Our willing volunteer to have a hop added is this gorgeous blued Colt Single Action Army.
First step, remove the side plate.
Then take out the spring and the hand (the part which pushes the cylinder round).
You can then pull out the centre pin and the drum.
There is a pin at the top of the barrel in the frame, push this out with a punch and a second pin that holds the ejector unit in place.
With both of these removed you can take the barrel off.
You can then slide the inner barrel out. At this point get out the o-ring and round needle file. The o-ring should be 1-2mm thick.
Then, start working on the barrel. Keep the channel the file produces on one side, by the time you work through to the inside you want about 3-4mm of the circumference removed from the inside. You may wish to give yourself a little extra space on the outside to hold the o-ring. Use a very sharp knife to cut the rubber roughly to size.
Once in situ, use the knife again to chamfer the edges of the o-ring so that it sits fairly flat against the barrel. There should be minimal space between the rubber and the barrel to preserve the gas seal. Through the barrel you should see just a flat, small line of rubber across the top. It doesn’t need to be much, just enough to catch the BB as it passes. If you can’t see it, file away a little more but go slowly, you can’t add material back on.
Use the collar at the back of the barrel to hold the o-ring in place. Check inside the barrel to make sure the o-ring hasn’t slipped in. It should be firmly wedged in place by the collar, depending on the pistol you may wish to seal it with electrical tape or PTFE.
Some pistols have a locating lug on this collar, which keeps the barrel oriented in a specific way. This gun does not, but if yours does then make sure the hop window is oriented correctly to the top.
Put it back in place, make sure the hop window is at the top.
Re-assemble the gun, there aren’t really any specific tips to put the SAA back together. While you have it open however, oil the moving parts with a little light oil (3-in-1 is perfect) and if you haven’t a CO2 cartridge in, put silicone oil into the cartridge pin and on the seal. Revolvers don’t need a lot of maintenance, but a bit of oil every now and then keeps them going nicely.
If you are so inclined, you could use a flat file and install a flat hop instead, though this fixed hop is quite adequate.