Re: gun angle

PD <TheDraperFamily@xxxxxxxxx> wrote:

> j...@xxxxxxxxxxxxxxxxxxxx wrote:
> > hfs2@xxxxxxxxx wrote:
> > > If you shoot a 22 rifle straight up, the bullet returns to earth at
> > > Vv, whatever that is. What is the minimum elevation of the
> > > rifle to insure that the forward velocity of the bullet, Vh, is
> > > no greater than (close to) Vv when it hits the ground; a speed
> > > that doesn't have a good chance of hurting anyone. No
> > > hills, just level ground all around.
> >
> > > With an elevation of zero, the rifle parallel to the ground, I suspect
> > > Vh is a good percentage of the muzzle velocity when the bullet hits.
> >
> > > I just need a general answer to this.
> > > Be off by xx%? This is not a homework assignment. I know
> > > that this must be a complicated question, because everyone
> > > refuses to answer it.... after telling me how smart they are.
> > > Maybe it can't be answered, maybe it can only be measured.
> >
> > > What info I have. Velocity is around 1,138 fps and the round is
> > > about 40 grains
> >
> > > Thanks
> >
> > After WWI the US Army got concerned about troop safety from airplanes
> > shooting at each other over the troops and troops shooting up at airplanes.
> >
> > The Army sent a guy named Hatcher, some troops, and some guns to what
> > was a deserted beach at the time to do experiments.
> >
> > In 1947 Hatcher wrote the first edition of "Hatcher's Notebook" which
> > among many other things dealing with small arms, gives the details of his
> > findings.
> >
> > The book is still available and can be found in some libraries.
> >
> > Short synopses:
> >
> > Bullets fall to earth with a lot less velocity than most people believe.
> >
> > A .30cal rifle bullet has a terminal velocity of about 300 fps; a .22
> > would be a lot less.

> This isn't obvious. The terminal velocity for a cylinder is
> sqrt[2*L*(p/p0)/C_d], where L is the length of the cylinder, p/p0 is
> the ratio of bullet and air densities, and C_d is an aerodynamic drag
> coefficient. The ratio of densities will be about the same for the two
> rounds, and L will be be higher for the rifle bullet (but don't forget
> the square root), and C_d I would wager would be within 20% for the two
> cases. I'd be suprised if the difference between the two terminal
> velocities is more than 25%.

Except bullets lose spin, destabilize, and start to tumble before they
start falling, which makes the calculations messy at best.

> > The muzzle velocity of a .30cal rifle is around
> > 2500 fps and on the order of 150 grains.
> >
> > A falling bullet is not a particular hazard unless it hits you on the
> > unprotected head.

> Which is 50% of your cross-section if a bullet is headed straight down.

> By the way, 300 fps is about 200 miles per hour, nothing to sneeze at.
> If you want a mental comparison, consider the sound of a pebble kicked
> up on the highway and hitting the windshield of your car at one third
> that speed.

> PD

Sounds impressive until you concider energy.

A 150gr bullet at 300 fps is about 30 ft-lbs.

A 40gr bullet at 300 fps is about 10 ft-lbs.

A standard 5oz baseball thrown at 90 MPH by a good pitcher is about
80 ft-lbs (unless I screwed the in-my-head math).

A fastball to an unprotected head could be deadly but to the shoulder
is a sore shoulder for several days.

--
Jim Pennino

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Relevant Pages

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  • Re: gun angle
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    (sci.physics)