Opinions please on a possible PCP conversion

[happymecanic]

Hi Bob and Lloyd, I have an Umarex Ruger 10/22 CO2 rifle, and I'm thinking about a PCP conversion. I don't intend to get big power, I just want reliability and good shot count. That would be nice to use 850 psi regulated pressure, but right from the start, I know it would be a totally custom build. The main parts are all made from what look like potmetal: the valve, piercing needle assembly body, and receiver case halves. The transfer tube is brass. I understand I can't use more than CO2 pressure with these parts, also I know I have to expect some valve lock due to hammer design, but I believe that may be solved with some tuning. I have nothing against the idea of designing and fabricating an entirely new system, i would just need some advices and directions, as I'm rather new to PCP.

I have some ideas, maybe use a CrMo steel tube the size of a Marauder tube to replace the actual CO2 cart tube, that would give me some choice for regulators I think? Would I need to replace the brass tube with a steel one? I'm also seeing the possibility to use a twin-tube setup in the butt stock by using a fabricated ''manifold'', similar in principle to the one you made to use two 88 grams CO2 carts as tanks Bob  . Was that on a QB gun?

What's your view on this project?

[Back_Roads]

 Wow I was just thinking if anyone has done this conversion, I was tempted to shoot mine yesterday but didn't due to the limited time to shoot and possible waste a CO2.
 My thought was to tether to a 850 psi air source. I was thinking just a custom end cap 

[rsterne]

I am extremely nervous giving any advice on a gun I have not seen, and particularly with one made from cast materials.... If we make the assumption that the gun was properly engineered to be safe with CO2, and you stay with 850 psi HPA.... and ONLY pressurize those components that were originally under pressure.... you should probably be OK.... My guess is that at the back of the gun, where the copper tube leads, the only part holding pressure is the CO2 cartridge itself, up against some sort of seal where it is pierced?.... I would NOT allow any pressure inside the black tube, or the cast part that it screws onto....

Make 100% sure that your regulator has a 1.8K burst disc installed on the output side, to prevent the pressure in the gun from exceeding that.... Even then, realize that while that does not exceed the pressure produced by a full CO2 cartridge on a hot day, you don't know if the gun is structurally capable of withstanding that load....

Stay safe, and remember, if you don't know what you are doing with HPA.... DON'T !!!!

Bob

[Back_Roads]

 The 10/22 does have a sealed 2 cartridge tube. Main issue is the rear piercing threaded plug is set deep into the stock, an option may be custom pressure tube as Crosman did with the 1077/ Wildfire type tube conversion, to make access to a fill nipple at the but of the stock easier.
 I had mine out today on 2 CO2, close to if not 90f little gun was hitting hard on some heavy .22 rimfire spinners at 10 yards.

[happymecanic]

I am extremely nervous giving any advice on a gun I have not seen, and particularly with one made from cast materials...

I totally understand that, that's why I asked for opinions, rather than advices

If we make the assumption that the gun was properly engineered to be safe with CO2, and you stay with 850 psi HPA.... and ONLY pressurize those components that were originally under pressure.... you should probably be OK....

I may try to tether it first to my regulated to 850 psi 48 cu in bottle, that'll give me an idea. When I did that with my 1077 i had valve lock, very curious to see how the 10/22 would do.

My guess is that at the back of the gun, where the copper tube leads, the only part holding pressure is the CO2 cartridge itself, up against some sort of seal where it is pierced?.... I would NOT allow any pressure inside the black tube, or the cast part that it screws onto....

This rifle works like the QB guns, the black tube is pressurized by CO2, so the cast part actually holds gas pressure, as does the valve which is also a cast part. If I ever intend to use higher pressure up to the valve, both the valve body and fitting would be re-made in SS, and the tube replaced with a 4130 steel one

Stay safe

Bob

Sir, this is the very reason I asked on here first before doing anything. Thank you Bob .

[happymecanic]

Wow I was just thinking if anyone has done this conversion, I was tempted to shoot mine yesterday but didn't due to the limited time to shoot and possible waste a CO2.
 My thought was to tether to a 850 psi air source. I was thinking just a custom end cap 

Hey great minds think alike! Would be nice to just re-fill with air as needed, however I don't worry too much about letting CO2 for a long time in the rifle, there is no face seal to crush like in a 2240.

The problem I'd see using an end cap is the interference when shouldering the gun. Maybe a 90° fitting that let the hose out through the bottom of the stock would solve that? What I'm after though, is to make it a true PCP with an air tank in the buttstock, regulated if feasible .

[subscriber]

Happymechanic,

If a pressurized part is going to blow, the size of the bang is proportional to the pressure and pressurized volume. 

The stress in the part wall depends on the diameter (if it is round).  For a given material and burst pressure (or safe working pressure), the wall thickness required is directly proportional to the tank inside diameter.   

Thus, pressurizing a 1/8" ID tube until it ruptures will be a much milder event than a 4" ID tank; even if they fail at the same pressure.  If they have the same wall thickness, the 1/8" ID tube will hold 32 times more pressure than the 4" ID tank.  Think about how thin the wall is for the fixed mounted tubing section of a vehicle brake line.

So, short fat gas bottle are more dangerous; because they contain a lot of volume and thus potential energy; and because their wall thickness needs to be quite large to contain the pressure.

This is a long winded way of saying that if the gas passages in that 10/22 are small in diameter, the parts would not be very highly stressed.  With all the unknowns, this still suggest that if the air pressure is similar to that generated by CO2 you should be OK.  Using a burst disc rated for twice that would be a very good idea.

If you exceed the rated working pressure by enough, something will blow.  If you want to know what will fail, and at what pressure, Lloyed might be willing to conduct a hydraulic pressure test to failure.  Then a safe working pressure may be deducted from that.

My biggest concern would be variability in the strength of die cast parts, and propensity for fatigue failure; assuming aluminum alloy.

Chrome molly tubing would be a good idea.  Seamless would be better.  This soft mild steel tube has a rated max working pressure of 10k PSI.  It manages this with a 0.1" wall thickness because the ID is only 0.125":  https://www.mcmaster.com/50295K717/

This 12 mm ID tube has a 1.5 mm wall.  It is rated for a max working pressure of 3100 PSI: 
https://www.mcmaster.com/50295K726/

More hydraulic tubing here:  https://www.mcmaster.com/hydraulic-tubing/steel-tubing/


For cheap chrome molly steel tube, see this:  https://www.aircraftspruce.com/catalog/mepages/4130tubing_un1.php

More sources from chrome molly steel:  https://www.mcmaster.com/steel-tubing/steel/easy-to-weld-4130-alloy-steel-round-tubes-7/


As this is structural steel for making aircraft frame parts such as engine mounts, it is not intended for internal gas pressure.  However, you could determine that, for a sacrificial length of tubing by means of a hydraulic pressure test to failure.  Never pressurize to failure with any type of gas.  The energy in the compressed gas is significant and dangerous, because it amounts to what can only be described as a bomb. 

This, while the energy contained in pressurized oil or water is much lower, due to their near incompressibility.  The risk of injury from a hydraulic test rupture is that a high speed jet of fluid would escape from the initial crack and enter flesh, or an eye.  At close distance it can be like that of a waterjet cutter.  So, hydraulic pressure testing is not something for the uninformed to play with.  Proper steel plate screening and stand-off distances are a must.


What ID /OD air tube did you have in mind?  Using multiple length of connected 3/4" OD tube will require a smaller wall thickness than a single 1" OD tube, for example.  I am guessing that the butt stock is only about 1 1/4" wide on the outside?

[happymecanic]

Hi Subscriber, thanks for all the detailed infos, that could come in very handy. I was planning to use seamless 4130 3/4'' ID tubing with a wall thickness that could take 2000 psi working pressure, in a twin-tube configuration. The buttstock is actually 1.250'' INSIDE on the lower part, and 1.030'' in the upper, narrower part. This is what makes me think there's enough space for a twin-tube setup. The big challenge I foresee is to make the pressure tube/transfer tube/valve assembly sealed without relying on the receiver to hold the pressure. Also I don't have any timeline on this project, I'd rather continue to educate myself for a little while before starting machining parts.

[subscriber]

Hi Happymechanic,

The more details you provide, the better quality advice you are likely to receive.  Else, the fact that the typical compressed air cylinder stores hundreds of foot.pounds of energy, and can cause lethal injury will be inclined to scare off smart advisors, beyond "there be dragons".

If you are sure that your tubing is rated at 2000 PSI working pressure, rather than rupturing pressure, then using it at less than that adds another margin of safety.  That said, for a sanity check, what is the OD of the 3/4" ID tubing?  Obviously small enough to fit into the stock...

4130 tubing is a good choice.  Does the manufacturer provide a yield strength for it?  Or a Rockwell harness value?  Else, I would assume that to be on the soft side at 20 Rc and 70,000 PSI; rather than maximum achievable for that material.  Soft is good for a pressure vessel, as it is much less likely to crack.  Assuming a softer material arrives at a thicker wall calculation - uncertainty applied here, should force safer choices.  Why assume 20 Rc? Because that is the most common listing I have seen at material suppliers.

Mild steel yield strength can vary from 30 to 50 kPSI; depending on the degree of work hardening induced in manufacture.  When unknown, I start by assuming the lower value.