I bought this airgun from Crosman Custom Shop. It is a .22 caliber co2 carbine, model 2400 which is the Custom Shop version of the discontinued (in America; Crosman may still sell it in the UK) model 2250. In the UK, this airgun is commonly called the "Ratcatcher."
As delivered, it is a great little air rifle. Velocity with 14.3 grain lead pellets is about 550 fps, and a co2 powerlet is good for about 30 shots. Powerlets are inexpensive too; about 50 cents each in bulk. I don't remember how much they cost in the early '70s when I had my Crosman 180 carbine, but they were expensive enough to cause me to add a pumper to collection so I could shoot more. At a guess, they might have cost 20 cents each. But 50 cents now is cheaper than 20 cents was then.
As cheap as co2 powerlets are, bulk co2 is much cheaper. And if you have the means to compress it to 2K psi or so (which I do in the form of my Hill HPA pump), the air around us is free. So I wanted to expand the capabilities of this little hunting airgun by converting it to use those power sources. At the same time, I wanted to retain the ability to use powerlets. That way I could charge the rifle with high pressure air and go hunting, with a couple of powerlets in my pack as backup in the event that I deplete the air charge while I am still in the field.
This video is a description of the conversion and the initial chrono test. Since I created the video I have tuned the gun further, gaining both efficiency and power. Now with the same 14.3 grain pellets I am able to achieve 700 fps for a dozen shots. I haven't yet tried bulk filling with co2, but by detuning (via the homebuilt velocity adjuster) to the original 550 fps I should theoretically get 100+ shots from a fill that costs mere pennies.
You can find the HiPac conversion here: http://www.powermaxhipac.com/