What makes me wonder is why 400fps is the magic number when I know MANY guns who shoot about 340-360 who are extremely accurate and durable.

Most anywhere, 400 is an upper safety limit, not a goal. A lower FPS lasts longer, is accurate if well done, and will not cost anywhere as much. Not to mention fewer headaches in upgrades.

But hey, what do I know, right?

Don't take it bad, but I think it's the first time I TOTALLY agree with Grey....
I am going at a game later tonigh with my 340 FPS G36K... accurate as hell...

range

Not that an extra 40-60 fps will carry your bb to the moon, but every bit helps.