Can a police officer purchase alcohol while on duty?

At one time any uniformed soldier or Marine could buy alcohol in most states and on base. Rules have changed and a 21-year age limit is enforced today.

If a foreign country has a lower age for purchase of alcohol, that limit is enforced on base.

If a base is located within 50 miles of Canada or Mexico, the base commander may set the legal age on base to 18. This is to discourage military personnel from driving 'suicide rides' across the border (i.e. binge drinking and driving back to base with a carload of wasted guys).

And yes, you can purchase it while in uniform. Purchasing and consuming it, however, can be a bit more complicated. Usually, it wouldn't be permitted. However, there will be events such as battalion formals where you will be able to purchase and consume alcohol while you're in uniform.