This came out of a conversation I was having with a coworker. If you were parachuting and shooting at the same time, how would that affect your drop? Would you start swinging a lot? A little? If you fired straight down, would it push you up appreciably?
I suppose it would depend on the weapon and the projectile, but I don’t know enough about the forces of guns when fired. Has this ever been studied by the military? I’ve never seen a war movie with paratroopers shooting their way down…
If my math is correct a 170 grain bullet given a velocity of 2000 ft/sec in one direction would give a 185 pound man or woman doing the shooting a velocity in the opposite direction of about 3 in/sec. max. Of course the butt of the gun against the shoulder, or whatever isn’t exactly an elastic system. That is, I think quite a bit of the recoil energy would be lost in deforming flesh.
Very little. Let’s look at an 80kg person moving at terminal velocity (about 120mph).
Momentum = 4300 kg m/s
KE = 115,000J
Handgun firing a 120 grain bullet at 1500fps (that’s on the order of a 9mm)
Momentum = 3.6 kg m/s
KE = 812J
The bullet has very little delta on either momentum or KE. I suppose it could conceivably give you a little spin if you fired to one side but since you can start (or stop) a very fast spin in freefall with only minor adjustments of your arms and legs, the handgun isn’t very useful as a rocket 
There’s some more gun physics here: Physics of automatic weapons, mass of 5.56?
Slightly related and maybe relevant is some things: I´ve read about attack planes, specifically about the P-51 and the A-10, some little math showing how many knots of speed they lost for each second of continuous fire from their machine guns and cannon respectively.
If anyone is interested I can dig it out of my file (I actually have a honest to goodness 50s era metal file cabinet that should weight a metric ton filled with stuff like this 
)
As for a handgun effect on a free falling gunner, I guess a machine gun could put some spin on the situatin, a M-60 for example would excert quite a force, for as long as the ammo lasts.
So the effect isn’t near what I expected - and my coworker was right. There’ll be no living with him now. Although, I wasn’t thinking about freefall - I was thinking someone hanging from a canopy, so their velocity would be less than terminal. Again, I don’t know enough about parachuting or shooting to figure this out.
I am interested. I had heard this too. Supposedly an A-10 (Warthog) flying at low speed has to pay attention to continuous fire of the main cannon as it can potentially slow the plane enough to stall.
That said I can find no cites for that despite a bit of Googling. I did find the following though so perhaps some other Dopers can interpret what it means:
I found another place that said each engine on the A-10 provides 9,000 lbs of thrust (in case that is useful in figuring how the plane reacts when the gun is fired).
For fun here is a picture of the gun on the A-10 next to a Volkswagen and a picture of one of the rounds it shoots (nearly a foot tall…quite large!)
http://www.wpafb.af.mil/museum/research/attack/a6/a10arm-3.jpg
http://www.wpafb.af.mil/museum/research/attack/a6/gau8-1.jpg
It doesn’t look like there’s room between the passenger seat and the rear engine compartment to fit the magazine drum. But if you mounted the gun along the cars’ right side, a 4 ton assymetric recoil would make for great fun in cutting kittys on icy parking lots.
how about a rocket launcher?
Rockets don’t have much recoil. In theory none at all since the exhaust gasses are vented backwards out of the open rear end of the launch tube, if there is one, and not confined as is the case with a gun.
Nitpick: Someone hanging from a parachute is in freefall and at terminal velocity. It’s just a much smaller speed than someone without a parachute
Not exactly. Free-fall as I understand the term is falling with the constant acceleration of gravity so that you are weightless. In a parachute jumpers are not weightless since they are falling at more-or-less constant speed and not constant acceleration.
But you are right in one thing. They definitely are falling at terminal velocity.
After some digging I found both articles.
For the A-10 it states that at 900 grames per bullet and 70 rounds per second the kickback is 5400 Kg, after a burst of a second or a second and a half the airspeed drops 5 knots. No wonder the thing can grind a tank to dust.
As for the P-51 Mustang, actually it says that the loss of speed is rather low, about a knot or less, but gives the math… and Og help me with this units:
The Ke of a 9000 pound P-51 travelling at 440 feet per second (300 MPH) it´s 27 million ft/lb, the Ke of all six guns (.50 machine guns) firing at the same time is 75.000 ft/lb. So an eyeball appreciation of that would be less than a knot of airspeed loss.
Then no object falling through an atmosphere can experience free-fall, due to buoyant and aerodynamic forces.
Hmm, a quick check of the Wikipedia confirms there are two general usages of freefall. There is a strict sense, meaning only being acted upon by gravity. And there is a more general use that allows a small amount of aerodynamic forces, often used in skydiving.
As cool as that would be, it’s not true.
From here.
Did I read that correctly? A 900 gram bullet?
Shit,… that’s a big bullet.
That´s odd, my cite is an interview to a Air Guard pilot called Dan Kuebler, around 1990.
I think another, independent site would be needed to settle this.
A person saying they are (or were) an A-10 pilot backs up what I say. But of course, it is just some guy on a forum, and have no idea if he’s telling the truth. He says it’s on his FAQ on his site, but after tracking down the site , I can’t find the FAQ, let alone the actual original text.
I think the rumour may stem for the fact that early on in the aircraft testing, the huge amount of exhust gasses released by the gun would cause the engines to flameout. (I’m not sure if they ever solved the problem, it may have only manifested itself during long bursts.) The easiest way to explain to a layman about the flamouts would be to say the engine was ‘stalling’, and many people don’t understand the difference between an engine stall and a wing stall (“If they shoot the gun too long, the plane stalls!”) And even if the aircraft was braked by 5 knots by the gun, the pilot would have to be flying damn close to Vs to actually stall the aircraft… Not to mention that the aircraft would already be in a nose-down attitude, which should cause it to be gaining speed.
But that’s just my theory on how this rumour was started… I have no facts to back it up… So it’s entirely possible I live in a fantasy world.
During these travels, I found a forum dedicated to the A-10. It looks like the man who did my cite is a moderator there, but the search doesn’t turn up any relevant info.
… I think I hit submit too soon.
He’s a crew chief, not a pilot. The pilot is “Ragman” “an FCF pilot who flew A-10s for over 15 years.” .
The post I mention to be backing me up, is the 23rd post down… You’ll have to count, sorry (Or you could hit ‘find’ and type in “Ragman”)
Sorry about that.