First, I just want to comment on this part. Ping doesn't represent lag, it's only latency. Usually round-trip time for data packets to travel back and forth between you and the server. Real lag would be packet loss; data packets that aren't getting through the whole way. This would result in various effects, such as "warped" movement or your actions not registering. Actions, such as your shots...
When you pull the trigger, you might see the gun firing at the client side, but the information about you firing your gun must reach the server before you can get a message about successful kill for example. If that information doesn't go through, or the response to that information doesn't come back to you, you're experiencing actual lag.
Any network router between you and the server (there can be half dozen of them, or thirty, depending on the situation) not doing it's job properly can cause this kind of lag. Another thing to consider is the raw power of the server machine. Are you sure it was powerful enough, especially if it was a full 64-player server. That requires quite a beast of a machine and it's not exactly uncommon that server admins are trying too much on their heat.
Note that I don't mean to say the problem with clip sizes doesn't exist. It may well be a legitimate problem, but there can be simple reasons for it. For example, lag. Good latency just simply doesn't mean lag related issues can't exist.
This sound similar to what I experienced. I tried to reload my MN9130 rifle, and the animation got kinda stuck and looping. It just kept going on for a loooong time before the reload was finally finished. There was no extra rounds loaded in though, it was just an animation glitch.