speech jamming gun can silence anyone instantly

This is mostly a way to startle someone into being quiet more than it is a security device of some kind. It breaks your concentration, and is much like first encountering a quick echo. It's not really going to be the "MIB weapon of choice" to 'save the day' in an edge-of-seat mission to stop some well-meaning whistleblower from unleashing pandora's box upon the world.

A simple rifle would suffice for that task.

This would be particularly useful in schools. When you get that mouthy student that never knows when to shut up, you use this nifty little gadget on them - then deck them in the midst of their instinctive "WTF-Why-am-I-hearing-myself" confusion.

If you have enough focus, however, this device is not going to render you silent. It's basically a way to create the silence necessary to interrupt someone who is difficult to interrupt without the use of a tazer or team of snipers.

Wow, kinda always saw this type of thing coming.. what a shame. The double edged nature of technology