If you look closely at the barrel, you should see it rests on a barrel plug, or
something similar. I saw something very close to this on a Police bike,
and it mounted on a barrel plug. No dirt.
Yes, no dirt in the barrel, but the reciever, action, ejection port, trigger guard and mechanism are all open. Those areas are very prone to contamination. The barrel plug is really for ease of mount and access. It's a good mounting system, it just leaves the weapon open to dirt that's all. And if an officer chooses to carry one, he is also responsible for keeping it clean for the guy on the next shift. Think of all the grime and stuff that accumulates on our bikes when go for a ride. Out here in SoCal the beach officers also have the high salt air content to contend with too. All that crud is also going into the weapon. Same with the sidearm pistols, although most of those are covered by the holster. I'm not saying it's a bad thing, like anything else it requires more effort to keep it clean and lubed.