How Much Shim Under The Scope Mounts?

[D.B. Cooper]

Background:

I'm replacing my riflescope on my Savage 243 Win with a Vortex Diamondback 4-12x40. Said scope has 70 MOA of elevation adjustment. I'm using a DNZ Game Reaper one-piece mount and rings.

Data:

There is a 1000 yard match coming up that I want to try out. My ballistics app says I need to come up 34 MOA from my 200-yard zero. That is almost half the total adjustment capacity in the scope. My previous scope used up almost all of its adjustment capacity, 39 out of 50 MOA, just to zero at 200 yards.

Possible Solution:

I'm thinking about adding a shim under the rear of the scope mount to give me some more elevation without using up the available adjustment on the scope. Yes, I know DNZ sells a "+20 MOA" mount that might accomplish what I want, and that's what I'm trying to emulate here. Honestly, I don't know if adding 20 MOA is enough to get me where I need to be. If the new scope uses the same almost 80% of its adjustment capacity just to zero at 200, I'll need 55 of my 70 MOA to zero, leaving me at least 19 MOA short.

Questions for the group:

1: How thick of a shim do I need for at least about 25 MOA?
2: What should that shim be made from for a blued barrel?
3: In what way and to what extent will this impact the validity of a ballistics calculator app?

 

[LoonWulf]

https://www.ssusa.org/content/how-t...herefore, a shim with a,0.00087 x 20 = 0.0174).

Theres the how and how much. Tho the few times I've shimmed up bases permanently Ive epoxy bedded them.

I use brass from ACE or soda cans primarily.

Your ballistics calc if originally accurate will not have changed, you simply re-zero at your prescribed distance, and then you have more adjustment range on the top end.

Im sure someone with alot more experience than I will be along before too long, but thems the basics (as I understand it)

 

[D.B. Cooper]

https://www.ssusa.org/content/how-to-raise-moa-with-scope-shims/#:~:text=Therefore, a shim with a,0.00087 x 20 = 0.0174).

Thanks. That pretty much did it right there. A .025" shim will give me 22 extra MOA.

 

[wild willy]

Be careful how much you shim you’re putting a lot of stress on the scope tube.Tighten the rear ring only and see how far off the front ring you are

 

[JFrank]

Must be a pretty slow moving .243 or a poor bc bullet.

https://www.bergerbullets.com/ballistics/php/bb.php

 

[D.B. Cooper]

Be careful how much you shim you’re putting a lot of stress on the scope tube.Tighten the rear ring only and see how far off the front ring you are

The article referenced in post 2 addresses that by suggesting this only be done with one piece rings/mounts, which will angle the scope along the entire length, rather than bend it in the middle. My mounts and rings are all one piece.

 

[CoalCrackerAl]

How much MOA does the hold over marks give? On that model scope?

 

[D.B. Cooper]

Must be a pretty slow moving .243 or a poor bc bullet.

https://www.bergerbullets.com/ballistics/php/bb.php

Interesting. The link you gave was for 308 Win, but I poke around and found 243. Many of their projectiles are VLD, which my rifle can't stabilize (1/10 twist-I need 1/7 or faster), however their 95 grn should work and does have a slightly higher BC. The interesting thing is it doesn't matter much until you get out to 1000 yards. At 600, it's a 1/4 MOA difference, but at 1000, it's 4.5 MOA, which is huge. The other interesting piece is it holds that 2000 fps (which is a benchmark I use because my Barnes projectiles require that to expand in game) all the way to 500 yards. That would push my hunting range out another 100 yards or so.

Thanks

 

[D.B. Cooper]

How much MOA does the hold over marks give? On that model scope?

All the way to 11 MOA, which, with my rifle and load, is about 600 yards. I thought about that. Just zero the scope to...say...850 yards, and then use the bottom hold over. I prefer the precision of dialing the turrets. Besides, I'm not sure if there is enough adjustment even to zero at 850. (I also have limited access, i.e. no access, to an 850 yrd range to zero.

 

[JFrank]

Humm, I was on .243 but you figured it out so that’s all that matters.
Get a 20 moa rail and you shouldn't have a problem getting to a thousand and be close to optical center, that will allow you to have windage adjustment available if needed.

 

[Varminterror]

The article referenced in post 2 addresses that by suggesting this only be done with one piece rings/mounts, which will angle the scope along the entire length, rather than bend it in the middle. My mounts and rings are all one piece.

When shimming, the bases MUST be bedded with epoxy too. Otherwise you’re effectively trying to torque your straight cylinder scope body tube into matching the shape of a set of stairs. A LOT of internal stress. A 1 pc mount alleviates some of that stress, but not really - the rail may flex less than the scope, but it’s still trying to flex to match the stair-step shape, rather than making a smooth, fully supported, full contact connection with the receiver.

For the work it might take, I’d consider spending the $50 for Burris Signature Zee rings, or $150 for Burris XTR Signature rings, and get the angle that way.

 

[D.B. Cooper]

When shimming, the bases MUST be bedded with epoxy too.

Stop right there. Hold the phone. Any quality epoxy is going to be nearly irreversible and nearly semi-permanent. Not sure I want to go there. If that's the case then...

I’d consider spending the $50 for Burris Signature Zee rings, or $150 for Burris XTR Signature rings, and get the angle that way.

Yea, DNZ makes a +20 MOA version of my mount/rings for under $100. I would rather go there/do that.

Here endeth the lesson.

 

[D.B. Cooper]

Humm, I was on .243 but you figured it out so that’s all that matters.
Get a 20 moa rail and you shouldn't have a problem getting to a thousand and be close to optical center, that will allow you to have windage adjustment available if needed.

Yep. @Varminterror just set me straight on that. Buying the right mounts is a better option in the long run.

 

[Varminterror]

Stop right there. Hold the phone. Any quality epoxy is going to be nearly irreversible and nearly semi-permanent. Not sure I want to go there. If that's the case then...

Nah, use release agent on the rifle, use it on the rail also if you want. Then you'll have a little epoxy sliver which would scrape right off if you ever chose to remove it. I generally only put release agent on the action, so the shim stays with the mount, and forever onward, every time I remove and replace THAT rail, I'd have assurance of the same footprint contact again. I actually bed almost all of my rails, whether I'm shimming or not (and I don't shim any more), just to know I have consistent, stress free mounting. Precision gunsmith and competitive shooter Jim See has a great white paper out there on how he beds rails and rings.

And maybe I'm also really comfortable with the idea that I'd have 20moa rails on everything I own - I can't think of the last scope I'd bought with less than 50moa of internal capacity, so adding a 20moa rail really has no negative consequence.

 

[wild willy]

The article referenced in post 2 addresses that by suggesting this only be done with one piece rings/mounts, which will angle the scope along the entire length, rather than bend it in the middle. My mounts and rings are all one piece.

Sorry I missed that

 

[MEHavey]

What's the distance between the scope rings ?

 

[berettaprofessor]

Signature XTR's can do 40...so that's the way to go.

 

[D.B. Cooper]

I actually bed almost all of my rails, whether I'm shimming or not (and I don't shim any more), just to know I have consistent, stress free mounting.

What epoxy and release agent are you using?

And maybe I'm also really comfortable with the idea that I'd have 20moa rails on everything I own - I can't think of the last scope I'd bought with less than 50moa of internal capacity, so adding a 20moa rail really has no negative consequence.

Yeah, and it's not like I need to zero to 25 or 50 yards, so I don't need any of the lower MOA adjustment. I ever shoot anything less than 100 yards and usually 200 plus.

 

[Varminterror]

What epoxy and release agent are you using?

I've used several kinds. A lot of Pro-Bed 2000, JB weld, MarineTex, Devcon/Devcon Steel. I throw a little black pigment powder in so it roughly matches the bluing/blacking of the mount. I typically just use Kiwi clear shoe polish/cream as release agent. Same gear I use for bedding actions.

Yeah, and it's not like I need to zero to 25 or 50 yards, so I don't need any of the lower MOA adjustment. I ever shoot anything less than 100 yards and usually 200 plus.

Centerfire guns will be about a mil below optical center (half of the adjustment) when zeroed at 100, with only ~1MOA between 100yrd and 50yrd zeroes, so 20moa on a 50moa scope means we're about 6moa from the top of the scope at 100yrd zero, about 5moa from top when zeroed at 50, and have access to ~45moa of the scope's total capacity. On scopes with taller internal adjustment capacity, we might still be a long ways from the top with a 20moa rail - most of the scopes I use have 80-110moa capacity, so a 20moa rail isn't even halfway to the top of the dial.

 

[MEHavey]

(Again) What is the spacing between the rings ?

 

[D.B. Cooper]

(Again) What is the spacing between the rings ?

3.8"

 

[MEHavey]

(I'll spell circumference better next time)
postscript: If the ring centers are actually 4", make 4 shims = 18 MOA
.

 

[Varminterror]

(Again) What is the spacing between the rings ?

Dude did the math a day and a half ago…

 

[MEHavey]

Then we agree
(....but only as long as 2+2 is still allowed to = 4 )

 

[kopcicle]

One piece base and it's just trigonometry.
Separate rings and it's a lapping nightmare.
20 MOA base? Gotcha.
No need for a zero inside of 200? Okay.
So, what's the problem?
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