A Player of mine asked this question (and yes he did to my amusement use the term big iron)
"Look if there are modern firearms on Midrea, why don't all the mages carry them and that if there is an anti magic field they can just pull out the big iron and go to town ?"
Well thats a good question. The reasons are
#1 Guns are expensive and that money can be used better for other things like making magic items, buying property or spell research
#2 It takes a time and money to learn firearms (even the cheaper black powder ones require a feat) and even more time to be good with them. Its not cost effective in terms of feats or GP investment (time and money) to learn to use something that is no more effective than a 1st level spell in terms of damage output. Sure it works in an anti-magic field but such effects are uncommon.
#2 The only reliable means of making ammo other than 9th level spells is Spanderals Duplicator a rare 7th spell.
#3 Very few Wizards can even manage that kind of magic. I figure about 1 in 200 persons. will have the intelligence, disposition and training to become a magic user capable of learning that spell. Of the casters, about 5% will achieve at least 13th level. Of those maybe 1% will know that spell. This means there will be about 1 person with that spell per 400k people. Of those maybe a 10th are likely to bother with firearms. Thus of a population of 80 million, 40 million adult humans there are about 100 gun using spell casters.
This seems like a lot but in reality its a tiny number and the actual users of "modern" guns number under a 1000 plus a few members of elite military units ...
The Return of Planescape? - Curious. There is a brief note on this 'countdown' at Enworld.
13 hours ago