Blank-only prop guns do exist. They can be had in full-auto, select-fire, single shot, semi-auto and revolver styles.
Good ones aren't cheap.
Neither is 8mm blank ammo.
Make sure the one you buy is approved by our betters at the BATFE before you purchase it. There are a few people out there whose "blank-only" props are not legal according to Massa ATF Agent.
Most interesting one I've seen was an MP-40 imitation that fed the ammunition backwards from the magazine into a firing pin in the "barrel" and was then ejected mechanically as the bolt was blown back. There have also been advancements in butane/propane fueled blank guns which are generally electrically fired and much cheaper to shoot.
BLANK GUNS ARE DANGEROUS. Idiots have been killed and injured playing around with them. Consider a good airsoft.
Take my advice with the usual grain of salt [tm]. I've only worked as a gun-wrangler for a few small-time productions.
