I've used AB on many prop replicas. The black parts of my ROTJ V2 hilt and Kyle Katarn hilt, carbon scoring on blaster muzzles, weathering on Wolverine dog tag, etc...
Before you begin, make sure your item is prepared properly. Make sure it is clean. You also want to make sure the item has not been polished to a ultra smooth surface. If so, the AB runs right off. You can take care of that with some light wet sanding or steel wool.
The trick is to apply in light coats. No not try to blacken in one or two shots. If you do, the AB does not adhere. It creates a coat which will easily chip away. You want to apply, then rinse (I usually have a container of water to submerge the item in). Dry again and apply more AB. Repeat the process unit you get the dark color you want. You can sand or steel wool to remove it if you do not like it or to give it the weathered look.
BTW: The Greedo Killer is a blaster that I have been growing fond of over the last few years. Would love to add one to my collection someday. Very nice work.
I did on this one... in fact I've cut it on ALL the greed killers I've done. But I don't remember exactly. The last one I did was with the real scope in my blaster build thread for myself and I used reference pics of the real prop to gauge the barrel length...Sorry... wish I had a measurement to give.