It currently holds about 11 rounds (R11 in the name refers to R[ounds:] 11) because further capacity increases caused green rods to easily slip out of place on the freefall downward; this could probably be fixed with larger ammo if only I had more Y connectors with which to make a larger bottom of the hopper.
I tried several times to make a hopper using sockets, but it didn't work out. Since Y connectors were relatives of the sockets, I decided to give them a try. Soon after, I found out how to make the rods stay in place using them. After that, I did some touch-ups to make it less of a top-loading one-shot barrel and more of a hopper.
I also tried modding this several times (to make it hold blue rods which failed due to my lack of Y connectors, for example), but none seemed to come out as well as this version.
I did not post the gun with this because the only thing I've made with this so far was a back & fire system (triggerless) that wasn't post-worthy, so I just posted the hopper. I plan to use this hopper for other projects of mine in the future, but for now, good luck building it and please...
MAKE THE HOPPER BEFORE YOU COMMENT! (unless you have trouble making it)
Step 1: Parts
2 Metallic blue pinless clips
2 1-way grey connectors
2 White connectors/Snowflakes
4 Orange Straight connectors
6 Red 3-way connectors
15 Yellow 5-way connectors
2 White 3 K'nex long rods
2 Blue 6 K'nex long rods
4 Yellow 11 K'nex long rods
8 Green rods
8 Grey 1.5 K'nex thick spacers
23 Black Y's
It's a small piece count, but remember that it's only a hopper, so don't compare it's count to that of a gun.
Step 2: Use Image Notes! Step 1-12 (all)