Advanced format ruling
XYZ-Dragon Cannon
Card effect (current official text)"X-Head Cannon" + "Y-Dragon Head" + "Z-Metal Tank"
Must first be Special Summoned (from your Extra Deck) by banishing the above cards you control. (You do not use "Polymerization".) Cannot be Special Summoned from the GY. You can discard 1 card, then target 1 card your opponent controls; destroy that target.
Current rulings for this card are maintained by Konami.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Advanced forum — sourced change requests get reviewed.