By B.B. Pelletier

This advice applies to both precharged pneumatics (PCP) and CO2 guns. The connectors that are used to fill these guns, and the caps that hold the CO2 powerlets, ARE NOT PICKLE JARS!

How a pickle jar seals
A pickle jar seals through force. The cap, with a flat synthetic seal, is pressing against the mouth of the jar. The seal is very flexible and fills all the small gaps of the jar mouth when pressure is applied. Same with a catsup bottle. But not a PCP or CO2 airgun.

How a PCP airgun seals
Both PCPs and CO2 guns use O-rings as seals. You DO NOT need to screw the connectors together tightly to make the seal. When the air or gas pressure rises inside the reservoir, it forces the O-ring against the walls of the connection, making the seal. The greater the pressure, the tighter the seal. In fact, that’s why PCP fill adapters have bleed valves.

The bleed valve is the large-headed
screw on top of the adapter body. The
refill gauge is underneath.

How bleed valves work
A bleed valve releases the pressure in the fill-adapter connection so the fillings can be removed. If there were no bleed valve, it would be extremely difficult to uncouple some fill adaptors. Why, you’d need a pipe wrench to do it! Which is why you sometimes encounter fill adapters that look like they have been manhandled with a pipe wrench.

If you just tighten the fill adapter finger-tight and allow the O-ring to do its job, you’ll find it a lot easier. The pressure of the gas is quite a bit more than you could muster by hand and it acts on the O-ring, forcing it into all the recesses where air could escape. The O-ring is in the K-valve on the scuba tank, in this case.

CO2 guns seal the same as PCP guns
CO2 guns can be charged in two ways – with a cartridge (either a powerlet or an AirSource cartridge) or with a separate bulk tank. The powerlet usually presses against a seal, though on some guns an O-ring in the cap of the powerlet reservoir does the job. The AirSource cartridge seals with an O-ring, and it does have a sort of reverse bleed valve that allows the gas to be turned off and the AirSource adaptor to be removed from the gun without losing the remainder of the gas in the cartridge. Neat!

If you use pellgunoil, as I recommended a few days ago, the seals will do their jobs for many years. So, tighten your fittings finger-tight only and keep the torque wrenches in the garage.