I have 7 GM p mags. Two leaked from new which I fixed with some PTF tape around the fill valve on one, and with the other a good dowsing of silicone oil in the exhaust port/gas router for the other. The silicone oil eventually softened the router internals to a point where it sealed. I then let the oil drain out and wiped any surplus away.
You need to load them with a speed loader, sliding the mag external cover down is a waste of time... bbs everywhere. I found it best to epoxy a speed loader tip onto a loader for zero movement, and then get the feed angle right. The mag lips do get easier, though hard to start with. The strongest speed loader for the job I found was a Viper 155bb loader at £6, other loaders like the 470bb TM mag loader simply cracked internally due to the pressure needed. Gas capacity is big, they do not like Abbey gas nozzles, but do like the Madbull propane adaptor. FPS is slightly higher. Recently in the cold weather at 4 degrees C using MAPP gas with the Madbull Adaptor, (equivalent of Guarder Black in power terms) I got 94 shots from one fill with good consistency.....at 1.6J. Quality is good, and they are noticeably lighter.