Ok i jsut had this issue with my KWA MP7
Lube your mags a little less then you would with your normal GBB mags
DO NOT lube your internals with silicone oil, use either grease or graphite filings. Both will work fine but remember to be light on the lube, use just enough. Remember to keep all the lube away from the air nozzel and the hopup. As soon as silicone anything touches the rubber it expands and there is no going back.
IF YOUR RUBBER SWELLS do not panic! You can order them directly from KWA for a couple dollars a peice. I recomend having them on hand if they do swell.
There are a couple videos on the internet which show you how to replace the rubber because this is such a frequent problem. This video should be useful. It is very easy to do this, just remember to be gentle with the gun and when you take out the mechanism from the reciever to tape the selector parts down so they dont fly out. The hardest part of the process is getting the trigger back in, make sure your spring looks like the one in this picture and is not bent all screwy.
kwa mp7 replace hop up disassemble and reassemble p1 - YouTube
If you ever need help with the assembly please shoot me a PM, I will be glad to try and walk you through the process.