Propane doesn't provide a flare of the right properties. Powder makes sharp short flares with burning particles. Rich propane flares (low oxygen) make the typical bright orange lazy flare. Balenced or lean propane flares (premixed oxygen) are blue or even purple.
Automaticly loaded rifles are typically gas operated. At some point late in a bullet's flight down a barrel, the bullet passes a side tap in the barrel. In Armalites the side tap is about 2/3 of the way down where the triangle sight begins. As the bullet passes the side tap, some gas from propellant combustion (30+kpsi!!!) shunts upwards into the gas tube (in armalites) or directly acts on a cocking rod (e.g. FAL or AK47) which pushes on the top of the bolt to power the recoil cycle.
Blanks do not properly cycle a gas operated automatic firearm because there is no obstruction in the barrel to generate a high enough pressure to work a rifle action. Gryphon is correct in applying Newton's 3rd law of balenced forces. However, I am not sure if Gryph is asserting that the reaction force of the bullet directly actuates the recoil mechanisms. Applying force to the bullet has no effect on the recoil mechanism until the bullet passes the side tap. Before this stage, the bolt in an auto rifle is locked against the breech by the bolt carrier or some similar arrangement.
The accelleration of the bullet exerts a reaction against the propelling gas which results in a high breech/barrel pressure. With no bullet, only light gas and air is accellerated so a low pressure develops which is insufficient to cycle the loading mechanism.
A BFA partially obstructs the barrel to generate a higher pressure to actuate various mechanisms in a rifle. A funny loading results though. Because the BFA is mounted to the flashhider or welded into the barrel, a net tension results which pulls on the barrel when the gun is fired. This does not occur with a bullet except for the frictional force in smushing a bullet down the rifling.
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