cant angle and bullet impact variation

[coolcruiser]

This idea has always puzzled me...that any minor inconsistent cant angle will result in a variation in point of bullet impact. Has anyone done any experiments to prove this point? It would seem that as great as a 90 degree cant angle would still line the point of aim up with the point of impact. Of course this assumes that trajectory is identical whether the barrel is directly under the sights or 90 degrees left or right....that is unlikely, right? It seems that maybe the point of concern is that line of sight is straight while trajectory is at best an arc. Any thoughts?

[KennyB]

The basic principal is that while it is in the air, the bullet will drop due to to influence of gravity.
Thus, in order to hit the center of the bull the BORE of the rifle must point ABOVE the bull so that the bullet drops into the center.
At 50 yards the bullet will have dropped about 4 inches compared to where the bore was pointing.
So while the sight line is pointing at the center, the bore line is pointing at 12 o'clock by 4".

Now if we cant the rifle over to 30 degrees anticlockwise, the sightline will still be pointing at the center but the boreline will now be pointing high at 11 o'clock (at a radius of 4") rather than 12 o'clock. The shot will then drop 4" from THAT point.

A bit of maths demonstrates that for a 30 degree cant, the bullet impact should be 2" left and a bit over half an inch low...

More disturbingly, the change in angle that will change a 10 into a 9 is about 4 degrees....

At 100 yards the drop is about 4 times the drop at 50 yards so the effect is much more pronounced (an error of 2.6 degrees will turn your 10 into a 9)

Explained with diagrams here: http://home.wanadoo.nl/jhogema/skeetn/b ... ntel1e.htm

Ken.

[coolcruiser]

Thanks Kenny...great explanation. My 100 Yard shooting has been done with no regard for cant. Perhaps my position was consistent enough to clean the target...or not! My regard for leveling is now greatly increased. It was the MEC catalog that called my attention to this. They wrote about it and featured a number of devices to help maintain level.

[Tim S]

Sprirt levels (or some other deveice like the "whiskers" inside Anschutz 1900 foresight tunnels) are very useful. I have a spirit level on top of the foresight, and find this easy to use. Other prefer the level inside the tunnel, in front of the foresight, or use somthing else. I check the position of the bubble during my shot preparation, not during release. If the bubble isn't centred, I'll check my position, NPA etc.

Tim

[nester]

Here are the long explanations and calculations:
http://home.wanadoo.nl/jhogema/skeetn/b ... ntel1e.htm

Example II

How much cant is allowed for small bore rifle at 50 m before a ‘10’ becomes a ‘9’? For this disciplene, the diametre of the bullet (0.22 inch) is 5.59 mm and of the ‘10’ it is 10.4 mm. The distance D needed to move from the centre of the '10' to the edge of the '10' is D = (5.59+10.4)/2 = 7.994 mm. With a drop of r=119 mm (see Appendix A), Equation (2) gives: ß = 3.85 degrees.

[dlinden]

Here's one more -

http://www.accurateshooter.com/optics/c ... of-impact/

8 degrees of cant will displace a shot 40 inches and off the target completely at 1000 yards.

Dennis L

[Redwagon]

wow, excellent description/quantification of the issue!

[daisy]

Yet another demo
http://www.arld1.com/impactpointvscantangle.html

[BAtarget]

i actually just read an article yesterday about 1000 yard shooting. David Tubb, 11 time national high power champion, experimented with cant. He and another world class shooter determined that 1 single degree can move a bullet 5 inches. As a rule of thumb, about .5 inches at 100 yards with a .22LR. Thus a mere 5 degrees will take you from the very center of a 100 yard target to the inner 9 ring. 8 degrees will put you on the outer 9 ring.

heres the article

http://www.accurateshooter.com/optics/c ... of-impact/