Disclaimer: This is an article for an airsoft gun only. None of this pertains to real firearms.
I have a G&G P90 (in tan, in case you were wondering) and like many others suffer from the hicap mags failing to feed. Half filling them kind of works, but the feed mechanism always seems to jam. I've got a half solution here in this article. It doesn't completely fix the problem, but my hicaps feed a lot more reliably now, particularly in semi-auto, than they did before.
This fix works by supergluing a couple of BBs into strategic places in the mag to agitate the BBs and make them fall into the feed mechanism more reliably.
First, crack open your mag by unscrewing every sccrew you can find. The mag has an outer shell and an inner box, both of which you'll need to open. None of the internals should spring out, take care during this step.
🔗Stopping 'back jamming'
I had problems where BBs would roll back into the feed wheel and cause it to jam. I fixed this by supergluing a BB into a spot in the mag like this:
The feed roller on the left rotates clockwise and must make sure the BB clears the spokes on the well so it can turn freely. About half a BB's width between the spokes and your glued BB is what you should aim for.
This is perhaps the more effective mod, and it should ensure BBs are fed into the spoked feed wheel better as they usually have a tendency to clump up from the spring pressure of the mag. I solved this using half a BB glued to the pre-feed wheel.
I cut a BB in half with some sidecutters and filed it smooth on the bottom so glue would bond better:
I then glued it near the center of the pre-feed wheel (gear on the right) like this:
Screw the mag back together (don't forget the filler hatch) and you should be good to go. It's quite difficult to break a mag using this method as the BBs can be snapped or sanded out if necessary, just don't glue any of the gears or wheels!
As with all stuff like this, your mileage may vary. This mod has worked out ok for me although I do still get some jams which are cleared by shaking the mag back and forth or giving it a good whack on the top. I take excellent care of my airsoft gear...