The Alberta Serious Incident Response Team says the weapon was a black Smith and Wesson 122-calibre BB gun.
ASIRT says a man walked into the detachment in Redcliff on Monday and pointed what appeared to be a firearm.
He was taken to hospital after he was shot and remains there in critical but stable condition.
The RCMP have charged 33-year-old Bernard Norman of Redcliff with possession of a weapon dangerous to the public and assault on a peace officer.
A court appearance has yet to be determined.