I've explained to Inconnu that the foci are an extension of the mage, that the karma invested is a type of "soul-bonding" of the item to them, and when the item is not attached to/touching them (at minimum in their tightest aura layer), that it no longer is juiced up. He understands the problem of "no magic bullet" or that a knife weapon focus loses juice if thrown (throwing it would be dumb too...that's one expensive piece of hardware to be handing over to your enemies...).
I understand his inability to understand why a gun won't grant improved aim, but let's put it another way:
It's not a living focus. It isn't even an agent with a "pilot" rating. It can't "see" or "aim". Yes, it helps in melee combat. One could say it gives a random chance of increasing the damage (rather than it's improving your aim per se) against the opponent -- like it has an Oomph!™ behind it that is lacking in a normal 'dane weapon. Because the effect of adding dice is not only improved chance of hitting (tru that...) but in the long run improved chance of bashing the fragger's brains out (and isn't that why we care?).
But the focus is part of your own aura field, when it's juiced up. Leave it home and it's a piece of slag any joe can pick up and walk away with. It contains a portion of your own will that -- perhaps through the power of intention & belief that fuels every tradition & magic itself -- has somewhat improved aim and damage (face it, you get 1 extra success per 3 dice on average....so a power rating 6 focus will tend to give you 2 more successes... not exactly a vorpral sword going snick-snack or anything...not even a vorpral bunny.).
So, sure, enchant a gun. Especially if you're a gunslinger. It will be handy when someone underestimates your melee ability and you snick-snack your gun up their nose before pulling the trigger and braining them from the inside -- because it WILL increase the chance that you can shove the barrel up their nose. Pull that one off, the GM may spontaneously rule that you get a few increased aim bonuses before you pull the trigger in the next IP. Just remember to wipe their boogers off the gun when you're done.
But when you say to the gun "hey, help me aim" when you want to shoot the guy across the street, the gun goes "Huh?" because really that's about the limit of its intelligence rating. "Me bash. Me bash. Me slice. Me chop-chop." It doesn't even make it to the Barbarian Counting System (One, two, many, many-many, lots, run-away.).
And that is why a gun weapon-focus doesn't help you shoot anything.