Miss K, words like "honor" and "chivalry" work quite well on the playing field, but we're talking about, even at a number ranging in the hundreds, a group of young girls who are certainly super powered, but still young girls, not Navy Seals, (probably a bad analogy, since the Seals do, in fact, use modern weapons) who are for the most part inexperienced. The only real experienced Slayers are Buffy and Faith, and a couple of dozen one-battle-survivors from Sunnydale. I'm not talking about "packing heat." Habitually and constantly going armed requires a mindset not learned in a few months, as witness the far too many accidental deaths caused here in my own country by the improper use of firearms. I'm merely saying that the Slayers should have the option of "evening the odds" with the properly applied application of modern weaponry when the need arises (as in with the Judge and the rocket launcher). As concerned as Buffy is about the welfare of "her" girls, which epitaph would you think she would prefer to have applied to any one of them: "She fought with honor and died young," of "She fought effectively and lived to grow old"?
And, for the record, Andrew's answer never struck me as much as an answer as the avoidance of one.
P S---If it ain't Spuff, it just ain't enuff.
Edit:
No, or at least on an extremely limited basis. There's a genre convention to stick with, of course, but from a realistic perspective, there's too much danger to innocent bystanders. Once a bullet leaves the barrel, it doesn't care who's in the way, can't be directed, has a much greater effective range than a stake, sword, or arrow, and can go any which way. I think we got enough gangbangers, cops, drug dealers, FBI, and BATF agents throwing lead in all directions already. (Disclaimer: I'm pro-gun ownership, and I've carried since I was 18; but this is the same argument I use against giving police departments these nifty little .22 machine guns that fire 1200-1500 rpm: don't the bad guys throw enough wild shots everywhere without the good guys adding to the carnage?)
(I didn't start carrying until I was 21, as is requires in my state, but since I'm now 60, I think I've been at a bit longer than you)
I agree with you about the "spray and pray" method of firearm use advocated by such experts as Rambo and the Terminator, but if you pay careful attention to the fight scenes in BTVS, ATS, or the current issues of Buffy Season 8, there are very few innocent bystanders to be concerned with, and under those circumstances, the responsible use of modern firepower would most certainly help the at least some of the Slayers survive long enough to draw their pensions.
(Besides which, the only time the hero misses the target in the movies is when it's integral to the plot)