Any ideas why Lefty Ruggerio aka 'Two guns' was also given the alias 'Half-Cock'??

Ey up mate, long time no see.

I don't know for certain and this is only a guess but I'd presume the name came from the fact he had cancer of the "prick".

That was the reason he gave in the film. The book also said his nickname was "horse cock," which somehow seems the antithesis of "half cock."

Sup Turi!
The only time i recall Lefty's alias' being mentioned was in an FBI document reporting Pistone's contact with lefty. I'm gonna watch the film again tonight and try and pick out some more info.
So is there not more detail in the book?

The book has tons more detail on everything. It's far superior to the film. You should read it if you haven't already.

Half-cock is kind of like the safety on a single-action revolver. The first click when you pull back the hammer will stay there, hense half-cock. It saves time and effort of cocking the hammer and firing because you are already halfway there. Every second counts in a hit, you know. If you were to accidentally pull the trigger on half-cock, there is not enough momentum in the down stroke to set off the bullet's primer. That way, you can have the gun kind of "loaded" without having to worry about blowing your kidney out getting your gun out of the side holster. So the only thing that I can think of why Lefty is called Half-Cock is because he his careful. What is it? 27 hits under his belt and he's not dead or been pinched? That's pretty careful, if you ask me. Fuhgiddaboutit!

Holy cow, good one Jackson, it now seems so obvious we missed it. That makes absolute logical sense.

By the way, welcome to BB.

Not a problem, my good man. Fuggiddabowdit! Thanx for the warm welcome to the site. Real stand up guy, right there.