Testing POI shift from gun cant

[Bedrock Bob]

Zero at 150 and shoot to 600. You will find out quickly how much cant matters.

Put a scope up there on the carry handle of an M4 rifle. Zero it at 50 and then shoot it at 300 canted 15 degrees. Take that handle off and mount the scope flattop. Do the test again. It will show that scope height does affect cant error.

It's exactly like mounting two scopes. Try it where there is a greater ratio between sight in distance and POI. The scope height matters. Leastwise that is a widely held belief supported by tons of observation.

I think scope height matters very little in the cant equation when shooting at pellet gun distances with little cant error. Try that same experiment with feet and inches of drop with a powderburner and I bet you arrive at a different conclusion.

Again I may be as full of stuff as a Christmas goose. I may be off base with my pants down. But I can hit my marks at 600 yards. Any old wives tales or voodoo magic I can use to those ends is solid science to me.

 

[Bedrock Bob]

Waaaaaaaaaat, two scopes, wait a minute....

It's sick and wrong. Nasty even.

It's unnatural coitus opticus. Photos like that should not be seen by little peep sights.

 

[nervoustrig]

Zero at 150 and shoot to 600. You will find out quickly how much cant matters.

Most definitely. Cant error is proportional to projectile drop, and since pellets have a poor BC and low velocity relative to a centerfire, the problem shows up at very modest distances.

Put a scope up there on the carry handle of an M4 rifle. Zero it at 50 and then shoot it at 300 canted 15 degrees. Take that handle off and mount the scope flattop. Do the test again. It will show that scope height does affect cant error.

Yes, if you use the same mildot for aiming to account for the drop, the POI for the low scope will indeed differ from the POI for the high scope.
However you would not want to use the same mildot. In other words, if you zeroed at 50yds using the low scope and then worked out the necessary aim point (mildot) to account for the drop at 300 yards, and then re-mounted the scope high and once again zeroed at 50 yards and proceeded to work out the necessary aim point (mildot) to account for the drop at 300 yards, you will find they differ. It would be perhaps 4 mildots down for the low scope and 3 mildots down for the high scope.

If you then cant the gun 15 degrees and aim with mildot 4 on the low scope and mildot 3 on the high scope, the resulting POI will be the same. Again, this is something that can be replicated either with the method demonstrated by the guy linked above (szottesfold.co.uk), or by installing two scopes as shown in my photograph. The two scopes is perhaps a little more compelling because not only can you verify the POI is same when using the correct aim point through each scope, but you can also clamp down the rifle and see that mildot 4 on the low scope and mildot 3 on the high scope superimpose onto an identical spot on the target at 300 yards.

 

[Ghostranger169]

There is a reason the BR guys use a level. Since the POI isn't the same distance for each side tells me your scope in not centered on your rifle. No big thing if you can keep the gun level.

Thanks for the input. I wanted to share a quick dirty test I see how my gun/scope combos respond to cant, but this thread sure has "grown legs".
I think the "right cant" (50yard) shot being farther right and lower than anticipated could be from unlevel shooting bench, unlevel bipod mount, or bipod cant being more "right biased", wind, bad pellet, etc..., as well as, scope not being centered, correct?

 

[Bedrock Bob]

Most definitely. Cant error is proportional to projectile drop, and since pellets have a poor BC and low velocity relative to a centerfire, the problem shows up at very modest distances.



Yes, if you use the same mildot for aiming to account for the drop, the POI for the low scope will indeed differ from the POI for the high scope.
However you would not want to use the same mildot. In other words, if you zeroed at 50yds using the low scope and then worked out the necessary aim point (mildot) to account for the drop at 300 yards, and then re-mounted the scope high and once again zeroed at 50 yards and proceeded to work out the necessary aim point (mildot) to account for the drop at 300 yards, you will find they differ. It would be perhaps 4 mildots down for the low scope and 3 mildots down for the high scope.

If you then cant the gun 15 degrees and aim with mildot 4 on the low scope and mildot 3 on the high scope, the resulting POI will be the same. Again, this is something that can be replicated either with the method demonstrated by the guy linked above (szottesfold.co.uk), or by installing two scopes as shown in my photograph. The two scopes is perhaps a little more compelling because not only can you verify the POI is same when using the correct aim point through each scope, but you can also clamp down the rifle and see that mildot 4 on the low scope and mildot 3 on the high scope superimpose onto an identical spot on the target at 300 yards.

You make a compelling case. You've done your research and you are supported on the shoulders of giants... Or at least a bunch of short guys with a lot more experience than I.

At this point I usually assume a guy is as full of mackerel as the captain's cat. But in this case I "cant". You have shaken my faith in a long held belief.

It's a great conversation on a high level topic with a lot of experience on both sides. I thank you.

I'm still going to pretend scope height matters with cant error. But whenever the discussion comes up on the range I'll have to mention your hypothesis. I simply "cant" discount your argument. It's a good one. I "cant" find fault with it.

 

[Peashooter]

TBH i might have to pay more attention to the bubble level on my rifle, CUZ i dont really make much use of it, and it could be why things suck some days.

 

[Ghostranger169]

Peashooter,

Yep, ya gotta learn to have the "scope eye" on the crosshairs and the "non-scope eye" on the bubble. Unfortunately, with the somewhat finicky nature airguns, there's A LOT of reasons why things suck some days. For all of us. Lucky we have a good community of dedicated airgunners and a forum to share info to hopefully get better.
I've read many posts from people asking help with "POI shift"/accuracy/consistency related problems. Most don't post picks of their gun/scope combos or even target groups which they've shot(even when they are asked to). Some that do post pics of their rigs, don't have any type of properly set up anti-cant device(bubble/electronic level on scope/rail/action).
Thanks for the input!

 

[boscoebrea]

Never used a level on a scope,always use my sense of balance and it works, though lately I have become somewhat unstable, and can not always get my prescriptions filled,I am now going to buy a bunch of scope levels and see if they help me, Thank you all for convincing me the attributes of bubble levels.

 

[Bedrock Bob]

Never used a level on a scope,always use my sense of balance and it works, though lately I have become somewhat unstable, and can not always get my prescriptions filled,I am now going to buy a bunch of scope levels and see if they help me, Thank you all for convincing me the attributes of bubble levels.

FWIW I don't use a level. I plumb the vertical crosshair in my sighting routine. And when shooting offhand Iron sights I often slide into a cant as the shot goes off.

A bubble level is great off a bench with a scope when shooting tiny pattrns. And it would definitely be an asset when shooting varmints long range. I don't use one though.

When hunting with a scope or shooting a rifle bare at reactive targets they are pretty much worthless. Some guys hunting from a stand might employ one. But I just don't feel (for me) it's a value added thing. Yes it may put me closer to the exact center of the intended POA. But that does not matter (much) in a hit/miss game.

I check the cant before I take up the slack in my trigger and focus on the front bead (or target with the scope). From there on out its about trigger timing.

If I take too long I'll slip into a cant and start wobbling. Then it's either let down and start again or drive the gun and rely on trigger timing alone. At that point your cant makes very little difference in a hit or miss. You are shooting basketballs and practice is the only thing that improves your hit/miss ratio.

If your checklist includes checking the cant just before you shift focus to the target it won't be much problem unless your form is bad or you are torquing the gun on the rest.

But that's just the way I handle it. If you don't have a good grip on balance you might benefit from leveling with a bubble.

 

[KatoKevin454]

keep up on the experiments, very informative....

 

[boscoebrea]

Yes Bedrock Bob,that is what I do ,I agree .I do find myself shooing from a good tripod and it has a bubble level in it,so now I will see how much better,if any a bubble level mounted on the rifle does .BTW i do check rifle level.with the flats on the rifle stock ,yea old school with a real stock..Oh I am the type that straightens out crooked pictures to.

 

[Ghostranger169]

I think it's important to remember that bubble/electronic levels are "cant indicator" tools. All tools can be used correctly or incorrectly.
Ideally, they need to be properly set up to align your vertical crosshair to be perpendicular to the guns action(at level) and centered on the bore line. A tall target test can help verify results.
There are several methods(plumb line, mirror, etc...) to help accomplish this.
However, even if you are using a non-perfectly mounted anti-cant indicator device, you will get much more CONSISTENT results, which are MUCH easier to find a way to correct, if they do not meet expectations.