CANT - fact or fiction???

[camac]

No doubt rifle Cant is real but we often read or get told about how big an effect it can have. If you actually plug in some cant on one of the ballistics calculators the effect is much less than imagined.

e.g. on the JBM site, 6.5x284 pumping out a sierra MK 142Gr at 2950, scope 2.0 inches high and 1 degree of cant gives 0.7 inches windage at 1000 yards - note, drift is 7 inches and every knot of wind is 7 inches, so a 5 knot breeze and drift can move you 42 inches, cant only 0.7. In fact if you add a negative 5 degree cant, you reduce the drift effect significantly right through the range giving a more perpendicular adjustment for range!!!!!

The common assumption is if you cant the rifle 1 degree then that will move the projectile 1 degree * drop (in this case 274 inches or "drop x tan (cant angle)" which would be 4.8 inches in the above case. I believe this is incorrect but am hoping someone with real knowledge on the subject can chime in and try and explain to the masses why??. EVEN if we can't understand "cant" at least we could then all understand the final effect and adjust for it correctly.

After much deliberation over the last few days I (think)can picture it but can't describe it.

CAM

 

[camac]

The more I think about it the worse I get myself into an angle tangle. I have attached (hopefully)a word doc with a couple of quick sketches. The top diagram is the basic assumption that if you sight the rifle in and then add an overexagerated cant. Bullet path (B) and sight line (A). I think in actual effect it is more like the bottom diagram. The bullet is launched up and towards the scope and therefore further away from the sighting line?? but I must be wrong!!

 

[queen_stick]

The top drawing in your word doc is correct. If you can't the rifle, the POA and POI will be different. The bullet path follows an arc that is vertical. If you can't the rifle, the bullet will follow the exact same vertical arc, but you're pointing the end of the barrel at a different spot than when it was not canted.

As an example...
In a perfect environment, with a rifle setup with no cant, you will shoot and hit the center of the bulls. In the same environment, cant the rifle clockwise 10 degrees. You will still aim at the center of the bulls on the target, but you're bullet will hit high and left of the bulls.
This occurs because the distance between the center of the sight (crosshair) and the center of the bore does not change when the rifle cant angle is changed. That means your sight follows the arc of the radius from the center of the bore to the center of the sight. When it's canted, the sight will move to the side and down at the same time (following the arc), changing your point of impact. Your point of aim did not change, but by adding cant, you changed your point of impact because the distance from the bore to the center of the crosshair has not changed.

I hope that makes sense. I had to re-read it multiple times to make sure I explained it accurately. Hopefully some others will chime in to the conversation.

Walt

ps... there was an article on cant in the daily bulletin within the past few weeks...

 

[camac]

Thanks Walt. That is the way I have always thought of it but why is the calculation in JBM so different? If it is like the top diagram then the 6.5mm projectile dropping 274 inches should move 4.8 inches for 1 degree of cant. in the calculation it moves only 0.7.

I actually use a scope level and try and get it as level as possible (or at least hold same cant once sighted in) and would have thought it was more significant than this but to be quite honest I have never shot in conditions that would let me really see this level of difference at 1000.
The interesting thing in the calcs is that we probably should be putting a little negative cant to counterbalance some of the drift so that the projectile falls down the plane of the vertical reticle (so that adjustments are more "elevation" and less windage). Thanks again for the input and I hope I can draw some more in. It is a hard one to describe as you said.
Thanks alsofor the article tip - nice to see some real life targets although quite exagerated cant.
A couple of the diagrams in other links were more like bottom one but that just means calculations are larger...

 

[BoydAllen]

Sometimes I think that we make too much of values that are calculated, but have not been verified. I may be incorrect but I would think that if you simply tested at 100 yards, you could extrapolate that to longer ranges. Recently, there was an item in the daily bulletin on this subject. It featured actual targets that had been shot. Maybe the curious could benefit from searching it out. Again, if you have a rifle that is reliably accurate, why not just test?

 

[camac]

I have no doubt cant exists, Just wondering what is the real effect (more for interest than practical purposes). But the calculations and peoples theories vary so much it would be good to get some discussion. There are quite a few articles by people written on the subject. Some with very simplified formulas and view of cant. (like my top drawing) others with more complicated ones (which I suspect is getting closer to the reality. As long as whenever you sight your rifle in you keep cant consistant then no problem (whichever way you do it) but there is a big differecne in 0.7 to 4.8 inches for every degree of cant. Who is right??? Verifying at 100 with exagerated cant (20-30 degrees) seems a little pointless. In real conditions we are talking 1-2 degrees (maybe 5) if you are aware of it. ANd obviously the long range guys should be affected most. But verfying at 1000 is pretty hard unless you find a perfectly calm day (doesn't happen here). I believe in it, do everything I can to avoid it but wonder how much it really affects us?? ie at 1-2 degrees.

 

[hmcsr]

Cam,
This site doesn't answer your question about JBM's numbers, but does discuss the effects of cant. Plugs their product too.
http://www.riflescopelevel.com/cant_errors.html#vha

 

[NateG]

Camac,

I believe the proper formula for horizontal canting error is drop * sin (cant). The vertical error is drop * (cos (cant) - 1). As your rotate the rifle, with sights still centered, just rotated, you trace a circle with radius equal to the drop at the range. (Using tangent means that at 90 degrees rotation, you approach an infinite windage error--sin means that at 90 degrees you are drop inches in windage error, and drop inches low (assuming zeroed with no cant))

So I end up with 4.78 inches of windage error in your example. (and .04" elevation error)

 

[camac]

Thanks Nate for the formulas and link hmcsr (although I am always a little wary of those that plug their own product) I will ignore the 0.04" elevation error at 1000yds ( ;D).

I also have entered the wrong figures in JBM (essentially came from me using JBM incorrectly - I have used it for a while for drop using my no wind zero of 100yds and calculating wind and elevation from there. My first attempts at CANT calcs were incorrect.) I had incorrect drop and zero.

Note - for correct drop, should have zero height on scope and sight in for 1m!!!! SHould have been 300inch drop

2nd JBM calc then for 2 inch high scope, sighted in for 1000 gives 5.2 inches and Nates formula gives 5.23 inches.

This essentially was where my error in JBM was too - my fault!!!!! not theirs. SORRY everyone....


SO 1 degree cant gives 5 + inches at 1000 -certainly worth wrorrying about!!!!