Wow, muzzle blast is similar in size (a bit smaller) than what one would expect from a .50 DE...
A bit large, you should only expect about an 60mm flash with a 20 - 30 mm circumferance from a 9mm, unless it is a self-loader load with a higher grain...
Even then, that size flash is only expected in very low light conditions.
Also, if you are looking for enhanced realism, add in a casing, and a small bit of smoke from the ejection port...
Being that you are in NY, and I am somewhat familiar with the regulations in the area, you should be able to get a permit to use a blank/prop gun with 24 hour notice if you are in the theatrical business or are working on a valid production (the definition of valid is VERY lax). You do need to hold a business license for theatrical productions though.
Aside from that, you could invest in an AEG (Airsoft Electrig Gun) or GBB (Gas Blow-Back) to get the blowback effect, then blur in the shell casing... A decent One would set you back around a hundred or so; a very good one can range into the thousands...
Being in NY, have you thought of going to your local police station, speaking to the duty sgt, and explaining that you are with a small film company, are working on a production requiring a muzzle flash, and were wondering if it was possible for you to visit the range and observe some live firing for comparisons?