Giffard carbonic gas (CO2) rifle.

Part 1

This report covers:

  • Resume description
  • The Giffard problem
  • To fill
  • Ooops!
  • At the shot
  • Adjustable trigger
  • Barrel comes off
  • Summary

Today we will complete our initial examination of the Giffard Carbonic Gas CO2 rifle. In the comments to Part 1 reader Airman of the Board linked to a short You Tube video of a miniature Giffard, and that video is so incredibly good that I’m making it a part of today’s report. You’ll see exactly how the rifle works, and much of what I’m showing today can be seen. The miniature is not 100 percent exact, but it’s close enough and it allows you to see the operation of the rifle.

Just a word on the operation. The builder fills his gun with some kind of airsoft gas instead of CO2. It could be green gas whose pressure is around 115 psi. Compare that to CO2 that’s 850 psi at 70 degrees F and you’ll see the difference.

It’s a shame no one builds a Giffard today, because it is a study in gas valves, reservoirs and how the hammer communicates with the valve. The video shows this very well, which is why I want you to watch it if you can.

Resume description

The question I was asked more than any other was how is the rifle filled. I thought it was obvious, but apparently not. The long steel tube under the barrel that serves as the forearm is where the CO2 is stored. That tube is unscrewed from the action for filling. And that tube also brings up a problem that I consider the kiss of death for the Giffard system.

Giffard steel tube
The steel gas reservoir is removed by simply unscrewing it from the receiver.

The Giffard problem

The big problem with the Giffard was that the owners had to send their empty tanks somewhere to be refilled. In my opinion, that was what killed the rifle. Even though the nation of France is slightly smaller than the state of Texas, people didn’t have cars in the 1870s, so sending in a tank meant either weeks without one or it necessitated buying an extra tank to cover the down time. The tanks are serial numbered like the rifles and most tanks numbers differ from the rifles they are on, for this reason. And they cost eight pounds, which was forty dollars at the time and incredibly expensive

I mention France because that was the country of the Giffard’s origin. But most of central Europe and the United Kingdom were its principal markets, and that region covers a territory more like the size of the United States. Even with multiple refill stations, it’s still a problem. Just ask Winsel who failed in 1950 from the same thing. Or Crosman whose model 197 10-ounce gas cylinder eventually gave way to the 12-gram CO2 powerlet.

Giffard Crosman 197 gas cylinder
Crosman 197 gas cylinder below and a paintball tank above.

To fill

To fill the removable Giffard tank with CO2 you need an adaptor. I would have made one to adapt to a 20-lb. CO2 tank, which is how I fill many bulk CO2 guns, but when I got the rifle it came with an adaptor that connects to a Crosman 197 10-ounce bulk CO2 tank.  Or at least that’s what I thought.


It turns out the threads in the adaptor do not fit the external threads on a 10-ounce Crosman tank. BB will have to do some work to fill his rifle.

Giffard  Adaptor
The Giffard fill adaptor no el-fitto the Crosman tank. The holes and shank are right, but the threads are off.

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At the shot

What opens the tank at the shot is a steel pin inside the receiver. This pin is driven by the hammer — not directly but via a linkage. The video shows this quite well. The CO2 valve lives inside the removable Giffard tank. The pin inside the receiver is not spring-loaded but the hammer drives it forward and the closing of the valve shuts it. There is a return spring inside the valve in the gas tank but the force of the CO2 gas inside the tank is what shuts it.

Adjustable trigger

The trigger is a direct sear type, where the trigger blade is what releases the sear to release the hammer. The trigger adjustment simply increases and decreases the contact area of the trigger and sear, and as such you want the sear to always have enough of a shelf for safety. That means you don’t want a light trigger. Remember — this is a big bore airgun. It may not be that powerful, but it could severely injure or even kill you if it misfired.

Giffard  trigger
The Giffard trigger is a direct sear with an adjustment for contact area. Don’t make it too light.

Barrel comes off

Yes the barrel also comes off. It simply unscrews from the receiver, though the rear sight must be removed to clear the steel pin that opens the valve. However, I don’t know if the bolt must also be replaced when the barrel is exchanged, for the hole in the bolt nose would not be correct for any other size ball — especially when going from 8mm to 4.5mm.


I have read other things about the Giffard in my research but they didn’t prove out. Things like the power adjustment wheel could be screwed out to act as a safety. I have checked that and don’t think it’s possible.

The gun will require a little tinkering on my part to charge it, and perhaps more than a little if it needs seals. But the design is so straightforward that I believe I can get it to work. I may even work up a high pressure air reservoir for the rifle.

This is going to be a fun series.