Some thoughts ...
While the gun companies can exert reasonable control over the design, materials, manufacture, production and assembly of their products, once a gun leaves the factory it's going to be exposed to influences over which the factory has no control.
Those are:
Shooter (grip/experience)
Ammunition (quality, under/under-powered, manufacturing tolerances, etc)
Maintenance (cleaning, lubrication, etc)
Conditions (in which it's used, including environmental factors)
Something else I take into consideration is the actual make/model of gun involved.
I'm very anxious to carry this gun, I've waited 3 years to find it ( hard gun to get).
What is it, and is it known for reliability, and for demonstrating that level of reliability with a wide variety of ammunition?
Is it a pistol that has been widely used in either LE or Mil circles, meaning it's been tested and observed in all manner of conditions across a wide variety of users & owners?
I typically buy and/or use pistols and revolvers which have been designed for modern use, using good quality ammunition, in the hands of average skilled users. They've been around a while, and the companies have enough of them out there that they've "proven" themselves.
So, knowing the guns are modern, major-maker guns, now I just want to confirm that the particular ones I decide to buy, or are issued, are normally functioning examples of an already proven product.
I used to have a figure in mind for the number of "confirmation" rounds fired (200-300), but that changed over time to take into consideration a very important part of semiauto pistol functioning. The magazines used.
Magazines are at the heart of pistol functioning. That being the case, I typically like to make sure that during my "confirmation" range session(s) I run
at least 25 rounds of the loads I'm actually going to use/carry with the gun, through each magazine. Preferably 50+ rounds.
If I only have 2 magazines at the time, then I up the round count to 50-100 rounds through each one. More magazines on hand, like 4-6? Then back to 25-50 rounds per magazine. I need the magazines to demonstrate they're operating normally.
I've encountered my fair share of issues with some brand new magazines. Some were materials, some were manufacturing and some were just assembly issues that were easily corrected (once recognized).
The major maker modern pistols seeing common service in LE/Gov circles have been produced to use much of the modern defensive/service ammunition. Some older designed pistols sometimes weren't necesarily as accommodating or tolerant of JHP ammunition, and were actually made with ball loads in mind, but newer guns have benefited from modern needs and requirements. What's yours?
Given my druthers I like to start out my confirmation range session going right to the carry ammo. Why not? It's not unknown for premium carry ammo to be on the higher end of the quality and "power" level, compared to budget/low cost "practice" ammo. Budget ball ammo might have velocities on the lower end of the usual range (lower powered), and since new guns have new, stiffer springs, why not break them in with premium ammo that runs more toward the higher end of the normal range?
I'll often test-fire to confirm functioning with either whatever current JHP load is available (as issued ammo), or, given my druthers, with a mixed assortment of current and past carry JHP loads. If it's a small pistol, like my LCP's, I like to run a mix of the commonly available JHP's through them 9and their mags), which
for me usually 4 different types of major maker JHP's.
Some gun companies will offer suggestions to LE agency customers regarding "break in/confirmation of functioning" practices, and some will defer to whatever the customer wants to do, or already has in place as a normal practice.
Revolvers don't need magazines, so I do a normal inspection of condition (assembly) and operation of the gun, and fire anywhere from 25, 50 or 100 rounds of the ammunition I plan to carry in it. Either one brand, or an assortment of loads. I have at least 3 major maker brands/loads I like for most of my revolvers, because you can't predict availability issues ahead of time. I check the firing pin hits on fired cases, too. I also like to check for signs of bullet pull, especially when using my lighter weight snubs and hard recoiling loads.
Now, as an armorer I've had to make repairs and corrections on both brand new and existing/in-service guns. The round count fired for confirmation of successful repair and restoration of normal function is another subject, in and of itself.