I've found a lot of information on what telescoped ammo is, but I can't seem to find info on the advantages and disadvantages of telescoped in terms of battlefield effectiveness and manufacturing costs. Any military nerds here care to help me out?
This is the first I've heard the term, but I did some quick research, and I'll accept the mantle of military nerd. No idea about manufacturing costs, but they're probably sky-high right now since it's a relatively new technology. An article in Popular Mechanics says that while telescoped ammo could take up less space, one company has decided instead to go for more range and power. Whether the military and/or the troops would be up for that is something that remains to be seen. However, the Wikipedia article mentions that some telescoped ammo is caseless. Caseless ammunition has been the holy grail for ages; it weighs less and has one less point of failure. If you're writing any sort of near-term military SF, or even just a couple years out military story, using caseless ammunition is a nice, believable way of showing that technology marches on that doesn't screw with your story the way blasters or particle beam rifles would.
I've definitely considered caseless ammunition for some of my guns, especially the bolt-action ones. The main problems with caseless ammunition is overheating because it lacks a heatsink that is the ammo casing, and that traditional reloading mechanisms wouldn't work with it because gases from the propellant would escape through the back. I guess a delayed gas-operated reloading mechanism or a rotating chamber could work, and I could stick a little heatsink on the guns for sustained caseless firing. (I'm doing this for a hard-scifi webcomic in which I plan to provide diagrams for the internals of most of the technology, so if I screw up the visuals, people are going to notice.)
The best use for case-less would be a specifically designed weapon, although from using the Steyr AUG, from memory at least, the bolt housing the firing pin would create enough of a seal to allow the gases to reset and reload, as the ejection port is only opened once the bolt is pushed back by the gas. However I could be totally wrong. As for telescopic, I think it's the preamble to standard case-less close-quarters. Snipers or ranged weapons will likely stay traditional, considering they've generally stuck with bolt-action, too.
So open bolt, fixed firing pin, dual propellant caseless (one explosive to give it some oomph coming out of the barrel, then the rocket kicks in) hollowpoint tracers. Done and done! (I might use that. 13mm BSC (Bat Shit Crazy) rounds)