Anyway, this doesn't concern big airguns. This post concerns a very nice, very tight .22 barrel that I was given last week. I opened my drawer and got out my beautiful Dave Manson match chamber reamer and found the pilot wouldn't enter the bore. The lands were too close together. Dave Manson's pilot is 5.5mm exactly. This barrel had the lands 5.36mm apart. So I had to make a chamber reamer.
I opened my other drawer and found no silver steel. (Drill rod, if you are an American Person, which I am not.)
Accordingly I had to engage both the remaining neurones and try to work out how I was going to make a reamer with woefully inadequate materials to hand. I did have a sawn-up, annealed flat 6mm file from something else, and it crossed my tiny little mind that this was all I needed.
I cut slits into two bits of 6mm mild steel rod and jammed them open onto a lozenge-shaped bit of annealed file, and after chamfering the pointy-bits, MIG welded each end.
Then I re-annealed it overnight in the fire lest the welding heat had hardened the file-steel. It came out grungy and crusty. They always do.
I polished it with wet-and-dry to remove all the ghastly brown rust or whatever it is, and ground off the corners on the bench grinder, holding it rather lightly to avoid heat build-up.
Into the lathe with the short end - which was to become the pilot - and I tried machining the long end true, but it was too slender and eventually caught on the tool and got savagely bent and I was forced to use appropriate words to mark the event. After I'd calmed down I cut that end off and machined it to a little 4.5mm stump and drilled likewise into a bit of 8mm and plug-welded this in place instead. (Plug-welding - you drill a hole through the tube from the side till you reach the middle and weld into it. - But you already knew that.) Then to the accurate machining. Because my lathe is 115 years old and isn't frightfully accurate I finished it by filing and polishing in the lathe, with lots of stops and lots of micrometer fiddling. Filing in the lathe? - all my engineer friends laugh at me. But I don't care.
When the pilot was 5.35mm and would slide down the bore, and it was 5.78mm at the cutting edge, I put it vertically in the drill press, rotating at the slowest speed; and propping firebricks up behind it to retain the heat, used a propane torch to take it to cherry-red. Then swiftly off with the torch, and a 2" steel tube containing oil raised up from below to quench it.
The steel tube is made out of a bicycle frame and retains a bit of the frame to act as a handle because the oil always catches fire and you only once want a baked bean can of burning oil all over your hand to remark on the need of a handle.
More polishing to remove the black oil-deposit.
Then a Dremel cutting disc to create the cutting edge. Lots and lots of gentle cuts so as to avoid heat build-up, with my bare finger touching the undersurface of the steel to give warning of heat. It took a long time to cut it out, a good hour or so. The cutting removed a scoop of circular section so I didn't need to grind all the way to the middle which I'd have had to if it were a D-bit, or half-reamer. Behind the cutting edge is a very slight taper, enough to give clearance but no more - measured 12mm back from the cutting edge the diameter is 5.7mm.
Because of the lozenge-shape of the file-steel the cutting edge turned out to be a bit of mild steel, and anyway it was a bit of mild steel that wasn't there - it was part of the unwelded gap. The customary word was uttered before I realised I could still use the left-hand edge as the cutting edge, provided I could remember to turn the reamer anti-clockwise.
I left it dead hard, and didn't attempt to temper it. How many more tight barrels will I ever need to chamber?
I rotated the reamer by hand with the barrel vertically held in the vice, and it took a hundred half-turns (ie 50 rotations) to cut about 2.5mm depth
Here we are with some swarf just visible on the cutting face. I used a dribble of ordinary engine oil both on the pilot to lubricate it in the bore, and on the cutting edge. Every forty half-turns it was removed, wiped clean, inspected under the microscope, and the bore cleaned. Then another dribble of oil and back to work.
Under fifteen magnifications there was no galling at all on the reamer, and the chamber is very smooth. I rather fancy that the dead-hard file steel gives a better cutting edge than dead-hard silver-steel. In spite of the accuracy of cutting, a cartridge is quite loose in it. Dunlap's minimum for a .22 is 5.788mm. - My Dave Manson match reamer cuts a 5.735 chamber and it's exceedingly tight - indeed some ammunition won't chamber at all. -
I shall machine a recess for the rim deep enough to make sure the bullet's nose digs into the rifling, and cross various fingers that this is sufficient to give accuracy.
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.)
. 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.