The agency can purchase any weapon it wants, and issue it to the officer for duty purposes, within the confines of state laws regarding LEOs and weapons (some states restrict which officers can carry some kinds of weaqpons, some require training and demonstrated proficiency, etc.). These weapons are property of the agency and cannot be transferred to individual officers. I have an issued MG. I take it home every night, and it stays in my house when I am not on duty. If I ever left the agency, I would have to turn it in. There is no way under the law for me to keep it (unless it were rendered permanently inoperable and removed from the National Firearms Registry).
If an individual LEO wants to buy an MG, suppressor, SBR, Destructive Device or AOW, he must abide by all of the same rules as any other resident of the state, though getting the CLEO signoff may be easier.