Tilting your rifle...bullet flight path?

[chris in va]

Here's something I haven't seen mentioned before.

If I shoot my AR in a normal fashion, my neck starts hurting, and I've mentioned this in previous posts. My other bolt guns don't cause this. I got to thinking and tried tilting my scoped AR about 20 degrees, and it is a TON more comfortable.

My question...how would this affect accuracy at say, 100 yards? Would the bullet's POI be thrown off? I don't paper punch, just steels and whatever I can find. Folks running 3-gun seem to shoot better with the rifle tilted but I doubt its a comfort issue.

 

[Jimro]

At 100 yards you won't see too much effect, at least with an AR shooting 223. The bullet is still "rising" at that point, so your point of impact will be just to the right and slightly above the point of aim, but easily still on target.

At 600 yards, that goes way out the window. When your vertical alignment is off the plane of the rifle, your bullet will impace WAY to the left because of the horizontal component to the vertical offset.

All of this assumes you are a normal right handed right eyed shooter, just swap left and right if you shoot from the left.

Now you can get good hits at distance as long as your cant is repeatable with the rifle (using a spirit level can help) and you verify your horizontal scope adjustments needed per range to correct for the cant.

Using some back of the napkin math, with a 2.5 inch height above bore, the horizontal correction should be 0.9 inches at 100, or 0.9 MOA per 100 yards. When you get to 600 yards you can see that you are 5.4 minutes off the target by then, or 32 inches. If someone wants to check my math, Tan(20)*2.5inches=0.9. It's a rough approximation because I didn't use a curved portion of the horizontal displacement and approximated a straight line. Could be my math logic is all screwed up too, but that is my first guess, maybe I can refine it in the future.

Hope this is helpful, or at least food for thought.

Jimro

 

[Bake]

Wrong Thread!!!

 

[chris in va]

My range is 300 yards and muddy as heck, so 100-150 yards is far as I care to hike setting targets. If it'll just be an inch off at that distance, I'm fine with that.

 

[skizzums]

I am curious about this subject myself. I shoot almost exclusively on big rocky hills out to 600 yard and sometimes its just impossible to get my rifle perfectly leveled on a bipod. Always wondered how much it changes things

 

[Snyper]

The effects will vary depending on the trajectory of your particular load, and the zero distance.

 

[Mobuck]

Tilting the rifle will put the POI off by varying amounts depending on the "tilt". Unfortunately, it's difficult to "tilt" the same every time so POI is variable. This is NOT conducive to consistent accuracy.
Firearms(especially the AR due to it's significant difference between sight line and bore axis) depend on the sights being consistently in the same relation to the bore. This doesn't necessarily have to be a vertical relationship but vertical is easier to replicate.
Picture the normal bullet path vs the sight line of the AR. When vertical, the bullet passes through the sight line at a given distance. Tilt the rifle and visualize where that bullet is now going(still trying to cross the sight line but now gravity is acting on a different plane). The bullet will hit somewhere left(assuming right handed shooter tilting sight to left) and low since the sights are now lower than if held vertical.

 

[g.willikers]

Folks running 3-gun seem to shoot better with the rifle tilted but I doubt its a comfort issue.

They're using multiple sights and one is mounted to the side, in addition to the one(s) on top.
Like this:
https://www.youtube.com/watch?v=cg94TKKv8no

 

[Panfisher]

Is the scope mounted too high for comfort to begin with, or maybe add a cheek piece to the stock. Might be able to eliminate the need to cant the rifle.

 

[DPI7800]

If I remember correctly Todd Hodnett of Accuracy 1st has found that with a 2.5 degree cant on a rifle the impact will result in a .5mil shift at 500yards.

 

[Bart B.]

Sine of the rifle's cant (tilt?) angle multiplied by bullet drop at target range equals horizontal shot hole displacement. Use the cosine of the angle to calculate the vertical displacement; it's always down but insignificant.

1 degree cant angle has a sine of about .017, drop at 1000 yards of 350", so horizontal displacement is about 6 inches; 6/10ths MOA.

Use a calculator to do your math for other cant angles and bullet drops. For all practical purposes, it doesn't matter how high the line of sight is above the bore axis.

I've used spirit levels on front sights calibrated so I knew how much to cant the rifle for a 1 MOA windage correction at 600 or 1000 yards in team matches so I wouldn't have to get out of position to make a sight correction. Such sights keeps windage shot spread minimized. Most people cannot discern any less than a 2 to 3 degree cant

 

[Jimro]

Bart B.

When the hypothenus and adjacent sides of a triangle are close in length (small angles) then either function should be close for calculating angular displacement.

Using sin would be Sin(20)*2.5= 0.855 inches of horizontal displacement, so only different from the tangent math I guestimated with by 0.005 moa.

If 1 degree of cant is 6/10ths of an MOA, then 20 degrees should be 12 MOA, which is a bit over twice the correction I estimated. But I'm also going to guess that at shorter ranges that relationship isn't as pronounced since the bullet should still be rising through the zero (if the rifle is zeroed with a 200 or 300 meter zero as is normal battlewight zero with AR-15s).

With a perfect zero at 100, the sights and bullet impact. So the angular difference between them is defined by the distance of height above bore. This means that the if the rifle were turned 90 degrees, the shot would be low and around 2.5 inches displaced horizontally. I just can't see a 223 being 54 inches off horizontal at 100 in my head.

Sounds like some experimentation is in order.

Jimro

 

[Bart B.]

Bullet drop from bore axis at target range is the number to use. At 100 yards, it's a few inches. At 1000, it's a few hundred inches. Sine of 20 deg is .34. That times a 2 inch drop at 100 yards is .68" horizontal displacement. For a 350 inch drop at 1000, a 20 deg cant causes more displacement; .34 times 350 equals 119"; almost a 12 MOA horizontal displacement compared to .68 MOA at 100 yards.

Canting a rifle through 360 deg shooting a bullet every few degrees will produce a group with a diameter twice the bullet drop at target range plus sight height. For the above examples, it would be 5.5" with a 1.5" sight height. 701.5" at 1000 yards.

It works like a giant compass drawing circles on paper with the distance from hinge pound to sharp end like the line of sight and from hinge point to pencil lead tip the line of fire (bore axis).

 

[Jimro]

Bart B.,

My understanding is that the shape of the impacts would be an elongated elipse with the very top of the elipse being when the rifle isn't canted. The actual dimensions of the elipse would be dependent on range, much like the beaten zone from a machine gun.

Jimro

 

[Bart B.]

That ring of shot holes is a near perfect circle. It's top is the point of aim and bottom twice bullet drop below it. Right and left limits are bullet drop distances equally to the sides and below point of aim. If the rifle's inverted 180 degrees, bullet impact is twice bullet drop straight down; 700" low at 1000 yards for that 350" drop below where the muzzle axis is.

 

[T. O'Heir]

"...my neck starts hurting..." Butt stock isn't right for you. Standard scoped AR's tend to have the scope very high and that puts your head in the wrong place. Lose the scope or change the stock to one made for scope use. A flat top is made for scopes.
"...seem to shoot better with the rifle tilted..." No they don't. And neither do the TV gangbangers who hold their stolen, illegal firearms sideways.
The physics of a bullet's flight doesn't change, but your scope's reticles are designed to work vertically and horizontally. Anything else will not allow you to sight in accurately.
"...Sine of the...the cosine...the hypotenuse and...displacement...tangent math..." I can find you guys and I have a stick. snicker.

 

[Jimro]

Bart B.,

That ring of shot holes is a near perfect circle. It's top is the point of aim and bottom twice bullet drop below it. Right and left limits are bullet drop distances equally to the sides and below point of aim. If the rifle's inverted 180 degrees, bullet impact is twice bullet drop straight down; 700" low at 1000 yards for that 350" drop below where the muzzle axis is.

I could see that at close range with a low line of sight over bore, or with a laser boresighter being used instead of bullets.

But with no cant the barrel is pointed UP, and when it is inverted 180 degrees it is pointed DOWN. The effect of gravity means that it is a net decellerating force (stopping the rise of the bullet) when the cant is betwen 270 and 90 degrees, and that gravity is a net accellerating force meaning the vertical travel is greater, which will "stretch out the bottom" a bit. I couldn't tell you how much but, I can say that as the range increases the stretch must increase.

The line of bore will make a perfect circle, but the point if impact has to get stretched out due to gravity.

Jimro

 

[Bart B.]

Jimro, with a bullet dropping only 350" at 1000 yards, the cone of fire is a bit over 1 degree. The shape of the circle of bullet holes will elongate an insignificant amount with the rifle spun about its line of sight to a horizontal target. Maybe 3% at most is all the bottom of that circle would extend.

Interesting thoughts, though, one could see the increase in bullet drop by comparing a ballistics program with the muzzle axis 35 MOA above the horizontal then 35 MOA below it and see what the bullet drop would be at 1000 yards for each.

Just ran Sierra's software, saw that a 20 degree down angle increased bullet drop from bore axis .17" more than when fired level; 356.5" bullet drop for a Sierra 30 caliber 220 HPMK leaving at 2400 fps.

 

[Jimro]

Bart B.,

Uncanted the line of bore is above the line of sight. The bullet will be rising or falling as it intersects the line of sight. Flip the rifle upside down and now the line of bore is below the line of sight, and there is no vertical up component imparted onto the bullet and what was the vertical velocity component is now transmitted down instead of up. Working with gravity instead of against it.

The Sierra software isn't measuring the angular disance between the sight and bore, it is measuring the angle of the shot, which is why there is so little difference. The cosine of the angle of the shot is the calculated distance for drop for scope adjustment because it gives you the effective horizontal distance for bullet flight. Setting the distance to 1000 and then the angle to 20 means that you already have the effective distance, but the scope adjustment would be for a 1064 yard shot.

Of course this is not something that would come up in real life very often, shooting a rifle upside down. For small adjustments near top dead center, a circle is a very good approximation.

Jimro

 

[Bart B.]

I set Sierra's software for a zero sight height above bore and zero range atd 10 yards do do the calculations; that put the line of sight virtually the same as line of fire.

If sight height was 2", that would make little percentage change for the size of the circle, at least in my mind.

Interesting web site page I just found:

http://www.riflescopelevel.com/cant_errors.html