Hey Guys,
Our team has been converting our TM M4 standards to 30rds, using the bbs before the spring method, for about a year and a half now - in fact its actually a requirement for our guys.
For those interested, it's easiest to take the mag all the way apart, place the 38 bbs and lay in the spring guide and spring in the one side - leaving the spring sticking out the top. Next put in place the opposite side, tighten all but the top two screws. Next use a screw driver as a guide to compress the spring down to the hole where the bbs come out, slip that black bb pusher back on the