Interesting that a lot of the same folks who are okay with us being able to own submachine guns/full-autos as citizens (as I am) aren't okay with the police having heavy stuff too.
My reasoning is twofold:
1) I'm of the opinion that if I can't own it, they don't get to own it. We're supposed to be a government of, by, and for the people, not a government that is over, above, and superior to the people.
2) I also have to ask what they plan to do with the things. What, exactly, is the role of the local police force? In order to pull it off, are they going to need the services of a crew served belt fed machine gun? Those are used for either fixed defense against an assaulting force or heavy suppression of an area while another force maneuvers against the enemies in that area. Short of a Mumbai-style attack (something highly unlikely), there's just no need for the police to have such things. In order to be effective with it they'll need to train with it, and if they're training for extremely highly unlikely events such as that (something that has yet to occur in the United States by regular criminals), they're NOT training for other events that they'll be far more likely to face. Therefore, with those weapons they're a distraction at best and a danger to themselves and the populace they're supposed to protect at worst. Submachine guns? Fine- even suppressed ones. Select fire capable infantry style small arms? OK, just know how to employ the things. Belt feds? That's probably a bit too much... better to hand that off to the National Guard- if it becomes necessary, they're going to be better equipped to handle it. The cops might as well have RPGs if they're going to use belt feds.
If I would have a belt-fed, my purpose would basically be amusement. The police have other, more pressing, needs to serve than just giggles while turning ammunition into spent brass and noise. If they're going to burn through ammo paid for by taxpayers, they need to do so to some useful purpose... amusement just doesn't rate for that.