THE CROSS PUNISHER - !!!Contains Spoilers!!!
The cross is actually three weapons. The first is in the longest leg of the cross. It is an extremely powerful machine gun with what seems to be a shit load of ammo. This would explain why one of the arms of the cross is never opened, which leads me to believe it's the ammo chamber. You don't ever see Nick load the machine gun ammo (at least I haven't), so I don't know how he loads it. The other arm of the cross is filled with about 8 pistols, all of them identical. They appear to all be .45 caliber pistols, each with an ivory or bone color on the side of the handles. The final weapon of the punisher is located in the shortest end of the cross. It's a very powerful rocket launcher, or grenade launcher. Whatever it is, it blows the crap out of anything it hits. Nick doesn't use this to often, but you can catch him using it when he battles Grey. Nick fashioned his gun after the one used by his mentor, Chapel the Evergreen. We find that the crosses are very simliar, but only by shape. Wolfwood's weapon is self contained, where as Chapel's cross is actually two different guns that come apart to fire.
A -This is the layout of one of the .45 caliber guns that Nick keeps in his cross. He has about eight of them, and they fit in the cross.(See B)
B - This is a sideview of the cross used in the anime, and a diagram of the arm on the cross with the guns. You can see that the guns slide into the arm, and then the cover slides back.
C - This shows the cross as it is seen for most of the anime, and in the manga. You can see how the bottom leg turns into a machine gun, and how the top fires rockets. You can even see where the rocket exhaust comes out of. This picture also shows the trigger configuration. You can see that the skull is actually several different triggers for the different guns and releases.