A 15# butt is quite large. Are you sure there's not 2 of them in a cryovac pack?
At any rate, I usually cut 7-8 lb'ers in half so they'll cook a little faster and provide more bark.
New Braunfels Hondo Offset
New Braunfels Bandera (Refurbished, Modified & Improved)
32 year old Char-Broil 450
1996 Blue Weber SS Performer
2003 Black Weber SS Performer
Weber 22.5 OTS converted to OTG with rotisserie & Smokenator
Weber SJS & modified stock pot Mini-WSM
STOK Island gasser