No, pennies wouldn't work. There are some methods that would, but pennies are not one of them.
As for converting an airsoft gun into a firearm. Well, with enough money and effort you can convert damn near anything into a firearm. However there are much easier ways of building an AR-15, even a full-auto one than converting an airsoft gun.
Sort of like converting a car into a plane. You're better off just building the plane from scratch.
80% lowers are readily available, and once you make them into a complete semi-auto lower, there's only one more small pocket to cut out to make it select-fire.
Hold up, aren't these airsoft guns battery powered? How would this even work?... I am thoroughly confused now....
Yes the battery-powered ones are infact battery-powered
Those you really can't do anything with, their entire design is completely different. Dimensions way off, they are too fat, in order to accommodate the motor and gearbox.
The gas-powered ones (those are the ones ATF kinda got twitchy about) are much closer to the real AR lowers in design and dimensions. You can actually mate a real upper to it. However the FCG pins (and the FCG itself) are slightly out of place. So If you used a real BCG, the hammer wouldn't strike it. Also the inner cavity is cut somewhat wider inside so real FCG parts, won't fit.
Again, with the amount of work necessary to make these lowers work, you may as well just take an 80%er and make a select-fire lower from scratch.
BTW I'm really not sure what ATF actually did about it. I guess they decided they had their head up their ass and these airsoft guns are not infact "readily convertible" Considering the fact that pretty much every major airsoft retailer in the country has been selling them for years now, and none of them have yet to get raided.