the USR-10 gas model will handle red gas for a few rounds before things break on you. just stick to propane ok? but hey the USR is a cheap gun to buy especially even cheaper in the states so who cares if you use red gas on it
(don't forget to tell us the results after)
for the spring USR, if you want to go the economy way, get a laylax 130-150 spring with some sear upgrades then teflon tape the internals to make things airtight, remove the "real shock" weights.