variation in scope base angle, scope zero angle

After sighting in my scope, I did some calculations that show the actual cant of my scope/base is 7 moa, not 20 moa. I'm kind of surprised because I bought what I think are quality parts: Farrell 20 moa base, SWFA SS rings, SWFA SS 3-9x42 scope.

Some background info: Rifle is a new Savage 116 FCSS Weather Warrior, .30-06. Base is a Ken Farrell standard aluminum Savage round top long action 20 moa base (whew!). Rings are SWFA SS standard low rings. Scope is SWFA SS 3-9x42. I used the Wheeler Engineering scope alignment rods to confirm the rings were aligned after mounting to base. I tightened all base/ring screws to Farrell recommended torque specs. I penciled in witness marks on the scope and verified that the scope did not slip in the rings while firing. I measured the scope centerline height to be about 2 inches (5.08 cm) above the bore centerline.

I used a laser rangefinder to range the target at 88 meters (8800 cm). Ammo was Federal Vital-Shok .30-06 with Barnes TSX 180 grain bullets, specced at 2700 fps muzzle velocity (did not verify with chrono). A ballistic calculator (Ballistic FTE iPhone app) calculates the bullet drop at 5.842 cm down from barrel bore centerline. Bullet impacts were 6.35 cm above point of aim.

When I do the calculations, I find the actual angle between scope centerline and barrel centerline is 6.7 minutes of angle. I was expecting 20 moa, since that's the base I ordered and installed. If the angle really was 20 moa, I calculate the bullet impacts should have been 40 cm (16 inches) above point of aim. With the effect of muzzle rise, I expect the actual point of impact to be even more than 40 cm above point of aim.

First of all, are my calculations above reasonable? If so, is a 13 moa total error (receiver top, base, rings, scope) normal?

I realize this doesn't matter much in practice. I'm just curious to know if this is within expected tolerances. I'm also wondering if I might get closer to the actual 20 moa angle if I undo then redo the base/ring/scope mount.

Ok, I guess I’m just having a hard time wrapping my head around how you did your test. You said that you sighted your scope in, not sure at what range, but then when you shot it at approx 96 yards it was 2.5 inches high, and this makes you think that your base has a 6.7 MOA slope to it. I'm not following you. If the bore of the rifle is completely level with the ground and given a flight time of approx. 0.107 sec to the target (that is assuming that the round is moving @ 2700fps and not factoring in that it has slowed to around 2500fps at 100yards) the bullet should drop close to your 5.8 cm figure (that also assumes that the target is completely level with the bore of the rifle). The factor that seems unclear to me is if your scope has been sighted in how do you know that the crosshairs are in the exact center of the scope tube? There are simply too many variables involved( any variance in the ring heights, centering of the crosshairs, scope tube straightness, etc.) to say that the base does not have the right amount of cant. I would think that a more accurate way to measure this would be to shoot a laser down the bore of the rifle, and then another laser from the top of the base and measure the difference between the two at your 96 yards where a MOA is for all intents close to 1 inch.

I guess a much easier way of checking it would be to pull the base and measure the difference in thickness between the front and rear mounting pads.

Quote:

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Originally Posted by Sundo

Ammo was Federal Vital-Shok .30-06 with Barnes TSX 180 grain bullets, specced at 2700 fps muzzle velocity (did not verify with chrono).

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- Rather expensive ammo for simply testing the cant angle of your scope base ;D.

- "Did not verify with chrono" Real life velocity out of your barrel is a very important input to your calculations. By skipping this step, you have reduced your ability to determine what is valid signal and what is simply noise (think signal to noise ratio) in this experiment. Try it again with accurate chrono data and actually using more than one mount and see if you can duplicate your results.

I like the laser idea.

As LabRat2k3 said, one other way to calculate the data you seek would be to precisely measure the difference in mount height by the front and the rear of your mount. Slope is equal to rise over run, so take the difference in height and divide by the length of the mount and you can calculate the angular displacement of that particular base.

What I mean by "sighted in my scope" is I left my scope adjustments in the middle of their ranges (as they came from the factory) and aimed for the center of the target. I measured the height of the bullet impact above the point of aim. I used a ballistics calculator to determine how high the bullet *should have* impacted, given a 20 moa base cant and no scope adjustments.

By comparing those two numbers I find that the total angle of the unadjusted line-of-sight relative to the bore is 7 moa instead of the 20 moa I was expecting from the cant of the scope base.

Although I don't have a chrono, I expect the actual muzzle velocity is not wildly different from the specification. I ran the ballistics calculator at the specified velocity and at 100 fps slower. That made a very small difference, about 3 cm at 88 meters, not enough to make up for about 25 cm lower than expected impact point.

BTW, I was using the expensive hunting ammo to sight in because I was preparing to go hunting. If I was just interested in target shooting I would have used expensive target ammo instead. :). Time to start reloading.

but was the scope truly at mechanical center? are the rings perfect? best to measure the base. I find it hard to beleive that Farrel would market a 20 MOA base that wasn't. what is it .020" over 6" span is 20 MOA or very close?

Does Ballistic Field Tactical Edition specifically allow correction for using a 20 moa base? You can actually enter the 20 moa base correction into the calculator?

I'm NOT blaming the Farrell base at all. The Farrell base is a fine piece of craftsmanship. I expect to buy and use more Farrell products in the future.

I did have the scope adjustments at mechanical center. I checked by turning the dials to both ends and recentering. I found the dial positions as shipped from the factory were already centered.

Ballistic FTE does not have a setting for base cant. I used the ballistic software to calculate how much the bullet would drop below the barrel bore centerline. The rest of the calculations I did by hand.

I realize there can easily be 13 moa of variation between the receiver, base, rings, scope, and mounting job. I'm just wondering if it's worthwhile to remount everything to get closer to the expected alignment. I'm rapidly moving toward NOT.

If the scope has adjustment for as far out as you wish to shoot, I'd let it alone.
Good hunting! ;D

How far do you plan to shoot, and how much total vertical adjustment do you have in the SS scope?

I plan to shoot as far out as the rifle/scope will allow me to shoot, without remounting anything. :)

Thanks to all for indulging my over-analysis.

This is all way over my head. That being said I would think you should start with a 0 base to get a baseline for what is not right.

Unless you know for a fact that the rifle is perfect, you can't make any kind of valid assumption about what the base and rings add up to. You'd likely get a different result with different ammo or on a different day or with a different scope. The "test" is totally invalid and not representative of anything meaningful. Hope you had fun spinning your wheels since amusement would be the only utility here.