I'd lean more towards examples that come from later books... I wonder sometimes if Jim didn't quite have all of his "settings" rules straightened out at the start of the series - probably by the third or fourth books, they were fleshed out quite a bit more in his mind and he was able to stay consistent.

Or maybe it was a potion made by him so it was "invited" or exempt. I dunno.

To get back to the point of the magic circle & vampire's gun... I'd say that's kosher. Fire away, oh bloodsucker with that B.F.G. Perforate said mage into swiss cheese.

But it wouldn't be terribly sporting if he whipped out that pistol in the climatic end-fight - if I was the GM I'd establish early on that he's a gun totin' backshootin' sort of bloodsucker, and he's got an unethical and unstereotypical approach to fightin'.

It's not just anything 'real' that breaks the circle:

"As a weapon, it wasn't much. But it was real, and it had been hurled by a real person, a mortal. It could shatter the integrity of a magic circle." (Storm Front)

So Murphy or Harry shooting out of the circle would break it, as would binder throwing something in. His minions - being summoned creatures - couldn't breach the circle. Which seems consistant with the rules laid out in Storm Front

That suggests to me that a magic circle wouldn't keep out bullets shot by a full vampire with a gun, but said bullets wouldn't break the integrity of a circle mid ritual. Of course the spell caster leaping behind cover (or falling to the ground in a twitching mess would probably do that anyway.