SpudFiles

I decided a little while ago that I would push the envelop a bit and see exactly how heavy of a round I could shoot out of my larger air cannon. It turns out what I ended up making was probably the heaviest bullet I could make weighing 12,600 grains or 1.8 pounds. The round consist of a mild steel or cast iron core (I'm not sure which), a lead tip, plastic jacket with a layer of aluminum underneath it and then a filler under that. We ended up shooting it a 1/8 steel plates and solid 4 in thick cinder blocks. I have a HS camera and we decided to try and figure out about how fast it was going. From what I can tell it was just going to slow at only around 120 FPS but had enough mass to give us a calculated energy of about 400 foot pounds and I think it was just going to slow do much of any damage to the plates or blocks.

check out the video of the round being shot:

Depends on exactly what you want from it.

I've fired 230g (~.5 lbm) projectiles from a 20mm air cannon at ~65m/s (~215 fps), which is visually impressive from the perspective that, even properly shouldered and fully braced, it'll shove you back several inches.

(This in itself presents a possible risk, as the recoil can damage the cannon or user).

It's not particularly impressive on target though, as it doesn't have the velocity for the really short, sharp, impacts that make for interesting explodey effects. It more shoves the target than hits it. (I wouldn't want to be in the way, though).

In short, probably. You'd likely get more impressive effects on target with a lighter projectile.

Ragnarok wrote:Depends on exactly what you want from it.

I've fired 230g (~.5 lbm) projectiles from a 20mm air cannon at ~65m/s (~215 fps), which is visually impressive from the perspective that, even properly shouldered and fully braced, it'll shove you back several inches.

(This in itself presents a possible risk, as the recoil can damage the cannon or user).

It's not particularly impressive on target though, as it doesn't have the velocity for the really short, sharp, impacts that make for interesting explodey effects. It more shoves the target than hits it. (I wouldn't want to be in the way, though).

In short, probably. You'd likely get more impressive effects on target with a lighter projectile.

I was shooting it out of my larger cannon that has a recoil system and it had no problem handling it but it was able to fully extend the 2 50 Lb springs providing resistance. I haven't bothered to see how fast it can get lighter projectiles going but I would guess mine would be able to get a 1/2 pound projectile going upwards of 400 FPS.

There is a rather lovely study done by a W.H. Noble of the Ordnance Select Committee in 1863 (referenced here but sadly it's poorly scanned text with no illustrations and jumbled up tables) which is quoted in Ian Hogg's "A history of artillery":

A 68 pounder smoothbore and a 7 inch Armstrong gun firing 200 lb shot had been fired at 4.5 inch plate backed by 18 inches of teak. The 68 pounders had penetrated the target, while the 200 pounder had made hardly any impression on it. Noble showed that the answer lay in the relative velocity of the two projectiles. The 68 punder had been moving at 1,425 feet per second when it struck, while the 200 pounder was loafing along at a mere 780 feet per second. On the face of it, the 200 pounder with 156,000 foot-pounds of energy should have out performed the 68 pounder with 96,900, but the low velocity of the heavier projectile allowed the plate to deform and resist the blow, whereas the higher velocity of the 68 lb shot tore through the plate before it could begin to absorb the blow.

As Noble said in his paper, "What is wanted is velocity; if you sacrifice it to weight you will only be able to keep knocking at the door without entering."

There are other aspects to penetration - projectile hardness relative to the target and sectional density for example, but if what you want is to make holes then a more massive projectile is not the solution.

MOO Tech wrote:I would guess mine would be able to get a 1/2 pound projectile going upwards of 400 FPS.

200-250 fps is more likely. While the projectile being a quarter of the mass means four times the acceleration (from the same force, anyway*), four times the acceleration means it leaves the barrel in half the time - hence, about twice the velocity.

*In reality, pneumatic cannons tend to get less efficient as velocity increases, due to flow losses, valve speed, and other factors, but at these modest velocities, it's not an awful rule of thumb.