This is weird I haven't experienced much on it but It worked once!!.
1) Pick wep of choice
2) press buy but dont "buy it"(clicking the ok button)
3) pick number of days
4) now let leave your comp alone til it kicks you out of the outfitting room
5) the buy message should be still there
6)click ok
7) it will not have a confirmed message and your screen will glitch a little
8) go to out fit room you should have it free with out GP charge and with your choice of time. if it dont work then but it again but for one day and join a room. when room done, you should have it.
I have not tested this many times. im basing this on today which i bought an L96a1 and did these steps and got it for 2 days for the price of 1. ill try to post screenies or maybe a vid if it is true what im saying.
Tell me if it works what you did with what wep and time you were going to buy.
And if it works also please THANK ME :P.