Attempt to make a cylinder fail (BB Mortar)

[Scout.308]

Here's another angle that I hope will augment DoubleD's excellent description.

Bernoulli's Principle of Fluid Dynamics states that:
"An increase in the velocity of gas creates a decrease in pressure."

Pressure is traded for speed.  This is what makes a carburetor carburet!

Within the chamber, the gas is not moving, and exerts extremely high pressure.  As gas escapes the chamber and gains velocity travelling down the barrel, it's pressure drops proportionately (and rapidly).  The tube, exposed only to higher-velocity gas, never "sees" the higher-pressure exerted on the chamber by static gas.

[Double D]

Here's another angle that I hope will augment DoubleD's excellent description.

Bernoulli's Principle of Fluid Dynamics states that:
"An increase in the velocity of gas creates a decrease in pressure."

Pressure is traded for speed.  This is what makes a carburetor carburet!

Within the chamber, the gas is not moving, and exerts extremely high pressure.  As gas escapes the chamber and gains velocity travelling down the barrel, it's pressure drops proportionately (and rapidly).  The tube, exposed only to higher-velocity gas, never "sees" the higher-pressure exerted on the chamber by static gas.

That's what I said.  Just not as well. 

[Scout.308]

This is why cannons generally  have greater velocity and range than mortars and howitzers.

I thought it had more to do with reduced barrel length and reduced charges, (but I probably don't understand something).

 Large bore cannon of the 18th century had powder chambers.  Following your line of thought, how would the ballistics vary (in general terms of course) between two identical guns, one without a chamber, and one with a chamber.

If I understand these correctly:


Mortars and howitzers are more efficient with their chambers: offering a very powerful but brief push for a given amount of powder (think: hitting a ball with a bat).  Perfect for stretching scarce powder resources.  They move large projectiles at moderate speeds.  Ideal for momentum-based blows at relatively short range engagements.


Canons burn relatively a bunch more powder and use their long tubes to get much higher velocity from that bigger, longer push (think: launching a warhead on a rocket). Their high-speed projectiles yield high-energy blows and carry for much longer ranges.  Since energy is equal to the square of the velocity, a little more velocity generates a lot more energy.  These hot-rods are less efficient, but powerfully effective.  Kind of like race cars.


Hope it's useful.

[KABAR2]

 The use of a chamber also has something to do with lobbing an explosive ball over a wall into a fortified structure where the cannon's velocity is used to deliver a smashing effect.......

For what it's worth, a cast iron shot of the diameter of a bowling ball would weight about 84 lbs, so even five 16 lb bowling balls at once would not equal one cast iron shot. On the other hand, if they never use more than one 16 lb bowling ball as a shot, maybe they are OK. Hopefully they will use cannon grade powder in the future.
 

GCaskill mentioned earlier that an iron ball of bowling ball caliber would weigh in at some 84 pounds but being a mortar the ball would be hollow and have a powder charge and weigh much less.... perhaps half as much?

[Artilleryman]

An eight inch iron mortar shell weighs 45 lbs.

[Cat Whisperer]

Good to see that we've proved that Gen. Coehorn figured out in the 1700's.
 

[GGaskill]

A flat or low trajectory gun needs a lot more velocity for its shot to keep it in the air until it gets to the target.  The math is d=.5gt² where d is the distance the projectile drops in flight due to gravity, g=32.2 ft/sec² and t is the travel time in seconds.  The travel time needs to be short so the velocity is high.

High angle fire from a howitzer or mortar uses a lot more of the powder force to counteract gravity so it can stay in the air longer.  Indirect fire from guns wasn't used until forward observers could report back shot fall location but the big CW mortars were fired at area targets over 4000 yards away.

[Webleys]

For what it's worth, a cast iron shot of the diameter of a bowling ball would weight about 84 lbs, so even five 16 lb bowling balls at once would not equal one cast iron shot.  On the other hand, if they never use more than one 16 lb bowling ball as a shot, maybe they are OK.  Hopefully they will use cannon grade powder in the future.

My BB mortar has a powder chamber that holds about 4 oz of powder. I use either fg or ffg. I have tried cannon grade powder which makes it look like something the Coyote bought from Acme; it kind of coughs and ejects the ball about 20 ft downrange. As an experiment I tried Pyrodex, it burned and just managed to dribble the ball clear of the muzzle, I thought I'd have to fish it out to swab the bore.

The pressure tank mortars make me nervous but they don't seem to fail. My mortar is very heavily built, the breech plug alone weighs about 100 lbs and the wall thickness is 1 1/8". The whole thing weighs about 700 lbs with the wheels. I don't feel bad standing 20 ft away when it goes off!

[Double D]

My BB mortar has a powder chamber that holds about 4 oz of powder. I use either fg or ffg. I have tried cannon grade powder which makes it look like something the Coyote bought from Acme; it kind of coughs and ejects the ball about 20 ft downrange. As an experiment I tried Pyrodex, it burned and just managed to dribble the ball clear of the muzzle, I thought I'd have to fish it out to swab the bore.

You put 4 oz of Cannon grade in your chamber and bowling ball on top and the ball only went 20 feet?  Do it again on video, I'll supply the powder. 

I am extremely skeptical.

[Webleys]

My BB mortar has a powder chamber that holds about 4 oz of powder. I use either fg or ffg. I have tried cannon grade powder which makes it look like something the Coyote bought from Acme; it kind of coughs and ejects the ball about 20 ft downrange. As an experiment I tried Pyrodex, it burned and just managed to dribble the ball clear of the muzzle, I thought I'd have to fish it out to swab the bore.

You put 4 oz of Cannon grade in your chamber and bowling ball on top and the ball only went 20 feet?  Do it again on video, I'll supply the powder. 

I am extremely skeptical.

No worries, I've got the powder! 

I've done this twice now, with the same results both times. Next time I have it out I'll try to get a video of it. I first tried it when I picked it up from the builder in Idaho (he told me beforehand what would happen) and then a couple of years ago at another shoot.

My normal load is 4 oz of fg but I also use ffg when necessary. The range and results are nearly identical.

The Pyrodex shot was comical. Lots of blow by and smoke and just when I thought I'd have to fish the ball out of the bore it just pushed it out of the muzzle end, it barely cleared the carriage.

Cannon grade works fine in my 3" mortar (even Pyrodex, with lesser results) which has a 1 oz powder chamber but that one also has a 48" barrel thus acting more cannon like.