Even without reading I can answer.
The big one.
South African Denel-Mechem NTW-20mm Anti-Material Rifle, arguably "the most powerful one" - firing 20 mm rounds
The question was the right rifle for a little girl, 5' 2" and with a small build. That's a much harder one, and I don't think we've got the right answer for her.
The big one.
I was gonna say the Henry Big Boy with the custom engraving...something girlie, like a big old Rose or something...very cute, stylish and not a little deadly...Sarah Palin just got one;
http://44magjournal.blogspot.com/2010/02/now-sarah-palin-has-henry-big-boy-44.html