[Yo-Yo]

Yep, you got me there, I meant siphoning the gases from explosion used to fire off the bullet. If this was developed earlier, why was the idea subsequently ditched?

What makes you think it was ever ditched? Gas operation is the most common semi-automatic and automatic high powered rifle action type. The M-16/AR-15 uses gas operation, in a novel direct impingement system rather than a piston, but it is still gas operated. Most semi-automatic shotguns are gas operated.

It's far from being 'ditched.'

[Draco88]

So why is AK credited as such a breakthrough in design?