What's the difference between an AR-15 firing pin and an M16 firing pin?  If I use a full auto bolt carrier assembly in a semiauto AR, will I have to replace the M16 firing pin with an AR-15 firing pin or the retainer pin? Can you use an AR15 firing pin/retainer pin in an M16 carrier or vice versa? Is one better than the other? TIA.

Tag.

I didn't think there was a difference but if there is, I'd like to know

Yes, there is a difference.  The full-auto pin has a larger diameter collar.

Quoted: Yes, there is a difference.  The full-auto pin has a larger diameter collar.

And no, you don't have to change the firing pin if you use an M16 carrier, or any semi type with a shrouded bottom.

The small collar pin was designed to use with the (rapidly disappearing) carriers that had the cutawy under the rear of the pin, and the notched hammer. It also works just fine in fully enclosed type carriers.

So. unless you have one of the carriers in which the underside is cutaway under the back of the FP, use whatever you've got for a FP

Diameter of the collar.  Makes no difference from a legal standpoint, does not affect full auto vs semi function.  

It has to do with the ramped "AR-15 type" bolt carrier.

Quoted: Yes, there is a difference.  The full-auto pin has a larger diameter collar.