm
Rick

[DoctorBill]

Rick459 - I want to lower the impact point by about 8 inches.

So, last night, I cut out three pop can metal shims and placed them under
the front screw-in mount.

When the rains go away, I'll see what that got me, at the range.

Back to the rifle and the cast bullet.

I have shot two of my five new Arisakas bought over this Winter Season.

I'm anxious to shoot all of them.

I've shot this .257 Roberts converted 6.5mm Arisaka with 12 gr of Trail Boss
pushing a cut back 138 gr Lee Cruise Missile (see link in post #1 previously).
6.5mm Swed - did not use a gas cap cause none will fit this hybrid bullet.

It shot really nicely at 80 meters.
Almost no "Kick" and a nice pattern (but high due to the scope).

Before that, I shot my 7.7mm sporterized Arisaka converted to .300 Savage.
THAT rifle also shot very nicely with a good pattern and right on with the scope.
Shot a 185 gr gas capped LEE cast bullet ( C312-185-1R ) using 11 gr of
Trail Boss. Little recoil and a nice pattern - 4 inch circle at 100 meters which
for me is damned good !

This Trail Boss is a nice low recoil powder for target shooting.

I will obtain velocities with my Chrongraph when the weather permits.

I have a real 7.7mm Military Arisaka to shoot, a 7.7mm sporterized Arisaka
converted to 30-06, and a Type I, 6.5mm Military Arisaka to shoot yet.

I'd like to "Scout Mount" the two Military rifles - maybe adapt a Mosin-Nagant
scout mount Base to the Arisakas. I need scopes for bad eyes.

DoctorBill

[swheeler]

Shim under front base

[Cap'n Morgan]

DoctorBill.

Next time you're about to move a sight, shim a scope or whatever - and confusion rears its ugly head - try this simple method: Just imagine you overdo the adjustment with a ridiculous large amount and you can easily visualize the result.

Did that sound like pure nonsense? Okay... Picture your gun with its scope shimmed a full two inches under the rear mount!! Now imagine you aim at something through the scope and you'll immediately"see" the barrel is pointing way over the target... Ergo, shimming the rear mount will make a gun shoot higher! Same thing when moving the sights on a gun. It doesn't matter if it's the front or rear sight: Just imagine you move the sight a couple of inches instead of a few clicks, and you'll never be in doubt again.

[cajun shooter]

As has been stated, you put the shims under the front so that the scope has adjustment to go down. It is best to go to Lowes and buy a piece of brass stock. You will have a better material to work with than the beer can.

[Von Gruff]

I always just remember FORS. Front opposite, rear same. Works for me.

I have a Marlin 45-70 that had the same problem. I decided that rather than shim the bases I went to the Burris Signature rings with the plastic inserts. You can get a set of offbeat inserts that allow you to basically get the same effect as shimming the scope but it is much easier. Works like a charm too.

Brad

That is a good way to remember it Brad. Simple and effective.

Von Gruff.

[hiram]

Bill-

You can try reversing the rings first. Put the front ring in the rear and the rear ring to the front.

For shimming, don't make your shims more than 1" long. If they are too long, they will wrap up the side of the ring and make the ring less than 1" wide making it difficult to get your scope in.

[mroliver77]

With this much shim used will the rings bite into the scope body? The bottom rear of the rear mount will be touching more than the front of that mount. I would think you would need to "bed" the mounts to keep from torquing the snot outta the scope. .040" is quite a bit!

Am I correct here or am I worrying over nuttin? ;)
Jay

[DoctorBill]

I went whole-hog.

Since these bases are not exactly what should have been on there (* ), I shimmed up
the front (3 pop-can metal pieces) and I took off the back base and sanded it down
with emery paper wrapped on a piece of 1 inch pipe.

The latter took the legs on each side of the rear base down a bit.

Now I can zero in the scope at about 100 yards with my Laser Bore Sight on
a neighbor's white shed (in the evening).
http://www.sportsmansguide.com/net/cb/guide-gear-universal-laser-boresighter-kit.aspx?a=733405

This device got me within about 6 inches of center on several rifles on the
first round !

The scope is now adjusted to somewhere between the vertical limits.

It is raining like the proverbial cow pissing on a flat rock, so the shooting range
will have to wait a bit....

DoctorBill

* - Weaver #46 (for the front) and Weaver #54 (for the rear) for all Arisakas.
Both are the proper curvature and height.....

[looseprojectile]

There are three places where you can add or remove material. I have had success when removing some material off of the bottom of the rings or the top of the base. As much as .020" can be removed without harm.
You gotta go by feel. Weaver one piece bases and rings are a snap. I use oiled hard card stock for shims under bases and between the ring and base.
I always measure several times and proceed carefully.
Scopes are just like iron sights. Raise the front , it shoots lower. Raise the back it shoots higher.

Life is good