Yup, good old slug-throwers are effective.
Still, it's like sending soldiers to fight in Iraq with muskets. 50 years old? K.. 100? oooooKaaaay.
200+ ??? now were' getting into weird territory.
Er, not really. I would be absolutely
terrified of assaulting a fortified enemy position armed with an assault rifle such as RK-95 or some M-16 series variant, while the enemy would be wielding something like well preserved good quality Lee-Enfields (1895), Mosin Nagants (1981) (especially Finnish manufactured versions) or K98 Mauser (1898) rifles or equivalent stuff... Especially if the terrain were open, and if the enemy were well trained. Those old rifles pack quite a bit of punch. Muskets and equivalent are more like... 300 years old.
Situation would obviously change a bit if it were a prepared assault with an artillery/mortar strike to soften the enemy up and an armoured transport to escort us close enough that the larger fire power of assault rifles would start to make a difference.
The weapons aren't the only thing that dictates an outcome of an engagement, or battle, or a war. It would be very possible to organize effective military/militia units armed with 100-year-old weapons, even these days - and in the future that will be even easier since A. Kalashnikov's design will probably be eternal. The limitations of the weapons would dictate some tactics, but using their advantages it would be possible to fight and win a modernly armed and armoured infantry unit. You just need to remember that all warfare is based on deception, therefore as long as you can deceive the enemy you can win even with inferior weaponry or lesser numbers. Of course, the fact that modern infantry is supported by very accurate and effective intelligence system makes deception a lot harder than it used to be in, say, WW2, Korea or Vietnam. Nowaday one of the still effective ways to inflict damage to a vastly more powerful and better organized enemy would be roadside bombs and, if terrain allows, guerilla tactics... though satellites and drone planes would be a serious problem. But, as long as you could minimize the advantage that the enemy gains from better equipment and larger numbers, and maximize the advantages of your weaponry and mobility of small groups, things aren't ever written in stone.
Underhanded tactics like using civilians as human shields and resorting to terrorism aren't really any means of warfare in my books, though, but criminal acts.
There was some work done on Earth that Was with laser weaponry - as demonstrated by the Lassiter gun, which is mostly a curiosity and antique.
edit: Also, talking about seeing modern day weaponry on tv series such as Firefly or Battlestar Galactica - yeah, it happens. It's more viable to get an assortment of replica guns than design all kinds of new weaponry; that usually leads to hand phaser/laser/twinkler guns that look like super soakers or toy lugers painted black. If you want to talk about unrealistic, how about the fact that Galactica crew uses modern day Earth weaponry. It doesn't need to be viewed as directly as that. Just view them as guns. Kinda like the fact that the characters in Lord of the Rings were actually talking Westron and Tolkien "translated" it to English, along with stuff like Hobbit names and so on...
Also, sawed off shotgun will probably remain forever the most effective close quarters firearm and it's ease of concealment will make it a continuous favourite of shady characters, just like Kalashnikov's design's brilliance will stay valid for as long as projectile weapons are used at all.
