High Pressure Air Conversions of the Crosman Designs

I am all for you getting the most out of your airgun experience, but there

are those that push the guns far beyond their limits. I have never agreed

with using anything except Co2 in the Crosman rifles and pistols including

the QB. I am not against using the conversion kits that allow you to use

external regulated compressed air in these guns, but even they are not

100% perfect. The tube in the Crosman is safe at pressures higher than those

generated by Co2 under normal circumstances, but in an unusually high

ambient temperature and in direct sunlight, the pressure can spike high

enough to prevent the valve from opening. I have seen where a few of the

more prominent modders have gone to great lengths to point out that they

have replaced the tube with one that takes high-pressure air. That's fine,

but what about the rest of the system? A compressed air gun does not get

its rating from one component, alone.

It brings up a flaw in many designs, both Co2 and compressed air. There is

no easy way to vent over-pressure gas, when the hammer and spring are

incapable of opening the valve. The normal way to do this with Co2 guns is

to cool it down until the hammer and spring are strong enough to open the

valve. An overly hot day and sitting in direct sun will not bring the air

in a compressed air gun to a point high enough to lock the valve. I am pretty

sure that if you accidentally filled a Huntsman with 4350 psi from a

carbon fibre tank, you might not be able to fire it with the hammer and would

have to resort to striking the valve with the guns' hammer, via an external pin

drift and regular hammer in order to reduce the pressure in the chamber.

The difference would be that the Huntsman can withstand that pressure without

failure, but you don't know if the Crosman rifle could take it and survive

in one piece. There is only one gun that I am aware of that has an

over-pressure burst disk and that is the USFT but there may be others.

A weakness of the rifle, when operating on high-pressure compressed air is

the actual anchoring point of the barrel and breach assembly near the point

where the air transfers from the valve to the barrel. This is a relatively

small, single screw and although the rear screw that mounts the breach is

fairly strong, the same cannot be said for the one right at the breach.

Under normal Co2 operation, the screw is more than secure enough to hold

the assembly in place for countless shots. However, once you start increasing

the pressure at that junction to double or triple that generated by Co2,

stress will start to take its toll on the screw and threads in its

attachment point. Remember, I am talking about a weakness in the design

when running them at high pressures, NOT when they are used with Co2.

While on the subject, the pistol presents a system even less tolerant to

high-pressure air as the tube is used for the attachment point of the grip

and the barrel assemblies. The screw is very small that holds the barrel

and breach point to the air tube. Unless the entire air tube is re-designed,

with an entirely different valve system and beefed up attachment of the

breach, this is not a good start for a compressed air conversion. A few

have stated that they have added additional screws to hold the valve in place

when used with high-pressure air. The next paragraph addresses what did

happen to a second-hand customer of one of these time bombs. The small

screws that are normally found in airguns are grade 2, which have low

shear strength. Even if the screws are increased to grade 5, the shear surface

is very small, and most of those shear points are on threads, not the

shoulder of the screw or the much more resistant head and shoulder.

A very serious accident happened to a fellow who owned one of these poorly

designed compressed air pistols that used a new pressure tube. It started

to leak at 2000 psi when he filled it, so he though that if he used his carbon

fibre tank to fill the pistol past the point that it started to leak, a

seal would be re-established. In less than the blink of an eye, he went way

past the pressure he wanted and ended up putting 4000 psi into the pistol. The

resulting high-pressure charge, blew the valve and hammer from the back

end of the pistol into one wall, severely damaged his hand. It drove the tube

and barrel into the opposite wall, and slammed the supply tank into his thigh.

The valve nearly took his index finger off and resulted in 4 hours of surgery to save his finger.

This was a perfect example of an improperly secured valve and a design

that had never been properly pressure tested. In this case, the tube was fine

as it could take the pressure but the rest of the system couldn't. It is also an example of

poor judgement and what ever is in the supply tank will ultimately end up in the airgun.

The maximum filling pressure for his pistol was 3000 psi. Do the math.

Let's look at a proven PCP rifle and what pressures have been used to test

the system. The Daystate Huntsman was designed from day one as a PCP

rifle, operated at a working pressure of 200 BAR (2950 psi). The cylinder was

hydrostatically tested at 300 BAR (4917 psi) in order that the tube, and

all components in it, would return to normal condition after that testing

pressure was removed. It does not mean that at 4918 psi the tank will

rupture, but it may not return to normal condition. The Huntsman's valve

is fixed in its tube with a very large hardened pin that goes through the

main tube and the valve from one side to the other (two points in shear), plus

two screws from the top (that hold the breach in place), and a large stud

that is the attachment point of the stock. In addition, the front face of

the valve must be tightened up, once in place, to squeeze an "O" ring into

place to seal the reservoir. This valve is more rigidly held than any

Crosman design. Why? It was required in the Huntsman but not in the Crosman.

What is the big draw to use these guns with compressed air? Some do it

because you want to see if it can be done, while others justify it as a

cheap way of getting a compressed air rifle. False economy. And then there

is the last group that do it because there are those that tell you not to do

it as you badly injure yourself. Human nature is to do what others tell

you not to because you don't like being told not to. Remember this, "Don't touch

the stove". Eventually, your curiosity got the better of you and you did.

The drawback of converting a Co2 gun to high-pressure air is that it can

have very tragic consequences. A false sense of security is created when

shooters tell themselves that they will never fill their airgun to more than

maximum. If you have a carbon fibre tank that is rated for 4350 psi, you can

be rest assured that one day you will fill your rifle, pistol, or cylinder to that pressure

The accepted burst disk pressure for a Co2 vessel is 2200 psi

and industry experts have determined that pressures over that are

dangerous. Remember that 2200 psi in the Crosman Co2 design is the same

level of safety that 4917 psi is to a 2950 psi in a compressed air design.

What air pressure are YOU attempting to run your Crosman designed Co2airgun?

Take care.

Keyrigger