Its a tough problem
There are devices called "thumpers" that make a loud report, gas operated, spoon actuated.. pretty good Physical simulator. No flash.. just a bang, and about $3.00 a shot after a $300 initial investment.
These are the typical LE solution.
MIL solutions.. are in many cases just a inert object chucked in to the room prior to entry.
Coming soon is an effective "airsoft" grenade that generates an nice report .. and sprays 300 bbs in 360 degrees with good force. Reliable timer, relativly easy reload ( yes 100% reusable ) with no parts to be lost ( except the pin, which does separate from the body.
These are very effective simulators.. I have be involved in the prototype testing.. these things are the answer to the grenade question.
Airsoft Innovations is the designer.. time line to production.. early 2008
CAPS Range Officer
Toronto Downtown Age Verifier
If the tongue could cut as the sword does, the dead would be infinite