Need help with a ballistics question

[Nightcrawler]

Okay, being an avid fan of the movie Aliens, a writer for Planet Aliens vs. Predator, and a closet gun geek, I've often pondered the good ol' M41A Pulse Rifle from the movie.

Now, the weapon in the movie was made from a dressed up M1A1 Thompson, with a Reminton 870 attached under the barrel to form the pump action grenade launcher.

In the movie/games, it fires a 10x24mm caseless ammunition. Now, 10mm is rather big for a rifle bullet, but I'm going to say that with 22nd century technology and propellants, it can be propelled to suitable velocities for a rifle and the recoil can be tamed by an absorbtion system of some sort.

Without stretching things too far, let's say that the 10mm bullet is pointed, to make it aerodynamic, with a nominal weight of 150 grains. Let's say the propellant pushes it out to 2300 feet per second, which is fast enough for a 300 meters and in assault rifle. Naturally, a lighter bullet could be pushed faster and would be flatter shooting.

I'm not sure about how to calculate ballistic coefficients, but can anybody give me an approximate trajectory from the numbers I provided? Remember, the bullet isn't flat or round nosed, like a pistol round, it's pointed, like a rifle bullet.

 

[Art Eatman]

Aw, split the difference between a thutty-thutty and a .38-55. The BC won't be as good as the .30-30, but the MV is better than the .38-55.

, Art

 

[Bruegger]

Nightcrawler -- do you have the Sierra reloading manual? They have a series of charts in the back that give ballistics data for bullets of known weight and BC at varying muzzle velocities. You could select a bullet that's closest to a 150 gr, .41 cal spitzer at 2300 and there you go. These charts also let you select various point blank zeroes.

I think 150 grains is probably a bit light for a 10mm bullet, but maybe the core is not made out of lead. I guess in the future, bullets will have to be nonpolluting.

Then again, maybe it's a discarding sabot design, in which case, it could weigh significantly less than 150 grains, maybe with a tungsten penetrator. This would make a lot of sense, since in the future, I'd bet the bad guys wear some really good body armor.

If you assume a DS design, just use the Sierra charts for an appropriate smaller-caliber rifle bullet.

Semper fi,
Bruegger out.

 

[Mike Irwin]

In order to figure the ballistic coefficient, probably the easiest way to calculate it is to record the velocity loss over a specified distance.

That's usually done with a pair (or even three) chronographs.

Knowing the velocity loss, it's a pretty straightforward set of calculations to arrive at the BC.

Remember, though, that the BC will change as the atmospheric conditions change.

Apparent BC will be higher in a thinner atmosphere, say one that is being created by process replicators, or lower in a thicker atmosphere, say one dominated by sulphur dioxide and methane.

Battle your aliens accordingly.

 

[hksigwalther]

Wasn't that round also stainless steel tipped for armor piercing?

 

[Art Eatman]

Hey, Mike! Isn't the ballistic coefficient of a bullet a function of the length, shape and diameter? Rather than the medium through which it travels? (Guess I'm gonna have to dig through my Sierra book.) Or is the medium built into the equation?

Obviously--I think it would be obvious--a cubical bullet will perform as well as a spitzer boat-tail if in vacuum; but, again obviously, the reverse is true in water.

(Maybe the equation(s) for BC was developed for a relatively narrow range of air density?)

Next question, for the physics majors: Given that a conventional chemical weapon won't achieve penetration of an armored space suit. Which would be most change an enemy's path, light and fast or heavy and slow, as to projectile?

, Art