Gas economy of the 1911 mags can vary greatly depending on how rapidly you shoot. Shooting very quickly can cause a lot of cool down and only getting 15 shots is normal.
Spacing your shots out when plinking, you can expect to get a full mag. So it can vary depending on cool down.
Regarding the gas spilling out, is the oring around the fill valve present? They can sometimes wear out, or go missing all together. The oring helps create a seal when filling so gas does not spill out.
And of course, as others have said, make sure that both the propane tank and fill valve are parallel to each other when filling.
If it is the oring I made a thread about it here
http://airsoftcanada.com/showthread.php?t=157657