impossible valve that works

This is a totally impossible valve. When it appeared on my drawing board I couldn't see how it could work but neither could I see how it couldn't work. So I made it, just to see what would happen. I doesn't appear in the book because it's far too dangerous. What happens is this. There is a long copper barrel, and it is brazed straight through a short fat copper air reservoir. The reservoir is capped, top and bottom, with meaty plugs silver-soldered in place. The only ingress to air is via the Schrader valve, and the only egress is via a (lateral) hole that sits in the side of the barrel, inside the air reservoir. A valve with two O-rings, one at each end, is pushed in from the back of the gun, such that one O-ring sits on either side of the barrel's lateral hole. The bullet, duly wrapped in thin paper, is muzzle-loaded down to the valve. In the photo, both bullet and valve have been pulled out of the barrel and lie to the side, exactly where they lie inside the barrel when the blessed thing's loaded.You pump up the reservoir via the Schrader valve. You use your bicycle track pump for this, and if you haven't got one you hie yourself down to the bike shop and buy one - they aren't expensive - because then you get a pressure gauge and can pretend to be doing physics whereas actually you're just making big bangs and annoying the neighbours.Now what next happens is incredible. You put on a stout glove, grab the back of the valve, and pull it backwards. As soon as the front O-ring clears the lateral hole in the barrel, the air tries to blow the bullet out. It also tries to blow the valve back where it was. A sort of unlikely sucking. And you can see why I put on the stout glove, because whatever you think, you cannot imagine that the valve will not be blown backwards just as hard as the bullet is blown forwards.But it isn't. It's sucked back into place, though the reservoir is emptied before it gets there. So at least you don't have a valve blasted through your wrist.At least, that's what always happened until last night when another physicist turned up and I had to demonstrate. The valve is fairly stiff, and I couldn't pull it out easily, so I got him to hold the barrel on a stool, the muzzle pointing at some rags that I keep for the purpose, and taking pliers to the back of the valve, I gave it a sharp blow with a mallet. This sent the whole valve all the way back completely past the hole, so there was now a bullet on one side of the barrel, and the entire valve on the other side. Whereupon, of course, there was an equal and opposite reaction so whereas the bullet hit the target, the valve blew across the workshop and snapped the back of it off on the door. (Which is why, in the photo, it is now so short.)

copper air rifle

Attentive readers will instantly recognise that this is the air rifle on page 159. Inattentive, or fiscally prudent, will have been wise enough not to have bought the book (which book, pray? Why, http://www.iop.org/EJ/article/0031-9120/41/3/M02/pe6_3_m02.pdf?request-id=6fe6bca2-f28b-4f25-a81a-9dfae4f8e0ac, of course. But don't click on that link cos it's a PDF, and I only include it because it flatters my pathetic little ego) so they won't have a clue.What you're looking at is a copper tube with a smaller diameter rifled copper barrel running down the middle. The front is sealed round the outside of the rifled barrel, and traps air between the outer and inner tube. Air is pumped in using a bicycle track pump via the Schrader valve, soldered in the middle (to a ring of brass, also soldered in place, to give it a bit of purchase). It's sealed at the back by the brass valve which I have very kindly pulled out of the back (at the bottom of the picture) to show how it works. The smaller diameter O-ring seals the back of the barrel, and the larger O-ring seals the entire back of the outer copper tube. Pump the bally thing up and this brass plug tries to move backwards under pressure, and you hold it in place with some kind of improvised trigger at the back, which is what the steel rod is all about. As soon as the smaller O-ring clears the back of the barrel, all the air belts the bullet out down the bore. Should this explanation have you faltering then there is the option of thrusting money at your bookshop or my publisher, which they and I will enjoy, and you will ever afterwards regret especially if your local constabulary disputes your need to inform yourself of these arcane adventures in physics. - I digress. - (But how else am I going to sell the wretched book? I'm not J K Rowling for goodness' sake.)This particular gun has an 18.5 inch barrel, and since performance relates to the square root of the barrel length, the pressure has to be fairly high. (Because performance relates directly to the pressure.) Unfortunately copper ain't very strong, and if you get carried away, the outer tube will burst, possibly around 380 psi, and you will be horribly injured in a spectacular way because compressed air is incredibly (I choose the word carefully) dangerous. - The bigger the diameter reservoir tube, the lower the pressure it will tolerate. This one's 3/4 of an inch in diameter and I've had it up to 250 psi using one of those tiddly little air shock pumps, whereat it gave me a satisfying 260 feet per second velocity with a half-ounce lead ball, which since it's 33 foot-pounds is something you May Not Do in England.

big airguns

Do not be deceived. This website is purely to induce you to thrust a hand inside a hip pocket, extract a wallet, pluck large denomination banknote therefrom, walk cheerily into your bookshop, and purchase a copy of http://www.merlinunwin.co.uk/bookDetails.asp?bookId=9&categoryId=1which, I now confess, will increase my royalties by a vast but indeterminate sum.The photo to the right is my first muzzle-loading big-bore airgun, a monstrous thing which my friends immediately called The Jezail. The barrel was a two-metre length of copper pipe extracted from the local plumber's merchant, and the air reservoir a piece of bicycle inner tube - cunningly incorporating the valve so it could be inflated hard - poked into a tube of sewn sailing webbing. The webbing prevents the inner tube expanding like a large and bulbous party balloon, so the air pressure goes up which is what we evil people want it to do. Air can't get out of the back of the tube because it is clamped between two bits of angle iron, and it can't go out of the front because there's another clamp in the way. This second clamp, however, can be flicked open by the sideways-poking toggle thingy, and then all the air is released instantaneously behind the bullet. If you happen to be a concert pianist then get someone else to do the flicking, because once it's past the centre of the various pivots that hold it shut, then the air snaps the lever open very hard, and it is quite indifferent to whether or not you want your fingers amputated.If you can't work out exactly how the trigger functions, the exact mechanism is neatly drawn on page 135 of the above-mentioned book - did I mention that I want you to buy a copy? - and, because I'm Hugely Clever, if you turn the page you can find an altogether safer mechanism on page 138. Actually, on page 136 there's a mechanism that can be built entirely out of wood. I'm perverse like that. I like to use inappropriate materials in gun-making. I can't see any reason why, when the Penan tribesmen of Borneo managed to make six-foot blowpipes in the jungle with nothing more than a steel stick as a drill, we should find it impossible to make an entire gun out of wood.I used a pressure of no more than 11 bar (160 psi) because that's what my bicycle pump was capable of. At this pressure and with two metres of plastic conduit for the barrel, and a larger air reservoir so that the thrust down the bore remained high, I shot a 16mm glass marble weighing 84 grains at 721 feet per second, and since this is the sort of thing that politicians around the globe are extremely keen that you do not do, it's probably as well to check your local laws before you embark on a wicked enterprise like this. (Where I live, outrageously powerful airguns are legal. Amazing but true. It's why I live here.)Should you have no chronograph, you can estimate velocity of a lead ball by shooting it at a brick. This can be quite exciting, especially for bystanders, because recovering the ball afterwards may involve searching the immediate environs which could include indentations in the bodies of the audience. The picture below shows what happens to a half-ounce lead ball shot at 192 feet per second at a brick wall, all the way up to 332 feet per second at a brick wall, which valuable information was obtained not without hazard to self and neighbourhood. A half-ounce ball is .527 inches in diamter, and wrapped in a whisp of tissue paper, fits neatly inside 15mm copper pipe sold for the more mundane purpose of plumbing.