Not at all. Harry's use of guns is quite clearly one of many many many many examples of the effect not doing anything. Hence why the wardens use them, Vampires them, Fae use them etc. And strangely the just keep on working even when 9000 people are flinging every sort of magic possible at the same time. It is very much the opposite of his singular use.
Harry never uses automatic weapons. To the best of my knowledge, he's used revolvers, a double-barrel shotgun and a lever-action rifle, each of which have far fewer moving parts -- and little to no "automatic" parts.
To my knowledge, we've never seen a wizard firing an automatic weapon. Even the young wardens appear to be using, at most, semiautomatic pistols.
The RPG books put a qualitative assessment on it, with a table on hexing. The higher the number, the more power a wizard has to put into the spell to deliberately hex things. "Conceptually complicated, more modern guns (automatic weapons, etc.)" are listed as 4, in the same category as older cars from the last few decades (like Harry's car) and "The exciting technologies 1967 had to offer."
At 5 are "Some smaller firearms may be affected, though conceptually simple ones still work pretty well, at least for a time."
It's not till 7 -- and the chart tops out at 8+ -- that you get to, "Simple guns may stop working at inopportune moments."
For further reference's sake, Harry's power level circa Storm Front has a base of 5, his rote Fuego spell is a power of 4, and he only gets stronger from there.
So that lays it out pretty neatly. The kinds of guns that Harry tends to use most often, revolvers, require more magic being flung around to malfunction, while more complex, modern, automatic weapons, are nearly half-way down the scale -- and they're right
under the threshold of his base power.
(Incidentally, when my players found out about that chart, it made any kind of encounter with mortal hired guns trivial, since they could just cast a zone-wide hex to wreck their weapons. It also made some encounters hilarious, such as when a wizard took out a mortal driver by hexing his airbag into going off.)
Whenever fully-automatic weapons are used, they're always in the hands of non-wizards and they are usually a considerable distance away from the wizards they're firing at. And out of the four instances I can think of where fully-automatic weapons are used in the middle of wizard throw-downs (Storm Front, Grave Peril, White Night, Turn Coat), they're specifically noted as jamming in two of them.
As for Changes, the automatic weapons we see there are in the hands of the mercenaries, who are all up on the walls and not down where people are using magic... and out of those 200+ hired guns, their impact on the battlefield is a grand total of "wound an apprentice with a stray shot, while the wizardly targets shrug off every bullet harmlessly, then literally kill every single one of us with a wave of his hand."
You're right that we don't see wizards having trouble using automatic weapons. But the reason jams don't happen isn't because it isn't a problem. It doesn't happen because everyone who it would affect is avoiding the circumstances that would create the problem.
Nobody in my family has had a skydiving accident. This isn't because there aren't real risks inherent in skydiving, but because none of us are crazy enough to jump out of a perfectly good airplane.