Rifle Scopes How does one calculate the effect a canted base

G

George Collins

Private
Minuteman
Apr 28, 2010
37
0
Mississippi
#1
I have a Remington 700P chambered in .308 topped with a Bushnell 4200 Elite. The biggest problem to owning this particular scope is that it only has 50 moa built into it (mine actually has 45 but the book says that it has 50).

To make up for this deficiency, it is mounted on a 40 moa canted base not knowing where the theoretical zero would be but since it was intended to stretch the .308 out to the extremes of what it is capable of (or to exhaust my skill), where the round would impact with the scope bottomed out seemed superfluous. Come to find out through experimentation is that with my present load, the 300 yard zero is 0.5 moa off the mechanical bottom.

Now that hunting season has arrived, I thought it might be a neat experiment to take all the tacticool stuff off the 700p, bolt it all onto a Ruger 10/22 and stretch the .22 out as far as possible. Towards this end, I would need to purchase a Picatinny rail to facilitate this transition.

But which one to buy? A flat one or one with cant built into it? The desired zero for the 10/22 is 30-50 yards. If Eley match grade ammo is used, how much cant should a Picatinny rail have, if any, to achieve this end and still allow the maximum possible elevation to stretch the .22 out as long as possible.

And as an aside, there is a YouTube video a fellow doing a demonstration for Accuracy International rifles where he fired a group, took his scope off, replaced the scope and fired another group on top of the first. If one were to take a Bushnell 4200 off a Remington 700p, mount it on a Ruger 10/22, hunt with it for a couple months, then reaffix it to the 700p, should one expect one's current range card to remain valid or should one expect to have to walk his 700p all the way back out?

Thank you in advance for any help that you can provide.
 
#2
Re: How does one calculate the effect a canted base

The range card should remain valid, once the scope is rezeroed at the original range, at least in approximately the same atmospheric conditions, assuming the same load is being shot.

With respect to the mount, a short range zero is composed of two elements. One is compensation for the height of the scope over the bore. That angle may be computed as:

angle = arctan (height above bore / distance to target)

In the case of my AI .308, for example, with a height above bore of 1.75 inches, at 100 yards (3600 inches) it's:

angle = arctan(1.75 / 3600) = .03 degrees or 1.67 MOA

The other component is the drop from the muzzle to the zero range. With my load at 100 yards, that's about 2.7 inches, or, using the same formula above, about 2.58 MOA.

Add those two up, and it's about 4.25 MOA or about 1.25 mil.

I don't know what those numbers would be for a .22 RF at short ranges.

In any case, if you really want to compute the maximum cant to achieve a short-range zero, do this, once you have an idea what the zero compensation (ZC) is:

Add the ZC to half the total travel of the scope. That will tell you how much "down" elevation you have to work with. Don't exceed that figure with the base cant, and you <span style="font-style: italic">should</span> be able to get a zero, assuming that the base holes in the receiver are reasonably aligned with the bore, so you don't need much windage adjustment to achieve a zero.

Example: using my figures from above, if I have a scope with 70 minutes total travel, the "down" elevation I'd have to work with would be about (70 / 2) + 5 MOA, or 40 MOA. So, I might be able to get away with a 40 MOA base, but I'd probably be better off with a 30 for safety.

To compute the total "up" elevation, <span style="font-style: italic">subtract</span> the ZC from half the total travel, and add the base cant. Again, using my figures from above, I'd have about (70 / 2) - 5 = 60 MOA total "up" with a 30 MOA base.

 
#3
Re: How does one calculate the effect a canted base

If I understand you correctly:
Sight above bore is 1.5"
Distance to target is 30 yards or 1080"

Angle = Arctan(1.5/1080)
= 0.08 degrees
= 4.8 MOA

Drop from muzzle to zero range = 1.4" at 30 yards for a subsonic .22

Angle = Arctan(1.4/1080)
= 0.07 degrees
= 4.2 MOA

Zero Compensation (ZC) = 4.8 MOA + 4.2 MOA = 9 MOA

Total travel of the scope = 45 MOA

ZC + 1/2 total travel of scope = the amount of cant not to be exceeded so:
9 MOA + 22.5 MOA = 31.5 MOA

So as long as I use a base with a cant less than 31.5 MOA, I will not be in danger of being unable to zero at 30 yards?

And with 20 MOA base and a 30 yard zero then there will be 11.5 MOA to spare on the bottom therefore freeing up 33.5 MOA of elevation which should take me out to beyond the 200 yard line (if a subsonic .22 drops 28.84 MOA at 200 yards).

Whew!!! PLEASE tell me I got that right.
 
#4
Re: How does one calculate the effect a canted base

Your math looks good. Bear in mind that scope adjustments are sometimes not what the manufacturers says, and if the mount holes are not in line with the bore, you may have problems with windage zero.

But also remember than you can hold over using the reticle, which gives you some more range.

 
#5
Re: How does one calculate the effect a canted base

Thank you Sir, immensely. I have been trying, in vain, to wrap my head around this thing for awhile now (with the headache to prove it) and I sincerely appreciate the time you took to help me out.
 
#7
Re: How does one calculate the effect a canted base

I know this will sound stupid but if you could tell me
youre running a 40 moa base on a 308 .
How far are you shooting out to?
 
#8
Re: How does one calculate the effect a canted base

The farthest I have taken it to so far is 900.

The theoretical limit based on the ballistics calculator I use is 1100.

This is based on:
MV = 2650
Zero range = 300
BC = 0.496

Plugging this into my ballistics calculator, I will need to come up:
41 moa at 1100 yards (~1100 fps) and
49.5 at 1200 yards (~ 1060 fps).

As mentioned in the original post, the scope only has 45 moa (0.5 of which is used to zero at 300) available so I should be able to get in the black at 1100 (assuming the black is large enough for me to hit it). 1200 yards will exceed the available 44.5 moa and will force a hold over. Since this distance will also strain the capabilities of my round (I think) I will probably stop at the 1100 yard line.

This rifle/scope/ base combo though will have served its purpose though as this was intended to be a learning experience. A poster and very experienced shooter on here gave some advice a while back (it may have even been LowLight ) to new shooters to buy a .308, wear the barrel out and by then you will know something.

That is the process that I am engaged in currently.
 
#9
Re: How does one calculate the effect a canted base

<span style="font-style: italic">Lindy</span> answered all the hard questions (thank God!), so I'll just answer one about the 10/22 base. Use a canted base. Unless your 10/22 has a threaded barrel/receiver connection, your 10/22 has barrel droop. Maybe a little, maybe a lot, but it has it. Barrel droop is an inevitability with the V-Block attachment system on the 10/22 so a canted base on a 10/22 is a no-brainer. As on centerfires a canted base really has no disadvantages.

And aside from barrel droop, the bore is 1.5" or more below the scope's LOS (Line of Sight), so the installation of a canted base will also help you get zeroed at the very short distances of 30-50 yards that you mentioned.

I've used the <span style="font-weight: bold"><span style="font-style: italic">Tactical Solutions 15 MOA Picatinny Base</span></span> as well as the <span style="font-weight: bold"><span style="font-style: italic">EGW Extended 20 MOA Picatinny Base</span></span>.

I found the finish of the Tactical Solutions rail better, and I liked that it is a "standard" length instead of being "extended" like the EGW rail, but I prefer to have the "extra" 5 MOA that the EGW rail has built-in.

I actually shortened an EGW rail by anout an inch, then smoothed it out and used <span style="font-weight: bold"><span style="font-style: italic">Birchwood Casey Aluminum Black</span></span> to blacken the exposed areas before mounting the rail on my MOA 10/22.

<span style="font-weight: bold">Custom MOA 10/22 w/threaded Kidd barrel - no V-Block! + VQ CNC Bolt and UNSHORTENED EGW 20 MOA Extended Picatinny Base:</span>
MOA1022BarreledAction8x6.jpg


<span style="font-weight: bold">Custom MOA 10/22 in McMillan 10/22 Sporter w/SHORTENED EGW 20 MOA Extended Picatinny Rail & Falcon 4-14X 44 FFP:</span>
MOA1022wFalcon4-14XRS8x6.jpg


<span style="font-weight: bold">Custom MOA 10/22 in McMillan 10/22 Sporter w/Tactical Solutions 15 MOA Picatinny Rail & Leupold Vari-X III 4.5-14X 40 SFP:</span>
MOACustom102258x6.jpg



Keith

 
#12
Re: How does one calculate the effect a canted base

Aries,

Thanks for the input. I ordered the EGW Extended 20 MOA Picatinny Base.

And please don't post pictures of such pretty rifles, it makes my poor ol' muddy, banged up, scuffed up, scratched up, basic ol 10/22 get all self-concious.

It's kinda like hanging up pictures of bikini models where the mother of my seven children can see em. Makes her feel bad cause she knows I'm looking at something I want really bad but because of her I can't have one.
 
#13
Re: How does one calculate the effect a canted base

Aries,

I received my EGW 20 MOA Picatinny base in the mail yesterday. I affixed it to my 10/22 last night and had an unexpected opportunity to try it out earlier today.

What I found was shocking.

Based on this information a 60 yard zero was settled upon for academic purposes. The ol' prognosticative function suggested that a come-up of about 10 moa from the bottom should have put a brother in or near the black - or at least on paper. Certainly not cutting grass which is exactly what I saw with every squeeze of the trigger.

I started coming up and found that 28.25 moa from the mechanical bottom of the scope was needed to achieve center-of-dot-on-milk-jug-sitting-in-a-lawn-chair-we-dug-out-of-the-dump-hole-punchage.

So 45 moa of available elevation less the 28.25 moa used for a 60 yard zero only leaves 16.75 moa. 16.75 moa will most decidedly NOT take a brother out there to ranges that makes rabbits in my neighbors' rye grass patches quiver with the amount of fear that I require.

I thought perhaps I had accidentally purchased the wrong base. I rushed home to check it out. The package says it is a 20 moa base. I looked at the base and even on gross examination of it, a decidedly thicker posterior aspect is plainly visible.

Question to you sir, can a 10/22's barrel droop THAT DAMN MUCH?

And, can anything be done about it?

ps - My neighbors' rye grass patches are on my daddy's land and even then, I did in fact get permission to bust caps on rabbits whenever they aren't actively hunting white-tailed Mississippi goat.
 
#14
Re: How does one calculate the effect a canted base

Are you sure you were doing that correctly? Are you sure, in fact, that the 28.5 MOA you measured was not the elevation you had left?

The reason that I ask is that if you look through the scope, and add elevation - as if to shoot at a longer range - the reticle is actually moving <span style="font-weight: bold">down</span>. The effect of the reticle moving down is actually to move the point of impact <span style="font-weight: bold">up</span>.

In other words, did you actually measure how much elevation you have from the zero?

 
#15
Re: How does one calculate the effect a canted base

I started out with the scope at the mechanical bottom. 

I then dialed ~11 moa of elevation by rotating the turret in the direction marked "UP" which on my scope is in a counterclockwise direction and when so manipulated, causes the turret to incease it's physical distance from the body of the scope. 

After making the above adjustment, the rounds impacted the ground as evidenced by grass flying. 

I dialed additional elevation a few times until I saw the rounds impact the lip on the seat of the chair in which I had the milk jug sitting. 

Finally having a reference point, I was able to get in the black by actually measuring the additional elevation needed and making the required adjustment  

After I was in the black, I counted the MOA required to return the scope turret to it's mechanical zero. That number was 28.5 moa.

And thank you again for your help.
 
#16
Re: How does one calculate the effect a canted base

That seems odd. It's hard for me to believe that something else than "barrel droop" is not going on. That makes me wonder if the receiver where the base is mounted is actually parallel to the bore.
 
#19
Re: How does one calculate the effect a canted base

Sorry Sir about the delayed response, Thanksgiving and all.

Barrel droop would certainly seem to explain the discrepancy. My son was in from college for the holiday and he and I did a little training to work off that large meal. Together, he and I started at the 20 and worked our way back to the 150-yard line. To get the group to center up on the POA at that distance required a come-up of 41 moa using standard velocity ammo. Based on a thread I read on The Hide earlier, I will need and additional ~ 12 MOA to get out to the 200 which I should be able to accomplish with a mil dot hold-over and the four MOA of elevation left in the scope.

Thank you for your help. It has been educational.

I told the friend of mine who and turned me on to "The Hide" and who serves as my LR marksmanship instructor that I had posted this question here. He asked if I had gotten any responses. When I confirmed that I had, he asked from whom.

I said, "Some feller named Lindy."

He said, "Pay attention."
 
#20
Re: How does one calculate the effect a canted base

OK, coupla points. While everybody is giving good advice, I suspect we have all just sorta glossed over the difference between slope and cant. Slope affects up and down. Cant affects the tendency of the reticle to appear as if it's leaning left or right. Aside from that, if we substitute 'slope' for 'cant', we're all on the same page in our hymnbooks.

Additionally, the overall amount of elevation can be reduced/limited by windage adjustments that are not centered, as might be caused by a mount that's slightly offset to the side in relation to the bore. Not certain if it was mentioned here yet, but before I tried to determine extreme elevation limits, I'd center the windage first.

Greg
 
#21
Re: How does one calculate the effect a canted base

Greg, thanks for the advice. Please tell me if I did anything wrong in mounting my scope.

Once I had the Picatinny rail affixed to the .22, I removed the 4200 from the .308 by detaching the scope mounts.

Next, the scope with mounts was placed onto the .22's Picatinny rail. Using my left hand, I imparted posteriorly and medially directed pressure to the scope for a snug fit between fixed metal components of the scope and rail. While holding constant pressure, I tightened the bolts on the scope.

Here is the data that I gathered from the range:

1. Federal hunting round
27 MOA for 50 yard zero
29 MOA for 60 yard zero
35 MOA for 100 yard zero

Go right 3 moa for each

2. Subsonic CCI .22 long - 38 moa up and 3 min right for a 30 yard zero. 

3. Bulk federal hollow point
100 zero = up 33 moa and right 2.25 moa
150 yard zero = up 41 moa and right 1.25 moa 

At first, I had to go right 5.0 with my hunting round but I was able to shave off 2 moa by noticing that the table from which I was shooting, when viewed from the rear, had a noticeable echelon left. I built up the left leg of my bipod with a sack of chewing tobacco and that seemed to keep vertical adjustments exclusively in the vertical plane.
 
#22
Re: How does one calculate the effect a canted base

I'm sorry, but I found out the hard way a long time ago that trying to do diagnosis over the Errornet doesn't work so well. Basically, if you can hit what you're aiming at, you're doing OK.

Greg
 
#23
Re: How does one calculate the effect a canted base

Want a Simple Solution?

Buy a Bushnell 74-3333 boresight-collimator.
Record your zeroed scope's reticle position before you move it
to the other rifle. Sight in when you mount on the other rifle. Record your zero before you move the scope back to rifle #1 or on to rifle #3.

As long as you haven't changed your ammunition or scope mount system, when you remount and reset your reticle to the grid location (on the collimator) you recorded before pulling the scope, you should be about right on your previous zero.

Maybe there will be some conditions related changes, but aren't there always?

About $80 or $90 for the most useful shooting tool you will ever buy....
 
#24
Lindy said:
Re: How does one calculate the effect a canted base

The range card should remain valid, once the scope is rezeroed at the original range, at least in approximately the same atmospheric conditions, assuming the same load is being shot.

With respect to the mount, a short range zero is composed of two elements. One is compensation for the height of the scope over the bore. That angle may be computed as:

angle = arctan (height above bore / distance to target)

In the case of my AI .308, for example, with a height above bore of 1.75 inches, at 100 yards (3600 inches) it's:

angle = arctan(1.75 / 3600) = .03 degrees or 1.67 MOA

The other component is the drop from the muzzle to the zero range. With my load at 100 yards, that's about 2.7 inches, or, using the same formula above, about 2.58 MOA.

Add those two up, and it's about 4.25 MOA or about 1.25 mil.

I don't know what those numbers would be for a .22 RF at short ranges.

In any case, if you really want to compute the maximum cant to achieve a short-range zero, do this, once you have an idea what the zero compensation (ZC) is:

Add the ZC to half the total travel of the scope. That will tell you how much "down" elevation you have to work with. Don't exceed that figure with the base cant, and you <span style="font-style: italic">should</span> be able to get a zero, assuming that the base holes in the receiver are reasonably aligned with the bore, so you don't need much windage adjustment to achieve a zero.

Example: using my figures from above, if I have a scope with 70 minutes total travel, the "down" elevation I'd have to work with would be about (70 / 2) + 5 MOA, or 40 MOA. So, I might be able to get away with a 40 MOA base, but I'd probably be better off with a 30 for safety.

To compute the total "up" elevation, <span style="font-style: italic">subtract</span> the ZC from half the total travel, and add the base cant. Again, using my figures from above, I'd have about (70 / 2) - 5 = 60 MOA total "up" with a 30 MOA base.
Click to expand...
How is 1.75 ÷ 3600 = .03
Am I missing a multiplier?

angle = arctan(1.75 / 3600) = .03 degrees or 1.67
 
#25
www.snipershide.com

How to figure the correct MOA angled scope mount base....

These questions seem to come up a lot with the rise of people shooting .22LR out to 200, 300, 400+ yards.... Do I need a 0 MOA, 20, 30, or 40 MOA scope mount base ??? Can I still get a 25 or 50 yard zero??? How far will I be able to dial with XX scope and XX MOA base??? This depends on 4 main...
www.snipershide.com www.snipershide.com
 
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