As for the what, I'm sure most of you are familiar with HEAL.
Behemoth was HEAL's predecessor (up to the point that HEAL was known as Behemoth 2 in the early stages of the design), built in October 2006.
As for the why, I've come across an old backup of files from a couple of years ago that actually has pictures of Behemoth in it. Up until now, I'd lost most of the records of Behemoth.
There were four revisions of Behemoth. The one you're probably most familiar with is Behemoth V1.0, one of the few photos I still had around before I found the backup:
At 190cm total length, and 7 kilos in weight, the name was not inappropriate. It was the largest thing I'd ever built.
It was also somewhat inefficient for its size - the barrel was only a metre long (so only just over half the total length), and although it had a 20mm piston valve, it only used 10 bar.
That said, with 3 litres of fire extinguisher to refill from, it could fire pretty fast with prepared ammo on hand. It was unregulated, so obviously, pressure dropped with each shot - still, it would fire about 10-12 shots before the pressure was too low for it to work.
You can see the crude system used to stop the heavy tank distorting the launcher here (electrical tape for the win!)
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Behemoth V1.1 took off the extinguisher - although fun, it was too heavy to be practical.
I did add a "bayonet" twist lock muzzle attachment system though, and some porting to go with it (which did help accuracy somewhat - but results did improve dramatically when I used larger holes in the porting)
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Behemoth V1.2 was a power upgrade - an extra half metre of barrel, and a pressure increase (Pressures of 160-200 psi were common). This brought its length back up to a little more than that of v1.0, but the 50-70% power upgrade was worth it.
I also upgraded the pilot, building a smaller piston valve for the job:
Also, the breech:
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Behemoth v1.2.5 was pretty much the same as Behemoth v1.2, except the pilot area was soldered to get me back into practise for building HEAL:
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Okay, the obligatory "damage pics" - and based on the picture numbering, these were done with Behemoth V1.2
Here we have a DVD player casing. 1mm thick steel on both sides.
A crapload of pumping, loading and shooting later:
My notes tell me that the X is where a paintball hit, but with no real mark; 1 was potato; 2 was a frozen grape; 3 was blu-tac (which, as it's dilatant, can be impressive at high speeds); 4 was a bit of threaded rod with nuts and washers; 5 and 6 were steel ball bearings; 7 was frozen potato; and 8 was a marble.
The threaded rod was interesting because of the imprint it left on the other side.
Yeah, no penetration through (although some close attempts, but at the time, the breach wasn't sealing very well. One new O-ring later, and I had two holes through the first layer to show for the efforts. (Sorry, I have no record of what all the other added dents are... at a guess, mostly potato & paintballs - the top left looks like an AA battery though.)
Still, pretty dull for a cannon of the size. 1mm steel isn't all that tough.
What about a sub-calibre projectile and 2mm steel...?
Better. Another target was a Quality Street tin with a 3/4" steel ball - quite thin steel, but it was full of water at the time.
The lid did fly about 15 feet up in the air from hydraulic shock though.
Also, a couple of drinks cans it took apart with potato slugs:
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It's an old cannon, but one I figured I'd share anyway.
im having trouble myself finding a piece for the back of the barrel to get sealed bt the piston.
