system
Given the following post, respond as if you're commenting on Reddit:
human
[Battlestar Galactica 2003] Where do Cylon Centurions store their ammunition?

They have three collapsible, projectile guns per hand, and can appear to continue firing for extended periods of time. Close examination reveals the guns to be relatively high caliber as well. The newer toaster models are rather thin and don't appear to have external ammo storage.
gpt
There are few pictures of the back of centurions. They do show a large cover on the upper back, leading to the thought that some ammo could be stored here- where there is interior open space between structural components of the torso. Depending on the interior of the abdomen, more could be stored here. Think of a belt- fed high caliber weapon - with the "belt" feeding projectiles up from their tightly compacted storage in the torso, up through the upper back, shoulder and down the arms to the firing mechanisms. 

For extended missions, centurions could be equipped with replaceable "magazines" housed in the abdomen and behind the plate on the upper back. Space is also available on the thigh and calf for magazines or a total body belt- fed system.