If I were an Armed security guard would it look good on an application when applying at a police station?

It wouldn't look bad. Some police departments would consider it good experience. More importantly is that you are a good worker and are able to get a good reference no matter what job you have.

Being an armed security guard could be good experience. It could show that you were a good employee, and it could show that you have some knowledge about firearms, and that you have the ability to handle them properly while on duty.