Trajectory Appreciation for Unconventional Positions

*This is in respons to a question in another thread that I thought would be a bit of a hijack if I replied in that thread, so I started a new thread.*

I have done a bit of shooting from weird positions to see what I can get out of them. To try to get solid info I did it with 1X optics and irons as well as with a 4X ACOG. There is a little bit of difference between the two categories, but mostly due to zero distance than magnification.

Here's how I explain it to guys that go through my courses-
When your rifle is in it's "normal" orientation the bullet travels upward at a determined angle to meet your line of sight at the initial point, continues up until it reaches the culminating point, and drops to meet the line of sight again at the true zero range (since we zero at the second cross-over with most optics to ensure maximum efficiency).

http://i296.photobucket.com/albums/m...rajectory1.jpg

However, when we turn the gun sideways (let's take the urban/roll-over prone for the example since it is at or very close to a 90 degree offset), the bullet's trajectory will never cross-over the horizontal plane of the sight other than as it leaves the muzzle.
Why?
Beacuse when the gun is rolled over the barrel sight relationship will still be the same, that is the bore will still point at the same angle to intersect the line of sight, however the trajectory of the bullet will be affected by gravity and the relevant angles differently than when upright. The bullet will still leave the barrel at the same angle in relation to the bore/sight relationship, and will follow that angle until the bullet comes to rest instead of being pulled "down" to cross the line of sight at the initial and zero points as when fired slightly upward.

http://i296.photobucket.com/albums/m...rajectory3.jpg

What this boils down to is that the bullet will cross the vertical sight plane (not the horizontal sight plane) closer to the muzzle (though not by a whole lot, depending on zero distance). It will be slightly low at close range because the bullet will essentially be launched horizontally instead of at a slightly upward angle and will therefore begin "falling" immediately upon exit from the barrel.

http://i296.photobucket.com/albums/m...rajectory2.jpg

Here are rough numbers if the gun is rolled 90 degrees to the right and fired with an Aimpoint at lower 1/3 cowitness zeroed for 200 yards.

RANGE__DROP__L/R inches in relation to Dot
10______0.0___L 2.2
20______-0.1___L 1.7
30______-0.2___L 1.1
40______-0.3___L 0.6
50______-0.5___0
60______-0.7___R 0.6
70______-0.9___R 1.1
80______-1.2___R 1.7
90______-1.5___R 2.2
100_____-1.9___R 2.8
110_____-2.3___R 3.4
120_____-2.8___R 3.9
130_____-3.3___R 4.5
140_____-3.9___R 5.0
150_____-4.5___R 5.6
160_____-5.2___R 6.2
170_____-5.9___R 6.7
180_____-6.7___R 7.3
190_____-7.5___R 7.8
200_____-8.4___R 8.4
(The above is from a ballisitc calculator, but is very close to what happens in actual shooting, in my experience)

The rule of thumb is from 0 to about 75 hold high center torso.
From 75 to about 150 hold high on the inside pectoral.
From 150 to 200 hold on the inside shoulder (that is-the dot is in the middle of the shoulder, not the edge of the shoulder).
This will get you good central hits only needing to rmember three general areas- close, over 75, and over 150.

If you use a 100 yard/meter zero things are a little easier (I will just show 25 yard increments). Here is the ballisitc calculator's numbers for a TA-31, zeroed at 100 meters, 90 degree offest to the right, from the tip of the chevron.

RANGE__DROP__L/R inches in relation to chevron tip
0______0.0____L 2.8
25_____-0.1____L 1.6
50_____-0.5____L 0.4
75_____-1.1____R 0.7
100____-1.9____R 1.9
125____-3.1____R 3.0
150____-4.5____R 4.2
175____-6.3____R 5.4
200____-8.4____R 6.5

(Vertical plane cross-over at 60 yds)
It's a bit easier in that you can hold high center out to about 125, and outside that just hold on the inside shoulder joint.

Of course, with practice and experience you will get a better feel for your holds for greater precision if needed and longer distances, but I really consider 200 yards to be the limit of effective use due to the rapid drop you will see past that as well as the difficulty you will have in the precise range estimation you will need for hold-off and hold-over. Throw a little wind into the mix and you are pretty screwed.

People will tend to see a distinct offset at closer ranges (out to about 30 yards, depending on shooter skill) because they will be forming identifiable groups. At longer distances the groups tend to loosen up, as well as the fact that they are shooting very close to the vertical plane of the sights. Since there is essentially no offset to be accounted for out to about 100 (depending on zero), on steel it all seems to be right up the center (and for practical application it is good enough).

Just added some pics, hopefully I haven't been celebrating our independance too much to make something useful :D.

Thanks, great info brother:D

F2S, this is the best info on the subject I have read to date. The only info in fact, that made sense in my mind. Thanks for the in depth explanation. :cool:

This should be sticked so the info isn't lost.

Thanks F2S, great illustrations and explanations!

Thanks F2S, great info. What ballistic calculator do you use?Mine does not have a cant function. I'll try to work out the same tables you did but for 15inch barreld Tavors.

very interesting - that's something i haven't given much thought as most shots i've taken rolled over have always been under 50 yards.
by the way, chuck norris doesn't need this info. when he cants his rifle, the earth's gravitional pull cants with him. :p

thank for posting this! never seen it brought up in depth.

Quote:

---

Originally Posted by pretorian

Thanks F2S, great info. What ballistic calculator do you use?Mine does not have a cant function. I'll try to work out the same tables you did but for 15inch barreld Tavors.

---

The barrel length is really one of the least important things. The biggest issues will revolve around zero distance and LOS over bore. What you really want to get is a feel for the POI, which the rules of thumb should do to get you close enough to get good effective hits out to 100 meters.

Quote:

---

Originally Posted by militarymoron

very interesting - that's something i haven't given much thought as most shots i've taken rolled over have always been under 50 yards.
by the way, chuck norris doesn't need this info. when he cants his rifle, the earth's gravitional pull cants with him. :p

thank for posting this! never seen it brought up in depth.

---

MM- yeah, it's one of those things that I wanted to intimately understand when I first learned it. The more I learned and the more I shot the more I realized that you don't want to be doing all kinds of mathematical work while you are on your side getting shot-up. My goal was to simplify the problem out to maximum effective range for the position under realistic conditions.

As such (depending on your job of course), 200 became my determination for being the farthest you can really expect to apply the technique, with 75 being the max for a dynamic gunfight- which is stil well within the "put the dot high center chest and apply smooth rapid reciprocating pressure to the trigger" zone.

ETA- there was a recent thread here or somewhere else regarding the use of alternate prones. I look at alternate prone positions as being applicable when the threat is far enough away that hampering mobility is preferable in order to be able to employ concealment/cover or observation. If the guy is 7 yards away and mobile, it's probably a bad idea to lie down unless you have a buddy or ten covering your ass. Just sayin.

Awesome write up F2S. This type of shooting is rarely addressed in terms of how it affects trajectory and this was great info. Thanks for taking the time to do the testing and share it with everyone. Great work!

Hey F2S,What ballistic calculator do you use?Mine does not have a cant function.

Quote:

---

Originally Posted by pretorian

Hey F2S,What ballistic calculator do you use?Mine does not have a cant function.

---

Using a ballistic calculator to determine your exact holds is kinda pointless given the nature of the use of the positions, thus the reason for the post- where to hold generally and why. All the time spent behind the keyboard would translate to skill better if you just go train.

However, I used the JBM Ballistic calculator, to actually answer your question.

Now, before everybody jumps over there and starts jamming numbers into the matrix I highly recommend that you look at the numbers I posted and how that translates to actual use and go shoot it.

For those interested, this calculator includes a Cant angle feature from -90 to+90 degrees.

http://www.jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

What if you used the windage marks on an ACOG or S&B?

Funny, I forgot about this for a while.

Re: windage marks on ACOGs amd the like:
Doesn't do much, since the projectile is still approaching the reticle from the bottom, and there isn't enough drop to make the side windage marks relevant.

There are some local guys that have been doing this for awhile for 3 gunning. They've told me to just hold high and towards the mag. Meaning if the rifle is on it's left side, hold high right. Of coarse we aren't getting shot at and our targets are 24x18 or something like that and we are usually making those type of forced shots at 100-150 yards.

I appreciate your explanation. A lot of techniques like this go untaught or out of thought with most people. I had never thought about it until 6 months ago when I was about to shoot a stage that had a forced 90 degree canted rifle shot. Then I thought my head was going to explode trying to reason where, why, and how the bullet was going to fly relative to my sight picture.