Lets say I have an AR15 that's zeroed at 100 yards with Mk262 ammunition (LT-104 scope mount and MIL turret adjustments).

If I were to fire this rifle from an inverted position, what should my turret adjustments be for a 100 yard shot?





ETA1:

This is the shooting position (unfortunately for Arfcom, this is not me.  I am a middle-aged, male engineer)


[photo (sort of) courtesy of http://kirstenjoyweiss.com/]

I have never tried to fire a rifle while bending over backwards.
Do you think I'll be able to hit a 5.5" target at 100 yards? And if so, in how many shots?

With this rifle, I can shoot 4.6 MOA(M193) / 3.9 MOA(Mk262) from the sitting position (15 shot groups).
If I can shoot 15 shots from this position, what do you think my max CTC spread will be?
What do you think the POI vs POA will be?

I'm also 40 years old.
Do you think I'm going to break in the process?

I have no Idea, but I am really digging the coffee table.

None...

Why make any adjustments?

POA will still be POI if the zero is for 100 yards.  In either case the bullet path is adjusted to intersect the line of sight at 100 yards.  The bullet will just be going down instead of up but will still intersect the POA at 100 yards.

Flip it 90·

Now have fun.

The TOF is 0.115 seconds.  

Normally, there would be a slight upward component to the velocity.  If I'm firing from an inverted position, there's now going to be a slight downward initial velocity component.

Also, I'd be holding the rifle upside down, with respect to gravity, so the recoil mechanics are going to be different.

I'm going with the negative of whatever elevation you are currently zerod at.

When I shoot M193, I dial in D0.3 MILs.

So, if I were to cheap out and do this with cheaper M193, instead of Mk262, you'd say U0.3 MILs?

+1

I'm gonna need a treadmill to figure this out.

Yup.  Barrel will be canted upwards slightly if zeroed and sights aligned. For purpose of experiment, cant barrel down until bore is aligned with target.  Scope is now aimed under target.  Rotate along barrel axis 180 degrees.  Sight is now aimed above target, but will impact below target.  Elev.  Will...

Changed mind... you would have to adjust elevation more upwards.

I'm drunk, btw.

Bowing out of this thread.

The firing position will be prone, unsupported, with the rifle being fired from the shouldered position. Technically, maybe it's kneeling?  Hard to say.  It's really not something I've ever seen in an Army FM.

I have never tried to fire a rifle while bending over backwards.
Do you think I'll be able to hit a 5.5" target at 100 yards?  And if so, in how many shots?

With this rifle, I can shoot 4.6 MOA(M193) / 3.9 MOA(Mk262) from the sitting position (15 shot groups).
If I can shoot 15 shots from this position, what do you think my max CTC spread will be?
What do you think the POI vs POA will be?

I'm also 40 years old.
Do you think I'm going to break in the process?

I don't think you'd have to change it at all.

The adjustment with the rifle at 90* is harder..

Op, are you actually going to do this?  I will go on record saying you will have to adjust upwards, just not sure how much.  Am interested to see the results.

If I were to try it, I would shoot between my legs with head on a chair.  I was mocking this up as an experiment and my five year old wouldn't stop running up and slapping my ass.

You haven't seen the shooting position.

Nope

My ballpark is like 1 mil.

You do not want to see the results.

A written transcript of the results?  Yes.  
Photographic documentation of the event?  No.

You will definitely be shooting low. I would think it would be by two times the time of flight squared times the acceleration of gravity.

You're tempting me to promote a new barricade position.

I could call it DA (Dat Ass) or KJWDY (Kirsten Joy Weiss Does Yoga).

See the update in the OP.  
Also, be happy that you are not a Life Member.  
As such, you can happily walk away from this site, and pretend that none of this ever, ever happened.

you mean the rifle is inverted as well?  If so then the barrel would now be  2" or so above the scope instead of below it so definitely need to adjust elevation.

I perversely hope you hurt yourself. In the name of science.

I thought about upping the ante and doing this on a bed of nails.

Unfortunately, this firing position pretty much turns it into a bed of nail.

Fail

 

Just a guess, but move the elevation turret about 1.5" 'down' as marked on the scope

You're only 40, you'll be ok.

I'm 46, in fairly not good shape, and 200lbs when I should be 165lbs at 5'10.

At 46, I can say assuming that position would be painful, and may cause permanent damage. I say that from experience, as last Tuesday I did assume that position, completely unwillingly and in a most rapid fashion.

You see, my son has bad headaches and a sleep disorder, so most times when he's home I can't play my bass inside, even unplugged. I go outside to play, where we have several cats.

Well, I went out and sat down in a folding plastic chair and started jamming, albeit quietly, but the cats kept rubbing up against my legs. So, I put my feet up on the table. Great right ? No, not great.

Now the bass body wasn't against the chair, so it wasn't resonating. Being a former genius, decided that the best way to fix this acoustic problem would be bone conduction. So, I bent my neck down, and put my chin on the body, and it was glorious. It was glorious until I reached the bridge on " New Religion" where there is hammering, slapping, popping, and harmonics.

Apparently, I grooved a bit too hard, which resulted in my assuming the above position in record time.

The chair collapsed, with me bent all backwards, over and sideways. Now covered in cats, I could not get up. My hip felt like it had been hit with a sledge hammer, and I was sure that due to the pain in my ass that I had been impaled by an icicle of hard plastic, and with the storm that was quickly approaching, I would bleed out or drown, and become cat food before anyone realized I was missing.

I did manage to get up by rolling around until I got my hands under me.

I'm still hurting,

I couldn't help you with your ballistics question, but so far as the position is concerned, just go really, really slow, have a witness, and remove all pets from the immediate area.

I think you'll be about forty inches low just due to gravity.

Explain thyself.  Will the pull of gravity suddenly increase 10 fold at all of 100 yards if the rifle is inverted?  300 feet traveled at 3000 feet per second means the flight time is all of 1/10 second.  A difference in dope for deep throat prone (I should copyright that!) will be noticed at distances different than the rifles zero (400m poa will be dramatically different for instance) but not at the zeroed distance.

The only thing that will affect point of impact will be the shooters inability to hold the rifle steady and handle recoil at such an awkward angle.  To at least eliminate the awkward hold on the rifle OP should rig up some kind of sling to hang the forearm on that won't affect the inverted sight picture.  Laying on a raised bed of some kind that his head can hang off of would eliminate the yoga contortions and maybe give rise to a new Dick Driver training position that Cory & Erica can cash in on.

PS - In thinking about this the rifle barrel will still be pointed up slightly assuming (I know) that the target is posted at a standard height of 5 feet or so.  The bullet flight angle difference (muzzle to target) when comparing standard prone to deep throat prone is nonexistent.



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lock the gun down on a bench at shoot at zero - barrel is canted upward slightly to compensate for gravity.  Eliminate gravity and you are shooting high - bullet travels a perfectly straight path.  Now, reverse gravity (in effect, turning the gun upside down) and you are shooting even higher.

That may be one of the hottest things I have ever seen!!!

Let's consider a firing range that is perfectly flat, and a rifle that is held in such a way that the LOS for the scope is perfectly horizontal (paralell to the ground)

A bullet starts falling the moment it leaves the barrel.  If you we to fire a rifle with the barrel held perfectly level on flat ground, and drop a bullet at the exact instance that the fired bullet leaves the barrel, they would hit the ground at the same time.

If the LOS of the scope is perfectly level to the ground, The barrel is pointed upward relative to the LOS of the scope to compensate for gravity.

When you turn a rifle upside down, the barrel is now pointed down (assuming the same perfectly level LOS for the scope)

POI will be lower than POA

Sadly, I do not have a suitable pair of yoga pants.

5/8" low

I really don't think I'll be able to group well enough to be able to discern a 5/8" POI shift, but thank you for that vote of confidence.

But we're dealing with the real world not a theoretical flat earth and the target is higher than the barrel regardless of scope orientation. Look at the chick in the photo demonstrating deep throat prone. What is the orientation of her barrel?

OP's scope is adjusted so the LOS will intersect the bullet path at 100 yards. That will still happen. I would encourage him to do some ball and dummy drills to test for drop caused by the awkward position not rifle orientation.

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By a lot

 

Oh lord

Jesus

so what would cause the bullet to climb?

the bullet path is an arch, that is cause by the barrel being canted above the line of sight.

when inverted, the barrel is now canted downward.

the LOS does not cross the bullet path at 100 yds, unless this is a magical bullet that laughs at gravity.

Ballistics on this site is really funny.

It will cross LOS around 25 yards, since it starts out above LOS, but will continue dropping under it, instead of crossing it again like it would normally do for a 100 yd zero.

The rifle is upside down.

The axis of the scope is a straight line from the shooter to the target. The axis of the bore has an upward cant to compensate for the effect of gravity on the bullet's path.

If you invert it, the axis of the scope is still a straight line to the target, but the axis of the bore has a downward cant. The bullet can't cross the axis of the scope.

Plus, gravity.

I have decided that I'm going to do this with M193, instead of Mk262, for two reasons:
1. It's slightly cheaper.
2. I have the words Don't Panic written in friendly letters on my box of M193.

Of course, there is also a 3^rd reason: more math.

For M193, I typically dial in D0.3 MILs for 100 yards, and my ballistics calculator tells me the top of the arc is going to be around 155 yards, with a TOF to 155 yards of 0.153 seconds.  The table tells me that the POI is +2.2 inches above POA.  So, +2.2" POI + 2.625" HOB = +4.825" above the bullet's original starting position at 155 yards.

For the upright condition, the bullet rises +4.825" in 0.153 seconds (per my ballistics table), and has a v_y of 0 in/s at the top of the arc.  Knowing this, we can work the problem backwards to determine the vertical component of the velocity, v_y0, as it leaves the barrel:
HOB + ?POI = v_y0·t_{TOF_155} + ¹/₂·g·(t_{TOF_155})²
where
HOB + ?POI = 4.825"
t_{TOF_155} = 0.153 seconds
thus
v_y0 = 61.1 inches/sec [at this speed, we can ignore aerodynamic drag in the y direction]

Now, let's turn the rifle upside down.  To simplify things, let's also keep the turrets at 0/0.  To really simplify things, let's assume that the rifle's recoil dynamics are unaffected by the upside down orientation [1].  Using this assumption, we can use the 61.1 inches/sec v_y0, and add gravitational effects to this velocity to determine the total shift in y at 100 yards:
delta-y = v_y0·t_{TOF_100} +  ¹/₂·g·(t_{TOF_100})²
where
v_y0 = -61.1 inches/sec
t_{TOF_100} = 0.096 seconds
g = - (32.17 ft/s * 12 in/ft)
thus
delta-y = -10.35 -7.64 inches

So, if I put an orange aiming dot on my target 10 3/8 " 7 5/8"above the X, I might be able to hold my shots in the 5.5" black area of the target.

If my shots fail to land inside the 9 Ring, then everything mankind knows about science - is a lie.
Or, I just don't have the physical or analytical skills to do this.
But, I'm going with the former - because I've always suspected that something was a bit wrong with the universe.


[1] I'm firing the rifle from an unsupported, shouldered condition, but my hand is now applying an upward torque on the rifle, rather than a downward torque.  Will the rifle's recoil still be in the "normal" direction, -y, "up" with respect to the shooter's point of view?  Or, will the rifle still recoil upwards, +y ?  I don't know.


ETA: I was still using a 155 yard TOF in the t^2 term of the equation in my Excel worksheet, and overestimated the delta-y for 100 yards.  Good thing I caught the error - maybe.

I would like to thank the NRA for sending me an Official 100 Yard Inverted Slow Fire Target.

For those of you who may not be NRA members, please make the time and effort to join - as you can see, membership has its privileges.

[after correcting the math error, the orange dot has now been moved to a position 7 5/8" above the X]

If the bipod were installed upside down on the top rail would it interfere with the sight picture? Might be an easy way to remove some of the shooter equation.

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Know how I know there are a bunch of nerdy homo's in this thread???

Really GD? I mean, really? Freaking yoga pants camel toe and boobs all over the OP....and you guys are seriously posting math formulas in an attempt to answer the question?

The answer is BOOBS!

The fucking answer is always boobs.....except for when it's ass, legs, or snatch.

You 1/2 PIE r square mother truckers (not pi, bitches.......PIE!

I am disappoint

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Sure.

It's definitely an option.  At 5x & 10x I can actually see past the top-mounted bipod with my standard nose-to-charging handle weld.  At 2.5x, I can't see anything.

I'm going to first try the unsupported shots.  If that doesn't work out, I'll relocate the bipod.

Tell you what, pal - you call her up and get her over to my range, and I'll happily change the direction of this thread.  Until then, you are stuck with me. Capisce?

If you can't handle the analytical hotness, I suggest you step aside ...
... though, I highly recommend that you stick around for the ensuing train wreck.