Can an ER doctor do Family Medicine practice?
Can an EM physician do primary care ? Yes.
'family practice' in the sense of advertising this on your shingle: No.
There are EM trained physicians who opened a primary care practice once they didn't care to work shifts anymore. I know one guy who does nothing but 'weight loss management' within a multispecialty group these days.
There are only few things you need the formal qualification of family practice board certification for. Mainly OB related stuff. If you don't want to deliver babies, depending on the community you are in, you should be able to get admitting priviledges at a hospital (or even better, find a hospital with a hospitalist service so you don't need to cover your own inpatients).
In the end, you are a 'physician and surgeon' and your scope of practice is what you make out of it. There might be only limited opportunities to be employed outside of your area of training, if you develop your own practice you can do what you want, particularly if you decide to go a bit outside of the competitive urban/suburban environments.
The only two factors that limit your scope of practice as a physician are:
- malpractice insurance: If you can't convince your insurer that you are qualified for the service you want to provide, they won't insure you (and you would be a fool to offer this service/procedure anyway).
- hospital priviledges: In order to be on the 'panel' for many commercial insurers as a PCP you either need to be on the medical staff of a hospital and hold admitting priviledges, or you need to have a formal agreement with someone who does. Depending on the community you are in, they might not let you on staff in anything but your specialty of residency.