Alright,

I have several AK's and variants.  I only have 1 that has had trigger slap, or I guess I should say that I've NOTICED trigger slap with, and I went to the supplied web pages and found the fix, grinding down the back of the disconnector so that it's not supposed to hit and be flat/vertical.

However, after having done that, I decided to check out the rest of my AK's and I have another AK (an SAR2) that ALSO has the disconnector with the huge rear hump on it, and yet I've never experienced trigger slap with this rifle, and I know that because it's my favorite one to shoot, so I shoot it often.

So what actually causes the trigger slap to happen with some rifles, and not others?

THanks...

Oh, and maybe a dumb 922r question too.  I have several other builds I'm planning, and I bought FCG's for them, but if I wanted too, I could use a receiver, all US furniture, a US brake/FH, and a US magazine and I would also be legal?  

This does a good job of explaining trigger slap: LINK

Yeah, that's the article I originally read, and used to modify the gun that had the slap, but I guess I just don't understand why a gun that has the same parts doesn't have trigger slap.

And I tried the trigger on the one I modified, and it still feels like it "slaps" me when I release the trigger for the trigger reset.

Maybe I will have to switch out the FCG because I can't find anything explaining why it would slap on resetting.

Quoted: I guess I just don't understand why a gun that has the same parts doesn't have trigger slap.

Since you already modified one of the disconnectors it will be hard to be certain, but it may be that the disconnector in one of the rifles is slightly different than the one you modified.
Century used several different disconnectors.There were at least three that I'm aware of. Apparently the design was a work in progress, until they gave up.
The fcg that came in my WASR has the smallest remains of a tail on the disconnector and didn't exhibit any slap. Instead my finger got sore from the trigger pull. I never measured it, but it had to be 20#.