The Marui AUGs are solid, but the mag well BB feed areas/hop up units are not overly strong.
The Thompsons and P90s are a couple of the most durable guns made (along with the G3's and AK's).
The metal body kits I have to disagree partially on though.
They're not "steel" just Aluminium or sometimes cast Zinc. They're _slightly_ stronger than the plastic bodies, but I wouldn't want to drop them onto a cement floor, or bash them into a tree repeatedly.

And they also weight CONSIDERABLY more than the plastic parts do. And as I get older, I find that the idea of lugging a heavy, all metal "toy" gun through the woods gets less and less appealing.
Try playing with a 15-20 lb Pulse Rifle for 10 hours strait and you learn to appreciate just how light a stock plastic body AEG really is. It's amazing the difference.
If you use common sense, and are not treating your AEG like a crowbar or baseball bat, they should give you years of operation.
Rook
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Rook's Castle, I dare you! What's a matter McFly? Chicken?