I have 40 cal. shells among others, and after a quick rummage through the pile, I realised that they all looked similar in dimension around the base.
Step 2: A quick check with the vernier.
A quick check with the vernier caliper confirmed my suspicions. As shown, the O.D. of these cases are: .270 WIN brass and plated - 11.9mm. .44-40 WIN - 11.8mm. .22-250 REM - 11.8mm. .243 WIN - 11.9mm. .303 British - 11.5mm. I have a good selection of prospective valve caps.
Step 3: Tools and materials.
Basic stuff found around the workshop, except maybe the pipe cutter. This one was only a few bucks at the hardware store.
Step 4: Mark and cut the shell.
I measured up the shell by sizing it up to the valve cap I wanted to use. The area around the base (strangely known as the head) is quite thick as you will see if you look inside the shell after cutting it open. You need to allow for this when measuring up to cut so that the valve cap will sit flush inside when inserted.
Step 5: Prepare the shell.
The shell needs to be deburred so the valve cap fits properly, and cleaned so the glue will stick to it. Flat file on the end, (hand 2nd cut I think). Small round file inside (poke it around inside the flash hole while you are at it). Then clean inside with scotchbrite and metho to remove burnt stuff. Don't scrub the outside or you will scratch it. Polish it up when it is finished if you think it is required. Ready for gluing.
Step 6: Test fit.
I drilled a hole through the end of the valve cap by twisting the cap back and forth on a 3mm drill bit held between my fingers. No power tools required. This along with the flash hole will provide a good "key" when filled with glue. Then I test fitted the cap without gluing yet, and removed it again by screwing it onto a tyre valve and pulling.