Re: .223 Semi Auto
Since full length sizing is the norm, the brass will get worked more, and the extractors are normally harsher on the brass in semi-auto's, so less brass life, as already mentioned.
The rotating bolt on an AR does not provide as much leverage as the bolt on a "bolt gun", so something that you could chamber and close with some resistance in a bolt gun will not allow the bolt to lock on an AR, as it doesn't have the extra force/leverage to do so.
I have never found a need to use small base dies for normal/SAMMI chambers in .223, .308, or .30-06 semi-auto's. You might be able to get away with just neck sizing for some chambers or with some brass, but probably not really worth doing so, and then having to test feed every round to assure it will chamber without jamming. And for semi-autos fed out of a magazine, you probably want a bit more neck tension than you are using in your bolt gun, just to make sure bullets in the mag don't change seating depth under recoil.