Okay... just gotta ask this: anyone here actually *tried* this out? Tested the theory empirically?
Not to naysay Uncle Sam's Misguided Children ;) or their book... but when I get enough different answers that don't agree even kinda close, they pretty much all become suspect in my eyes.
The above mentioned field manual supposedly mentions 'over 55 inches', as cited on mountsplus.com's advertising blurb for their Anti-Cant Device.
Scop-Level says 'Just a 6° cant, or one minute on a clock, will cause a miss of over 36 inches at that range'
. When pushed for how they came up with that number, they said they got it from Sierra's exterior ballistics info.
Deros Level Grouse says that '...a cant of just one degree can move a bullet's point of impact as much as 52 inches at 1000 yards.' In one of the 'reviews' that they cite (apparently in lieu of actually publishing any hard data about their product under the 'Information' tab on their site) the shill... I mean reviewer gets confused by the teleprompter... he says that 1° of cant is worth about 1/2 moa @ 1000yds (okay, I'm onboard with that), and could cause you to miss by as much as 52" @ 1000yds (nope, ya lost me)." So *thats* where all the 1/2 moa guns on the Internet come from! ;)
Running the numbers thru one ballistic program (Quick Target, companion to QuickLoad) agrees pretty closely w/ the 36 inches.
Running the numbers thru another ballistic program, JBM, shows a radically different number... more like 5-6 inches. Enough to be annoying when trying to say, zap a prairie dog at distance or snag an 'X' in F-Class, but not so much as to take you literally off the paper @ 1000yds - we use 6' square targets, so 3' or 36" to the edge. The 52" mentioned elsewhere would seem to make hitting the target nearly impossible - as little as one degree off plumb either way would have your shots impacting in the dirt on either side of the target, or on the target next to yours if the frames are close together *and* you fudged the wind call.
I do usually use a level of some sort. From time to time during a string fire in F-Class I'll find that my gun (F/TR, shot from a bipod) has twist or shifted somehow and the level is a little off (I use a Harris 6-9 swivel bipod w/ PodLoc & PodPaws) - I'll readjust and keep shooting. I guaran-freakin-tee you I don't get blown clean off the paper due to the small amount of cant (probably 5-10 degrees). The wind may kick my hiney royally now and again, but I'd like to think I'm experienced enough to be able to tell when the value marker comes up over in the '6' position instead of a '10' or an 'X' because of a missed wind call vs. some cant error. I also shoot NRA High Power, ala conventional Prone & Palma - trust me, it's a whole heck of a lot harder to hold the gun consistently stable with just a coat, sling, & glove, yet somehow we manage to hit the center of the target with reasonable success & regularity.
Sadly, I'm not conversant enough with the math involved to be able to crunch out the trajectory numbers myself. Maybe if I get really bored at work I'll work 'em out on the back of a napkin. Maybe.
An article published on the 'Net from the Varmint Hunters Association magazine discusses the factors involved *and* the author did some basic empirical testing. Granted, he used some flat shooting calibers - .260 Rem & a 7mm RM, vs. the cannonball-like .308 Win-168gr-SMK used in some of the earlier examples. But his numbers were far closer to what JBM predicted than any of the others - 5° cant netted about 1 moa horizontal displacement @ 400yds & 700yds. Where he got a nasty surprise was the *vertical* displacement as the trajectory skews off to the side and no longer rises as high above the LOS.
This is something I've been meaning to get to for about six months now. One thing or another has kept that from happening. After this weekend, I should have more time to experiment. Not necessarily fewer demands on my time, just less urgency and more flexibility to shuffle things around. I also have a *lot* of brass that I need to fire-form for my main competition 'team' F/TR rifle. I'm thinking of making a bracket to fit in the fore-end rail (a surprisingly convenient attachment point) and hold a protractor and/or plumb bob. Since my bipod locks at what ever angle I want, it should be relatively simple to align it, lock it down, and fire a few groups at various cant angles. I know I can shoot 600yds easily enough; whether I can get set up and zeroed for 1000yds here locally on a day when the wind plays gently will be the big question. If the winds howling the difficulty in separating wind drift from group displacement due to cant could be an exercise in futility.
Just so I'm clear: I don't disagree that holding the rifle *consistently* is a key element of good marksmanship and essential to accurate shooting. Shooting with a cant is fairly common in NRA High Power... inboard (to the left, for a right-handed shooter) in Standing and Prone, and outboard for Sitting, due to the mechanics of the human body in getting situated in a good tight position. Doing it consistently is
tough. I just take issue w/ some of the more... 'dramatic' claims being made; they smack of fear-mongering to me - i.e. making up wild claims to promote sales of various widgets and gadgets and gizmos. Who knows, I might be completely wrong... but then someone would have to do the testing to prove that, wouldn't ya
A few links from what I mentioned above:
Scoplevel Anti Cant Leveling Device
Welcome to Alamo Four Star
Cant Errors - Long range shooting