I would strongly advise against using a pressure as high as 1250psi with a standard main tube. As Doug indicates, it's some way above the maximum pressure of CO2 at the valve lock-up temperature. You might get lucky, but I certainly wouldn't do it myself!
In the past, I've talked to the factory in detail about the design, materials and manufacturing specifications for the main tube and have always received somewhat evasive answers about any potential use BEYOND that for which the tube was designed. This tells me that while the specs are perfectly satisfactory for normal use with CO2 - as we all know - there is no certainty of safe operation if higher pressures are used. This is why I always advise against any increase in pressure with these guns unless a high quality replacement main tube specified for a higher pressure is used - such as the Jack Haley conversions.
Stephen Archer
Archer Airguns Inc.
http://www.archerairguns.com
