In similar manner, your new car was driven from the assembly line onto a storage lot, then onto a truck, then off the truck and onto the dealer's lot. It might have 1.4 miles (or .6, or whatever) on it when you go to look at it at the dealer's showroom. If it has 126.4, that is different.
I have read that Corvettes and some Honda motorcycles are also chassis dyno'ed before leaving the factory.
Just as the above argument only applies to certain parts of the country, making generalized statements about factory test-firing (or proof testing) is only situationally applicable, as well.
Some firearms companies don't proof-test or test fire their products. After all, they don't have to - there is no requirement to do so in the United States.
A couple companies have been rumored to fire only standard-pressure ammunition for their test firing.
And, a few companies sometimes go above-and-beyond, actually performing a rough sight-in and accuracy check.
Some companies will swab residue out of the chamber(s) and barrel afterward, but most do not. With any near-new firearm, just getting rid of fouling in the bore and chamber will make it look "New, Unfired" again.
Just like cars...
Around here, a Ford vehicle is transported by rail, driven to a storage lot, then transported by truck to the dealership. The average Ford/Lincoln/Mercury has 17 miles on it, brand-spankin'-new and fresh off the truck.
Yet, their neighboring GM/Cadillac/Chevy dealers will have vehicles with an average of 38 miles on them. That's because the GM dealers don't transport from the storage lot by truck - they send a van full of salesmen and detailers up to drive the vehicles back (it's cheaper, and makes their "destination charge" more profitable).
But, if you go downtown to the Maserati dealer, the new cars coming off the truck show 0.0 miles on the odometer. Even though the car was given a far better post-production inspection than the GMs or Fords, then dyno'd and track tested, the odometer was not activated until the receiving dealer re-flashed the computer.