Older guns are expected to have been fired. As such, you firing it isn't going to reduce it's "Value" at all. But while on the subject of "Value", there's only 2 types of "Value". Monetary and personal/sentimental. For something to have monetary value, you must be willing to sell it. Are you willing to sell it? If not, then it has absolutely no monetary value at all. The value is strictly personal/sentimental. As such, you should take the gun out and shoot it. Enjoy the gun for what it was intended to do. Enjoy the fact that your father-in-law gave you a gun that he once used. Enjoy the history behind it. That enjoyment can not even be compared to that of pulling a gun out of a cabinet once in a while and just looking at it and cleaning it. That's not what that gun was made for.
Obviously there are 2 types of guns that I wouldn't shoot. One would be a collector type gun that is definitely going up in financial value. I wouldn't fire it because I want the value to go up. I would have every intention of some day selling it. The other type might be a true antique. Something more than 100 years old that besides being valuable in history could also be too fragile to take a chance on shooting it.
The gun was meant to be shot. If it's a safe gun and doesn't possess any historical value and you weren't going to donate it to a museum, then enjoy shooting it. Later... Mike....
