Scope cant

[Zen_Seeker]

I was setting up my new new scope and mount for the M&P15 last night when I started thinking about cant issues.

I use a flat level table, or the kitchen island when the wifey isn't around, as my start point. I sit the mount on the table and place the scope into the mount without the top rings installed. I either place a bubble level on the turret cap or remove the cap and then place it on the turret. I adjust till level, turn the mount 90 and level again. At this point I put on the top rings and lightly tighten a few screws so I can move it without loosing level. Then I get the rifle it's going to be mounted on and make it level, place the mount on the rifle and check that it still remains level. After adjusting, when required, I tighten down the ring screws and do a last check for level before replacing the scope cap.

Once in a while the scope or the way I hold the rifle seems to be slightly canted. (Some hunting rifles just seem to be more comfortable when I hold them that way or the stock and action might be slightly off, don't know.) So these days I hold hunting rifles the way I shoot them then open my eye and check the scope to see if it looks straight and correct before tightening. Then I aim at a plumb line to finish things off.

So my question is; since I don't have anyplace I can shoot further than 300m how much of a difference would a slight cant make on a scope when making field windage adjustments after zero?

 

[BigUglyMan]

It will make more difference to your OCD than it will to your shooting, if you're talking about close-range stuff like 300m. Personally it would drive me insane to have the crosshairs out of plumb.

 

[Trinimon]

Unless your AR can shoot cloverleaf groups at 100yrds, there shouldn't be a whole lot of impact on cant at 300m.

I usually start off by hanging a plumb bob either in one corner of a house where I have the longest line of sight or hang it on a tree outside. Then mount the rings on the rifle that's on a bipod or secured on a benchrest pointing towards the plumb bob. Level the rifle using an accurate level, insert the scope onto the mount, check to ensure I have proper eye relief while aiming at the plumb bob. Verify that the rifle is still level then rotate the scope crosshairs so that the vertical line is aligned with the string on the plumb bob. Carefully snug down the screws on the scope. Verify again that nothing has shifted and if good, torque screws to spec.

Without moving the rig, install a scope level.

 

[Zen_Seeker]

Thanks gentlemen. Yes, it does bug me, I hate things being off. But as it's for a basic AR, and I don't expect it's going to be a tack driver, I thought I'd ask. Most bolt rifles I have I can correct after shooting 2 3RND groups and never touch the scope again. My MR1 is a little fussy and I expect the M&P to be about the same. Just wanted to check before I make another trip to the range for my zero. At least this time I'll be able to see the target and not just a small dark spot at 100m.

 

[jethunter]

I was setting up my new new scope and mount for the M&P15 last night when I started thinking about cant issues.

I use a flat level table, or the kitchen island when the wifey isn't around, as my start point. I sit the mount on the table and place the scope into the mount without the top rings installed. I either place a bubble level on the turret cap or remove the cap and then place it on the turret. I adjust till level, turn the mount 90 and level again. At this point I put on the top rings and lightly tighten a few screws so I can move it without loosing level. Then I get the rifle it's going to be mounted on and make it level, place the mount on the rifle and check that it still remains level. After adjusting, when required, I tighten down the ring screws and do a last check for level before replacing the scope cap.

Once in a while the scope or the way I hold the rifle seems to be slightly canted. (Some hunting rifles just seem to be more comfortable when I hold them that way or the stock and action might be slightly off, don't know.) So these days I hold hunting rifles the way I shoot them then open my eye and check the scope to see if it looks straight and correct before tightening. Then I aim at a plumb line to finish things off.

So my question is; since I don't have anyplace I can shoot further than 300m how much of a difference would a slight cant make on a scope when making field windage adjustments after zero?

I can't handle a scope that's not plumb, drive me batty.

Is it the scope that's canted on the rifle, or are you canting the rifle when you shoot? It would be best to have the scope mounted level on the rifle and adjust your hold, instead of the other-way-round if that's what's happening.

I use roughly the same method you do for scope mounting. Tightening evenly on both sides of the mount is part of the trick.

I do occasionally need to make fine adjustments at the range, so I carry a torpedo level in my range bag to check the target and the rifle are level.

 

[tvanember]

I had a canting issue with a cheap cabellas scope. I "leveled" it using bubbles to the action. Everything looked good.. until I started using the elevation knob for drops. It was mounted on a 22 used for silhouette shoots. I had to rotate the scope so the elevation would track true. The scope was way out of level when shouldered, but the elevation knob tracked true. I junked the scope and upgraded shortly after. Doesn't really relate to your question, but I would say if you're just zeroing and shooting, just level your duplex. If you're dialing dope, make sure your reticle and elevation are true to eachother

 

[Zen_Seeker]

I can't handle a scope that's not plumb, drive me batty.

Me to.

Is it the scope that's canted on the rifle, or are you canting the rifle when you shoot? It would be best to have the scope mounted level on the rifle and adjust your hold, instead of the other-way-round if that's what's happening.

I try to make it level to the rifle first, always. But depending on the rifle and stock it doesn't always workout. We're only talking about hair out, one way ot the other. When I line up the crosshairs on or beside the plumb line the bottom might be a tad right or left.

I use roughly the same method you do for scope mounting. Tightening evenly on both sides of the mount is part of the trick.

I have the Burris P.E.P.R. mount so it's like doing two tires at the same time. I lightly do the middle screws first until I think it's good then I tighten in a star pattern just a bit at a time to about 25ip with my fat wrench.

I do occasionally need to make fine adjustments at the range, so I carry a torpedo level in my range bag to check the target and the rifle are level.

I'm probably going to be doing that next week if I'm lucky. See how the weather holds.

Thanks,
Zen

 

[Zen_Seeker]

I had a canting issue with a cheap cabellas scope. I "leveled" it using bubbles to the action. Everything looked good.. until I started using the elevation knob for drops. It was mounted on a 22 used for silhouette shoots. I had to rotate the scope so the elevation would track true. The scope was way out of level when shouldered, but the elevation knob tracked true. I junked the scope and upgraded shortly after. Doesn't really relate to your question, but I would say if you're just zeroing and shooting, just level your duplex. If you're dialing dope, make sure your reticle and elevation are true to eachother

It's just a $300 Bushnell AR optic. Grabbed the 4.5-18x40 because of the 25% rebate, so I could "try" and drive tacks with the M&P15. But if that doesn't work out I can still use it for groundhogs on my other 5.56 rifles. I'm at the point where I need to shoot it and see how it's hitting on paper. As this is my first AR I suspect how I'm holding it, like I do a bolt rifle, is a little off as well. If it looks good and hit's well off the bench I'll be able to adjust myself when holding it freehand going forward.

 

[Sunchild2071]

I'm gonna go for getting crapped upon by the precision guys for this, I'm sure.

But, how does your rifle know it is canted? It doesn't.

In your natural hold, is your rifle canted?

Does your barrel have an indexing mark for vertical alignment to plumb? I'm betting not.

You can do all the level table and multiple bubble levels you want. When you take up the differences in manufacturing tolerances, a thou here and a thou there, and add them all up, what you think is level and plumb, likely is not (to 100.000% accuracy).

I find if I take all those steps, when I shoulder my rifle naturally and comfortably, the cross hairs are not level and plumb. When I twist myself to adjust that, I shoot like crap.

Changed my approach. I shoot how the rifle feels comfortable and natural. Then I use the plumb bob method to set the scope. And by whatever miracle, I shoot good.

Ever see the 45* co-witness irons? I'm pretty sure the rifle doesn't care that it is canted when the shooter uses them.

Works for me. I'd rather shoot comfortable and natural than twist myself up to try and match what some bubble levels made by the lowest bidder tell me.

Not saying this will work for everyone. I just like to set things to fit my body, not the other way around. YMMV.

 

[anchor3593]

Yeah - s'true. If I sight my rifle in at 200 and my scope is off-plumb with the bore by ¼", but is hitting dead on at 200, then at 400, it will be off again by ¼."

 

[CraigNConnor]

Feeler gauges

 

[kamlooky]

I bawt one of them Level Level Level jigs and even with the level clamped on the bawerll awll prawper,
next day I think I'm out.
I go frum right shoulder to left shoulder.
Sometimes I can kawnvince meself it's awll gooder and other times I shake me tete.
Put it away for a day oar two and try the shoulder aim and it's good.
Dem sneekie gun lawker kritters gartzs tewls me pense.
Play head games on the shewters.

Drives me fruck'in nutzs.

 

[WobblyHead]

One simple trick for checking your levels during a scope level is end for ending your level. Simply level your scope then turn your level 180 degrees, if your level has any error it will show double the error by doing this it is one of the tests used when setting bubbles.

As some people aready pionted out here nobody holds there rifle perfectly verticle and gyroscopic draft will always make your bullet have a drift to the right with a rh twist even if you have your rifle perfectly verticle.

 

[tiriaq]

Why not conduct an experiment?
Fire a careful 10 shot group at 300m.
Fire another 10 shot group with the crosshairs carefully centered on the target, but with the rifle canted randomly for each shot.
See if there is a difference in group size...

Or carefully fire a group with the reticle level/vertical.
Then fire another group with the rifle uniformly canted.
See if the groups are in the same place...

 

[Zen_Seeker]

Been there, done that. (Not with the fancy levels but you get the idea.)
Then add to it how the handguard, bipod, or even flat bottomed PCV magazine might be a hair off and compounds the issue. I set my 30-06 25 to 30 years ago and have never needed to touch it again. But in the last 10 years with the military style rifles and quick disconnect mounts I seem to tweak them more over a longer period before leaving them alone. (Until I switch to a different rifle and start all over.)

Which is why in the end the final step is using a plumb line when on a table and then I close my eyes and hold the rifle the way it feels good before opening my eyes and again looking at the plumb line. I'll live with it being a hair off but I'd be lying if I said it didn't bug me a bit.

We'll see how things go with testing Monday.

I bawt one of them Level Level Level jigs and even with the level clamped on the bawerll awll prawper,
next day I think I'm out.
I go frum right shoulder to left shoulder.
Sometimes I can kawnvince meself it's awll gooder and other times I shake me tete.
Put it away for a day oar two and try the shoulder aim and it's good.
Dem sneekie gun lawker kritters gartzs tewls me pense.
Play head games on the shewters.

Drives me fruck'in nutzs.

 

[Zen_Seeker]

I use a round post/bullseye level first and a small normal level if I think I need to double check. Either way I always turn 90* and then 90* again, but only when putting the scope in the mount, not when the mount is on the rifle.

And your right, gravity works the same way no matter how you hold the rifle or mount the scope. I just wanted to double check with those that know if it really makes that much of a difference when it's only up to about 300m. (Or in the case of this restricted rifle 100m.) You know, for when the time comes that you somehow miss that groundhog at 200m or so and you see no other possible reason for it.

One simple trick for checking your levels during a scope level is end for ending your level. Simply level your scope then turn your level 180 degrees, if your level has any error it will show double the error by doing this it is one of the tests used when setting bubbles.

As some people aready pionted out here nobody holds there rifle perfectly verticle and gyroscopic draft will always make your bullet have a drift to the right with a rh twist even if you have your rifle perfectly verticle.

 

[Zen_Seeker]

That's sort of what I do now in the bush with my NRs. But this is the only restricted rifle I have and 100m is as far as the club range goes.
It doesn't seem to bother me with the hunting rifles I have in wooden stocks because I try to get them centred for how I hold them. But when I do that with the M&P15, Benelli MR1, or even the Ruger SR-22, where the body of the rifle doesn't end up perpendicular to the caps of the scope I can't seem to leave it alone.

But after reading all the feedback it seems that most try to live with it working they way they like and try to "live" with how it looks when mounted. If you can get over the OCD thing or not seems to be the biggest issue.

I'll be doing a 50m to 100m test and add a ladder or tall target test if I have time. One set level as I can keep things and a second set as comfortable to shoot as I can make it. No 200RND dumping this time. Break-ins over.

Why not conduct an experiment?
Fire a careful 10 shot group at 300m.
Fire another 10 shot group with the crosshairs carefully centered on the target, but with the rifle canted randomly for each shot.
See if there is a difference in group size...

Or carefully fire a group with the reticle level/vertical.
Then fire another group with the rifle uniformly canted.
See if the groups are in the same place...

 

[stubblejumper]

Shooting an M&P at zombie targets, cant likely won't be of any concern. but shooting the heads off of ground squirrels at 300 yards with an accurate varmint rifle, cant can be a significant factor.

 

[JoKa]

Ever see the 45* co-witness irons? I'm pretty sure the rifle doesn't care that it is canted when the shooter uses them.

You are correct.... IF the rail on the receiver does not have any MOA built into it.
If it does not then it is more important that your scope is level when firing then it does being level with the rifle.

 

[Can-down]

I mount the mount or rings to the rifle first, and if the rail is one piece I tighten it fairly snug but not fully torqued. Then I drop the scope in place and put the caps on tight enough that the scope does not flop around but I can still move it.
Next I set the fore and aft position for eye relief
Finally I place the gun in a cradle and level the gun, and then while maintaining the gun level I level the scope and tighten the rings. double check everything and then torque everything down.

Final check is to shoulder the gun naturally and aim at some door trim in the house and see how it looks.