I've got a project that only using a #209 primer and a lead ball. It currently is muzzleloading for legal reasons, but it could just as easily be breech loading.
Are there any breech loading #209 powered airguns that are breech loading?

My project
https://www.ar15.com/forums/armory/My-Suppressed-Short-Barrel-Volley-Gun-Project/16-506173/?page=1

Muzzle loader if loaded from the muzzle.  If loaded from the breach it would be a firearm* that does not use a self contained cartridge.  Since there is a burning propellent, it is not an airgun.  For something to be an airgun, room temperature gas (at the start) must be compressed somehow and released to propel the projectile.  Now the gas is usually air, but can be carbon dioxide, or helium (or other gas).

* Now whether the ATF considers such to be a firearm, that may not be the case.

I noticed while the ones I was looking at were break action, that was only for loading the 209 primer. They also had a ramrod for loading the pellet. So they would be muzzle loaders.