Board Thread:General Nerf discussion/@comment-27035444-20160821193230/@comment-27306930-20160829161928

That's right Flat, AAAs, AAs, Cs, and Ds are all 1.5 volts. The difference is the capacity, or the total amount of energy stored in the battery, and the maximum current. Larger cells provide more current, and can by extension provide longer life.

In reality it's based on the milliamp-hour rating on the battery. A 1000mAH battery will deliver 1000 milliamperes of current for an hour. Alternatively it can provide 500 milliamperes of current for two hours. See how that works. So the battery life is based on how much current is called for by the blaster.

The Rival benefits from both the extra current D cells are able to provide, AND to some degree extended battery life because D cells have a higher capacity.