ôMuzzle velocityö 8

[Veemax]

I have a question about the “muzzle velocity” of projectiles fired from a gun barrel! I’ve always assumed that the projectile is still accelerating- providing the rate of burn and energy of the charge is sufficient to continue to overcome friction and inertia as it leaves the muzzle- that the projectile velocity is still increasing, even when it’s left the barrel. Am I correct in saying that only the rate of acceleration decreases, ultimately of course- the velocity decreases.

I’ve read several articles in various shooting magazines, that this is not the case! They appear to suggest that the velocity decreases as soon as it leaves the barrel and that maximum velocity is at the muzzle!

Are we to say that the charge and weight of projectile are perfectly balanced- so that the velocity is constant as it leaves the last section of barrel? If this is not the case, then the projectile could still be accelerating at many tens of g’s, as it leaves the barrel.

 

[btrueblood]

This thread needed one more post to hit 100.

This video shows a muzzle blast exiting ahead of, and out-pacing a (admittedly subsonic round) 45-cal pistol bullet.

http://www.youtube.com/watch?v=iTc9Oky4mzA&feature=related

 

[ornerynorsk]

5 ms will equate to a 1.98 inch drop. It's still 33 ft/sec regardless of forward momentum.

 

[IRstuff]

It's actually 32.174 ft/sec², but I'll give you a pass on both the value and the units.

I suggest that you run this through your calculator again:
1/2*386.0886*0.005²

It comes out to 0.0048 inches on mine, or 0.1226 mm. You seem to calculating velocity, and not distance:

386 in/s² * .005 in = 1.93 in/s

TTFN

FAQ731-376

 

[handleman]

Norsk:
Please check your understanding of the basic equations of motion, then post your English units answer.

Bonus to restore engineer cred:
Calculate the start time of the 5ms period in which the bullet actually does drop 1.98 inches.

-handleman, CSWP (The new, easy test)

 

[ornerynorsk]

Yep, a little too quick punching the keys. Feeling kinda stupid right now :>(

 

[IRstuff]

With or without lift? ;-)

Without, it's about 101 ms

TTFN

FAQ731-376

 

[handleman]

IRstuff:

I didn't ask how long it takes for the bullet to drop 1.98 inches from the start. Worded slightly differently, how long does it take for the bullet to accelerate to the point where, during the next 5ms (and taking into account acceleration during that 5ms) it drops 1.98 inches.

Yeah, and without lift.

-handleman, CSWP (The new, easy test)

 

[eadwine]

When it comes to hitting a target, it's all about windage. elevation, charge and holding the weapon steady on target. Unless of course you're using a projectile with a built in thrust & guidance system.

Now, if F=ma and F/m=a, then if "m" is held constant or even if it changes and "F" goes to zero it must follow that "a" goes to zero. Unless, of course, we're using the "New Math" and the Time/Space Continuum isn't a factor here.

 

[IRstuff]

OK, 1.0232 seconds

TTFN

FAQ731-376

 

[oldfieldguy]

Former highpower rifle competitor and handloader of ammunition here. Couple of points:

Most chronographs operate from photodetectors noting the passage of the projectile. They still need to be placed well away from the muzzle of the gun barrel, though, or muzzle gases will affect accurate readings.

The propellant ("gunpowder") is a variable. Fro a given caliber, projectile weight and velocity, one can chose powders with differing burn characteristics, reaching peak pressures sooner or later in the firing sequence. This used to be particularly important with certain models of semi-automatic firearms that used gases tapped from the barrel to operate their mechanisms. The characteristic is also a variable in achieving maximum accuracy.

Aside from those two points, and a hundred-odd posts, the short answer is that the bullet starts slowing when the force behind it pushing it faster is equal to the force in front of it slowing it down. Giving the gases a little credit for mass and velocity, it would appear to my electrical mind that this point would be a very short distance in front of the muzzle, on the order of a couple of bullet diameters.

old field guy