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.
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