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Old 07-04-2019, 07:56 AM
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Shooting Downhill from My Deck



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I have plenty of room to shoot whatever bows and firearms I want. I have lots of options for shooting but one that I want to pursue due to extreme convenience, is shooting from the deck of my house.

My deck railing is 16 feet from the ground. My immediate yard falls off about 100 yards to the woods edge. It would be very convenient to set up a shooting bench on the deck to plink at targets.

I worry about the utility of sighting in and shooting downhill and what that would mean to my accuracy on level shots. Although as a squirrel hunter, most of my shots are far from level...straight up or very steep, greater than 45 degree angles usually.

I did some googling and found lots of information. Some of it took me back to trigonometry days.

I got out my archery rangefinder, that provides slope distances compared to horizontal. I ranged level, well up a tree on the woods line. It was 93 yards. Then I ranged to the ground at the bottom of the tree. It was 96 yards. Wow, only a 3-yard difference. I'm very comfortable with that not being enough of a difference to impact POI enough for my purposes. I was surprised the difference was so small.

I checked this using the math. (A sq. + B sq. = C squared)

B=93 yards
C= 96 yards
A= ~22 yards (this would be how high I am, up the hill, standing on my deck from the base of that tree (66 feet - this was the unknown in the equation)

Sound right or am I missing something?
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Old 07-04-2019, 08:02 AM
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Quote:
Originally Posted by bowwild View Post
Sound right or am I missing something?


It takes a very steep angle to make a significant difference.
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Old 07-04-2019, 08:57 AM
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Your math seems correct.

I've been thinking about doing the same thing, that is moving one of my portable shooting tables to the deck. My deck, however, is only about 4' off the ground. I had considered placing the shooting table on the ground beside the deck, but you have convinced me that on the deck itself would be more convenient (no steps to climb.)

You must have a really neat range finder.
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Old 07-04-2019, 09:19 AM
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The OP's shooting angle to the bottom of the tree is Inv(Cos(93/96)) = 14.36 degrees downhill. Not very extreme.

In addition to needing an extreme angle, you would need a fairly long distance to make much of a difference.

It makes logical sense to me that there would be more bullet drop aiming downhill than aiming uphill, but ballistic calculators report the same drop either way. So I did the math for my 30.06 at 200 yds and 30 degrees up/down. I was correct. The bullet does drop more aiming downhill. But the difference was only 0.1". Not enough error to make a difference in the hunting angles and distances I'm ever likely to encounter. Just for fun, I extended the range to 1000 yds and found the error only increased to 0.5".

The reason the calculators can ignore the error factor is the difference in angle between LOS and Bore Axis is very, very tiny.
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Old 07-04-2019, 09:35 AM
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"Sound right or am I missing something?"

Hi Bowwild, I'm a salesman not a mathematician but I don't believe that you can use the Pythagorean Theorem on a scalene triangle.

My breakfast napkin diagrams aren't helping much except to lead me to agree with you and Sophia that the effect of such a small angular difference isn't going to mean much in terms of POI.

My own experience with deck shooting is that other humans and animals tromping around on it while you shoot will affect accuracy much more than any minor elevation difference.

Frank
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Old 07-04-2019, 11:55 AM
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You are correct LTCrunch.

Don't know why, but I was thinking right triangle with my problem, but my yard is sloped downward and all the angles are different as are all the sides.

Thanks for pointing this out.
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Old 07-04-2019, 12:19 PM
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Quote:
Originally Posted by bowwild View Post
I got out my archery rangefinder, that provides slope distances compared to horizontal. I ranged level, well up a tree on the woods line.
I assumed this rangefinder had some way of determining level. If true, and if this tree grew pretty vertical, a right triangle was formed - not scalene.
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Old 07-04-2019, 02:14 PM
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Seems like this had been said but the angles and distances the OP is talking about are not significant enough to make a difference that would require much correction. It is a good exercise and practice to shoot at different angles, from a practical perspective.

I shoot an archery match in a canyon a few times a year and most of the shots are uphill vs level. The more extreme the angle the more correction (hold over) is needed, both uphill and down. Uphill moreso with an arrow as the flight path is more linear than a bullet not to mention arrows are heavier and travel slower in most cases.

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Old 07-04-2019, 04:09 PM
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Yeah, with a bow its a huge difference, but with a gun, its not as much as people think, then they over think it then over do it. For instance, when shooting down hill, you are basically shooting the distance from the base of your shooting platform at the ground, to the target, Just like in archery. So if the distance only changes 5 yards, now imagine if you would make any aim point changes just for 5 yards.
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Old 07-05-2019, 09:06 AM
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Shooting down hill would have an advantage of being somewhat safer, IMO.

When I want to check a gun for function, I just walk to the edge of the deck and shoot almost straight down.
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Old 07-05-2019, 10:01 AM
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You can't JUST use the horizontal vector to your target. The bullet is still traveling 100 yards, or whatever your rangefinder tells you... So, with a slow bullet like a .22LR, even though slope and gravity become factors... their affect is semi mitigated by the fact the bullet is still traveling (and slowing) through the full 100 yards of air resistance.

In other words, it takes a serious angle to make a significant difference in aim point. Archery is a different ball game, as they have a seriously 'arching' trajectory.
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Old 07-05-2019, 10:08 AM
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Just for giggles I look at this with my ballistic calculator. Using a .22LR round at 1057 FPS and 100 yards I found this:

angle Angle drop
10 .26
14 .51
20 1.04
30 2.30
45 5.03

If I used a 1250 FPS round the difference would be:

angle angle drop
10 .20
14 .39
20 .79
30 1.64
45 3.83

If the angle is downhill the "angle drop" would be high and if the angle is uphill the "angle drop" would be low.
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Old 07-05-2019, 11:25 AM
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Quote:
Originally Posted by Test_Engineer View Post
I assumed this rangefinder had some way of determining level. If true, and if this tree grew pretty vertical, a right triangle was formed - not scalene.
I'm sure it does. The one I have tells you the angle as well as figuring that into the drop (based on a library of trajectories selected by the user). Cell phones have levels built into them nowadays; part of knowing when the phone is being lifted from the table. if your phone is like mine, it can actually be used just like a bubble level (there's an app for that!).
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Old 07-07-2019, 04:38 PM
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I have a Vortex. It has the function to tell you the exact distance or the apparent distance due to the angle but it doesn't tell you the angle. If you think about it, you don't really need to know the angle just the apparent distance.
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