Math wizards: a challenge for you

[Monkeyleg]

I'm sure in high school I could have arrived at the answer to this question easily, but I'm not that smart anymore.

My 6.5x20 scope is zeroed 111 clicks from the uppermost limit of reticle adjustment, and 43 clicks from the lowermost limit of adjustment. This is when the scope is zeroed for 100 yards.

I can't think of any circumstances under which I'd need to lower the reticle more than a few clicks, but I suppose having ten or so available for lowering wouldn't be a bad idea.

The reason I'd like to increase the amount of reticle travel for adjusting for bullet drop is that I want to shoot out past 500 yards, maybe out to 1000, and 111 1/4 MOA clicks is not enough.

So, I'd like to put a thin shim under the rear base to raise the reticle maybe 20 or 30 clicks (5-6 MOA).

How thick should the shim be? The center of the scope is 1.75 inches above the bore.

[AmbulanceDriver]

what's the distance between the front and rear base?

Actually, lemme back up.   is this a once piece base, or is this a two piece base?   Apparently from what I've been reading, you cannot shim the rear base of a 2 piece base, it will end up doing bad things to your scope tube. 

[Monkeyleg]

Apparently from what I've been reading, you cannot shim the rear base of a 2 piece base, it will end up doing bad things to your scope tube.

That makes sense, although I wonder how much the scope would have to be elevated at the rear before it would damage the tube.

[Perd Hapley]

42

[zahc]

I need to know

how far apart are the scope tubes

How many total vertical clicks does you scope have (all the down to all the way up)?

I agree that shimming the back is probably suboptimal. You can buy wedge-shaped scope rails that tilt the scope while leaving the rings paraxial. I'm not sure if it's a big deal though given tolerances and the small amount of shimming that is likely necessary.


Here's a general formula:

R/S = D/T

or

T = (D*S)/R

R=range of interest (100 yards maybe)
S=point of impact shift desired at that range
D=distance between scope rings
T=shim thickness

use consistent units throughout. If you want an answer for the shim thickness in inches, then you need to express the range and distance between the scope rings in inches.

[Monkeyleg]

Thanks, Zahc. I counted the number of 1/4 MOA clicks earlier this evening, and I seem to recall there being something like 160 total, or 40 MOA. That's not a lot, certainly compared to scopes made today.

If I want to move the reticle down 15 MOA, I'd need to shim the rear .0333 inches. I can't imagine that small a change would damage the tube.

Fistful, sorry, but 42 isn't the answer this time.

[Tallpine]

I'm sure in high school I could have arrived at the answer to this question easily, but I'm not that smart anymore.

My 6.5x20 scope is zeroed 111 clicks from the uppermost limit of reticle adjustment, and 43 clicks from the lowermost limit of adjustment. This is when the scope is zeroed for 100 yards.

I can't think of any circumstances under which I'd need to lower the reticle more than a few clicks, but I suppose having ten or so available for lowering wouldn't be a bad idea.

The reason I'd like to increase the amount of reticle travel for adjusting for bullet drop is that I want to shoot out past 500 yards, maybe out to 1000, and 111 1/4 MOA clicks is not enough.

So, I'd like to put a thin shim under the rear base to raise the reticle maybe 20 or 30 clicks (5-6 MOA).

How thick should the shim be? The center of the scope is 1.75 inches above the bore.

Well, first of all - I'm not sure what you mean by uppermost and lowermost, because you raise the reticle to lower the POI. and vice versa.

Secondly, a MOA at 100 yards is not the same as a MOA at 500 or 1000 yards.  One click at 100 yards might be equivalant to (not stopping for any math) 5 or 10 clicks or more at longer ranges.

I've really never messed with this exactly, but I know from trying to shoot gophers at long range (>50-75yards) with a .22 LR that the apparent holdover is amazingly small compared to the diameter of the scope view. 

.22 LR has a lot of drop out past 75yds so it's somewhat comparable to CF at much longer ranges.  Let's say (I forget, now...) you need 8 inches holdover at 125 yards, but that eight inches is mighty small in the scope view at that range.  My experience was that the apparent holdover stays pretty constant beyond about 75 yards, but the target gets a lot smaller.

I guess what I'm saying is that I wouldn't go mucking with my scope mount till I had determined empirically that the reticle adjustment was not adequate.

[Harold Tuttle]

why use math?

get some shim stock and start noodling

[Bob F.]

All the above (well, most) sounds good. But keep in mind a minute-of-angle at 100 yds is still a m-o-a at 500yds. i.e.: What's 1" at 100yds is gonna be 5" at 500yds. So 4 clicks is gonna raise ya' 5" at 500!

Have fun, stay safe.
Bob

[French G.]

Can you just drop back and punt the credit card, buy a 20MOA base from someone that already did the math?

[Monkeyleg]

Can you just drop back and punt the credit card, buy a 20MOA base from someone that already did the math?

I was just reading about those. Are those one piece or two piece bases? I would think it would be one piece unless the rings are included and have been lapped for the change in angle.

[Tallpine]

All the above (well, most) sounds good. But keep in mind a minute-of-angle at 100 yds is still a m-o-a at 500yds. i.e.: What's 1" at 100yds is gonna be 5" at 500yds. So 4 clicks is gonna raise ya' 5" at 500!

Have fun, stay safe.
Bob

What I was trying to say, only you said it better. 

[Fly320s]

Most are one-piece Picatinny rail-type mounts.  The rear is elevated slightly, of course, but the rail portion is all on one plane so that when the scope rings are mounted they will be aligned with each other.  In other words, there won't be any stress on the scope tube that isn't there with a normal mount.

[Chuck Dye]

To answer the original question, the angle of deflection produced by a shim is the arc-tangent (tan^-1 on many calculators) of the  thickness of the shim divided by length between the contact points of the mount.  Because the contact points will be somewhere within the length of the rings, that may be hard to measure accurately enough for  precision, but you should get on the paper.


Edit to add:  Over a five inch base a .001 inch shim produces .6875 MOA.

[Hawkmoon]

what's the distance between the front and rear base?

Actually, lemme back up.   is this a once piece base, or is this a two piece base?   Apparently from what I've been reading, you cannot shim the rear base of a 2 piece base, it will end up doing bad things to your scope tube. 

Well, you can shim it, but then you have to "line bore" the two bases so there's a 1" diameter hole that's in perfect alignment through both.

[Bob F.]

Tallpine: RE: your "empirically determine", I put a BIG piece of cardboard out at 500 yds w/ a 1' square orange aiming point at the top. Shot a group and measured. Cranked in the clicks and shot over the target! Was figuring 4 clicks to the inch instead of 4 clicks=5" at 500yds. Got that, OP??

Just loaded 50 rds of the same load, waiting for the weather to co-operate!

Bob

[zahc]

the angle of deflection produced by a shim is the arc-tangent (tan-1 on many calculators) of the  thickness of the shim divided by length between the contact points of the mount.

My way is better because it uses the small angle approximation, so no need for trig functions.

[CNYCacher]

What are the bullet drops for a horizontal trajectory at 100, 200 and 500 yards?

[Monkeyleg]

What are the bullet drops for a horizontal trajectory at 100, 200 and 500 yards?

At a muzzle velocity of 2600 fps, an altitude of 900 feet, 69 degrees temperature, 50% humidity, barometric pressure at 30.00, bullet BC of .496 (a Sierra .308 168 gr HPBT Match King), and a scope sitting 1.75 inches above the bore, the drop from the muzzle is 2.7" at 100 yards, 11.2" at 200 yards and 82.5" at 500 yards.

[Fly320s]

At a muzzle velocity of 2600 fps, an altitude of 900 feet, 69 degrees temperature, 50% humidity, barometric pressure at 30.00, bullet BC of .496 (a Sierra .308 168 gr HPBT Match King), and a scope sitting 1.75 inches above the bore, the drop from the muzzle is 2.7" at 100 yards, 11.2" at 200 yards and 82.5" at 500 yards.

That must be with a level barrel, not one zeroed for a certain range.

My dope, using similar atmosphere and 2700 fps, shows:
100 = 0
200 = -4
300 = -14.5
500 = -60

[AJ Dual]

We're all aware that they make long-range scope bases, right?

Like a 10 or 20moa base that has an angled bias to get a scope zeroed at a longer range without using up all it's internal adjustment.

The DIY thing is strong in us, and I get that. Just saying... \

http://thefiringline.com/forums/showthread.php?t=298817&referrerid=36767

(edit)

Saw French G. beat me to it. 

[Fly320s]

I agree that a dedicated 20 MOA scope base is the way to go.

Something to consider with shimming:
- The shim will tilt the screw hole which will cause the screw to not be vertically aligned with the scope mount and receiver hole. That will cause extra strain on the screw
- A front shim at the same angle will be required to prevent bending damage to the scope body and/or erector mechanism.
- If all the parts aren't on the same plane, the scope is more likely to be damaged during recoil.
- One piece 20 MOA bases are pricey, but still cheaper than a new scope.

[Monkeyleg]

Fly320's, if I put 2700 fps in my calculator, I get numbers very close to yours. The ones I gave were from the muzzle.

I bought this rifle more than 15 years ago, and haven't shot it in 13 or 14 years, I think, so I've forgotten a lot about it. I just looked closely and saw that I already have a very thin shim under the rear base. I think I'll have to find a tapered base. I'm good right now out to about 600-650 yards.

[Tallpine]

Are you quite sure that you don't want to move the bucket a bit closer?

 

[Monkeyleg]

Are you quite sure that you don't want to move the bucket a bit closer?

I guess I'm even more dense today, because I don't understand.

I did a little checking and found that Burris makes "Zee" rings that have composite inserts in them that can rotate slightly so that a scope that's not mounted perfectly doesn't get damaged. They also have inserts that will give a vertical offset of .005", .010" and .020 for mounting the scope on an incline to increase reticle travel up or down. Since I can get them at cost and they'll fit my existing bases, it's a nicer solution than a one-piece 20 MOA base, the looks of which I wouldn't care for anyway.