You get a 1 MOA change per .001" Shim?

[CaptainMal]

Recently put a Nightforce BR on a BAT target rifle I acquired. That action had a flat picatinny base on when I got it. Wanted to use it at 1,000 yards so selected a set of Burris 30MM rings with the offset plastic inserts. You put a -10MOA on in the bottom of the front ring and a +10 MOA in the bottom of the rear to get 20 MOA.

Shot it yesterday at 1,000 and did have enough elevation to go 1,000 yards.

Barely.

There was less than 1/2 MOA adjustment left in the scope.

After the shoot I took the rings apart. Measured some plastic container thickness and found one that measured .013". Cut the plastic to fit and slipped it under the bottom of the +10 MOA insert in the bottom of the rear ring. Did not notice I had left a little bit hanging out that you can see in this picture.


Did set up a good bore sighter with moa grid lines before starting and never touched it. To me it was "happenstance" to see about a 13 MOA change in elevation. That coincided with the .013" thickness of the plastic shim I cut and fitted.

It worked. I have full use of the app. 40 MOA adjustment built into the scope as it looks like the 100 yard zero is a few MOA above the bottom of the adjustment. Now I can easily go up 25 MOA to a 1,000 yard zero and have a good 10 MOA left to go up even more.

Both an observation and question. It worked out that .013" of shim for this particular scope, height over bore, ring spacing etc. came out to app. 1 MOA per .001" shim. Is that some kind of standard that might repeat for other applications?

 

[jds holler]

That's pretty cool, but I think that figure would change depending upon the distance between your rings. jd

 

[Geno C]

It will absolutely change with the distance between the rings. Farther apart and it makes a smaller change, closer it will make a larger change

 

[Lapua40X]

I ain't no ingineer but if .001 = about .05 degrees I'd say you gained 3 moa. How much you compressed the shim material in the rings will also make a difference IMO. Just sayin' .....................

 

[BoydAllen]

The ring insert offsets are in thousandths not MOA. As others have said the change per thousandth varies with ring spacing. Figuring this out is a simple proportion problem.

X (change at 100 yd.)over 3,600 (100 yd. converted to inches) equals .010 ring offset (for back ring, thicker insert on bottom) over ring spacing (front to front or back to back equal center to center) Cross multiply and divide both sides by 3.5 (arbitrarily chosen ring spacing. X (change at 100 yards) equals 10.29". If you use another set in the front installed thick on the bottom the change will be doubled. If the ring spacing is less the change will be proportionately greater, if more, less.

 

[CaptainMal]

Agree and understand. Did state:

"It worked out that .013" of shim for this particular scope, height over bore, ring spacing etc. came out to app. 1 MOA per .001" shim."

Boyd got me checking. Those ring inserts are marked 10+, 0 and 10-. I measured. There is about a .010" difference in thickness between each piece. Meaning that thickest to thinnest is about .020". As you buy that set from Burris they advertise them as a 20 moa offset.

Just curious if that was happenstance or there would be some rule. What I did from the standpoint of spacing is probably pretty typical. First time I ever quantified the change into concrete numbers. Peaked my curiosity.

 

[MrMajestic]

1 MOA=288 PPM so take your ring spacing times, in inches if you're working in imperial units, times .000288". So 10" would require .00288"(round to .003") and of 5" spacing would require half of that.

 

[Alex Wheeler]

20 moa is about .0055" per inch of spacing. multiple your ring spacing by .0055" and add that shim under the rear of the scope= 20 moa.

 

[CaptainMal]

5" ring spacing. Been using the calculator ( too much rain for fishing today) and came to the conclusion ...

I'm too stupid to figure it out what the .0275 means. I only put a .013" shim in and, from the looks of the marks in the boresighter,
I got 13 MOA. Will test that on the range this weekend.

 

[expiper]

OP,,,,to answer your ???....YES....
Roger

 

[noload]

I'm more intrigued why you only have 1/2 moa left @1000yds with the original setup. Though a caliber /chamber size was not mentioned.

 

[snert]

All I can say is clearly you guys did well in math while I studied the liberal arts and girls! Thank goodness when I need something made or an answer guys like you all have the answers! i tried to follow, but I was lost at moving the distances on the rings!

snert

 

[BoydAllen]

To figure the change at 100 yards, multiply 3,600 times the total ring offset (.010 down in front and .010 up in back equals .020) and divide that by the center to center ring spacing.
Example: 3,600 times .020 equals 72 . Divide that by the ring spacing and you get:
Ring spacing 3" gives you 24" at 100 yards
Ring spacing 3.5" gives you 20.1 " at 100 yards
Ring spacing 4" gives you 18" at 100 yards
Ring spacing 4.5" gives you 16" at 100 yards
To convert inches to MOA divide by 1.047

 

[Ledd Slinger]

20 moa is about .0055" per inch of spacing. multiple your ring spacing by .0055" and add that shim under the rear of the scope= 20 moa.

Use this equation to determine your shim size for elevation adjustment. zfastmalibu taught me this a while back and I have used his equation a few times now with perfect results.

So if 20 MOA = a shim size of .0055" per inch of ring space (distance of rings on scope center to center)

You would take .0055" and divide it by 20 to get the shim size for 1 MOA which equates to .000275". Knowing that, if your rings are 4" apart, you would have to have a shim thickness of .0011" (.000275 x 4) to achieve a compensation of 1 MOA. Now being that 1 MOA is a shim size of .0011" with rings 4" apart, you would have to have a shim thickness of .011" to achieve a compensation of 10 MOA.

Make sure you measure your ring distance carefully and convert the fractional measurement to a decimal point before punching it into a calculator. ie: a measurement of 3 and 3/8" would equate to 3.375". Of course you could use calipers to measure and get your decimal measurement right off the bat. Measurements from front side of ring to front side of ring or rear side to rear side are the same as measuring center to center and is a more accurate method to determine that distance.

It is also a good practice to bed your rings with something like JB Weld because once you shim your scope, it will be setting at an angle in your rings. This lessens the amount of ring contact to the scope and can cause stress on the scope body that may create unreliable windage and elevation adjustments. Bedding the scope in the rings allows you to re-achieve full scope contact with little to no stress.

If you use this equation to a "T" and carefully and methodically calculate what you want while triple checking your work, you can achieve any desired amount of elevation adjustment you desire. With this new skill I learned from zfastmalibu, purchasing compensating rings and bases have become a thing of the past for me.

 

[CaptainMal]

Boyd, Ledd and NoLoad -

Rifle is a custom BAT chambered in 6 Dasher. If it zeroed at the bottom of the adjustment in the Nightforce BR, there would be plenty of adjustment to go 1,000. It did not and zeroed in the middle of the adjustment range, hence the problem.

You do not touch the scope with the shim using the Burris adjustable ring inserts. They kind of swivel inside the metal ring mounts to compensate for angular changes. It's a good system and literally never marks the scope tubes.

Boyd and Ledd - I am going to copy your last contributions, print them out and add them to my main loading manual. Good stuff. Actually, the stuff of a "sticky" for others that will bump into this.

 

[Nomad47]

Posted by Captain Mal = "Rifle is a custom BAT chambered in 6 Dasher. If it zeroed at the bottom of the adjustment in the Nightforce BR, there would be plenty of adjustment to go 1,000. It did not and zeroed in the middle of the adjustment range, hence the problem."

Sounds like you might have a similar problem to one I had. One of my sets of 10 MOA inserts was marked backwards. I had installed two 10 MOA inserts expecting 20 MOA of scope change but instead got virtually no change, since one insert was cancelling out the other one. Really had me scratching my head until I figured it out.

 

[Outrider27]

To make it simple (if you have the option ), 0.001" will equal 1 MOA with a center to center ring spacing (front to front or back to back is easier to measure accurately and is the same, as mentioned in Ledd Slinger's post above) of 3.6" using signature ring inserts, with or without added shims. If you can't hit that spacing, you're back to doing the arithmetic to get the shim thickness, as laid out by BoydAllen and Zfastmalibu.

 

[Tempest]

Yep. There is a Trig equation to calculate this. I've done it numerous times to know that it's very close to .001" = 1MOA if the rings are spaced as above.
It's close enough to not bother with the calculation to get you on target. If I can find the calculation in my one of my folders I will post it up.

 

[CaptainMal]

Right. My supposition turned out correct. We should call it "Boyd's Corollary" or maybe "Alex's Law".

1 MOA works out to about .001" of scope shim with about a 5" ring spacing. Creative people can re-write that to look more authoritative.