Something I'm surprised no one has mentioned: Set up a series of scenes that show the firearm being used in a setting and manner which has something to do with its intended purpose.
If it is a service sidearm, set up some basic practical drills -- El Prez, Bill Drill, Mozambiques, etc. -- and show us how effective the gun is for defensive shooting like a police officer, military officer, tanker, pilot or other original intended user might need to do. Tell us how easy or difficult you thought it was to accomplish the task. Is this a sidearm you'd be comfortable relying on in a tight spot?
If it is a target arm, set up something that is close to the original intended target course, or a portion of it at least, on the intended target form, and show us how that gun works under those conditions. What features of this gun assist you in precision accuracy? Tell us a bit about the specific shooting sport this gun was designed to excel in and what makes it good or bad at that task?
If it is a front-line military rifle, get a pile of stripper clips or mags and run a few simple courses of fire. Operate that bolt and reload a few times while prone behind cover. Give us a rapid-fire kneeling string on a ~50 yd. target. How smoothly does the system work? How fast can you reacquire the target and how fast can you put a magazine-full on the "enemy?" Yes, you can shoot it off the bench and show us what kind of accuracy it can produce, but shooting the weapon as it was intended to be shot is much more useful information.
As a side note, try to put together a decent assortment of the accessories and gear that would have been issued or delivered with that weapon. How well does the whole system work together? How smoothly and effectively can you recharge the empty weapon from the issued web-gear? Are there tricks to getting the stripper clips to feed smoothly?
In other words, put the gun into context as much as possible. I can see pictures of one anywhere. Show me how it really WORKS.
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Oh, and for heaven's sake, write all of this info down ahead of time, rehearse your spiel, and don't post your first take. Speak clearly and calmly and stick to your script. Give us relevant info without rambling and repeating yourself. If you do a VERY good job planning out your demonstrations, explanations, and detail shots, you should probably have between 5 and 8 minutes of information. Practice, make several tapings, and don't post it until it is honestly watchable.