Oh boy, that's are hard one to answer.
Different departments issue different weapons. There is not a national standard in the US for what Law Enforcement can and cannot issue to their officers.
It comes down to what ever they want. Not all departments have a SWAT, SRT, CERT, HRT, or what ever name is out there for those groups.
My department issues it's officers GLOCK pistols, we can choose between the GLOCK 17, GLOCK 19, GLOCK 21, GLOCK 22, GLOCK 23. We carry Remington 870 Shotguns and AR-15 Rifles/Carbines in the trunk. The shotgun and rifle can be department issue or personally owned. That is just the patrol officer.
We had a SWAT team in the past but budgets made us disband the team. They used the samething as a patrol officer plus H&K MP5, GLOCK 18, Remington 700.
Miami-Dade County Sheriff issues it's officers with Sig Sauer P226 and allows them to carry either the Remington 870, Remington 1187, or Mossberg 500 shotgun, for rifle they are allowed to use the Colt 9mm Carbine only. No 5.56x45mm rifles for patrol officers expect for SRT and RID. To my knowledge those are all personal purchase. Even the sidearm. They are only allowed to carry a specific model of firearm but they have to pay for it out of their own pocket.