Ilford Rule wrote: (SNIP) a pressurized tank at a planes cruising altitude would almost definitely explode.

No, the oxygen tanks used in high altitude aircraft are EXACTLY the same as the HPA paintball tank...Kevlar, carbon fiber or fiberglass and all.

A tank that holds 5000 psi inside is not affected by lower pressure on the outside. Atmospheric pressure pushing on the outside of the tank at ground level is only 15 psi...even taking the paintball tank into the vacuum of space would have no impact on its ability to safely contain the 5000 psi air inside.

Ilford Rule wrote: Are you sure? Then why is it against FAA regulations to carry presurized cylinders? (is it to prevent impact damage and resulting explosions?) also, the outside pressure does matter. thats why bicycle tires will burst if they are at their full ~60 psi load.

Uh, where do I begin. Lets see, Middle School science class or, hmmm, maybe High School physics class.

A tire explodes because its structure is designed to hold a specified level of pressure. If you pressurize it to its maximum rating then hit a pot hole or it is degraded in strength because it has been exposed to UV for a year or there was a fold in the tube. Just like a balloon, the tube has a physical size limit. If the lowering of external pressure causes the tube to grow larger than its design physical limity it will fail. A tank designes for thousands of psi, 15 lbs of differential has no impact on its structure.

It is against FAA regs to transport pressurized cylinders because:

1. A cylinder that leaks or ruptures its safety disk  in an enclosed area (cabin of a plane) may displace the oxygen. (CO2 Cylinder!) Causing asphyxiation.

2. A cylinder may have a defect that could cause it to rupture. Such an explosion in the confines of an aircraft (having nothing to do with the fact that it is in an aircraft) could bring it down.

All the possible dangers of high pressure cylinders that might cause injury or damage on the ground become critical problems in aircraft due to damage, depressurization, or close proximity of passengers.

It simplifies things just to not allow them in any state of fill or containing any gas...allow them on board only if verifiable empty.

And again, atmospheric pressure squeezing the tank at  sea level is only 14.7 psi. The tank can contain thousands of psi. It makes no difference what the outside pressure in  wherever the tank is...at sea level or in the vacuum of space. Oh, BTW if your bicycle tire were in good condition and mounted correctly on the rim, it could be taken into the vacuum of space with no problem (there are some considerations due to the physical structure and maximum internal pressure...but of no consequence in this level of discussion)

Ilford Rule wrote: this still doesnt explain why bicycle inner tubes DO burst when outside pressure is lowered.

At the low working pressures of tires/tubes differences of 15 pounds of pressure can take the tube to its structural limit. If the bicycle tire has a maximum working pressure of 50 psi, the addition of 15 psi may take it beyond its structural rupture point.

Filling that tire to 50 psi at sea level then taking it into space, in this case of maximum structural expansion, the tire will expand as though you had added 15 more psi, making the tire larger and possibly causing failure.

Also, the cargo hold is not pressurized. Actually, in some the air is pumped OUT (not a full vacuum, but almost) to prevent fires.

You get a kick out of off the wall statements, don't you? Some cargo holds are pressurized and some are not. Cargo holds are not pumped down. If they are not pressurized they assume the same atmospheric pressure for the altitude at which the plane is flying.

Most high altitude airlines run pressurized so Aunt Mimi's pet poodle in the cargo hold doesn't die from lack of oxygen.

A fully pressurized tank exposed to the low pressure of an un-pressurized hold will not explode because of the low pressures around it. The dangers it presents are always present, having nothing to do with atmospheric pressure surrounding it.

You do know, don't you, that people climb mount Everest carrying oxygen bottles. The top of Everest is 29,035 feet, which is about the altitude at which many airlines fly. Those people are not in pressure suits. The tanks are not in pressure suits. They, the tanks, don't blow up because the atmosphere pressure is lower at 29,000 feet than it was at sea level (now people at such low atmospheric pressure do have problems). Tanks that are within their hydrostatic test period and are not damaged don't usually blow up in any situation. The FAA restricts them so they don't have to verify that they are in good shape or within hydro-stat. If the tank is un-pressurized and they can see that there is no C-4 stuffed inside they will let you transport it to your tournament.

BTW, did you get into the "legality of silencers" discussion? If you miss it, you missed a great opportunity to make inane statements on another subject.