A few questions. First, are there normal 30 round mags for the MP5s, or only 24 rounds? Second, how do I buy explosives? I have some dynamite and grenades, but I' not able to research them.
Only 24. For more powerful explosives you need Promotion II, which is acquired by acquiring all important members of staff - the Researcher, Xenologist, Military Envoy, and Engineer. If you're really stumped, hit Q to bring up the Tech Tree Viewer, search for "Promotion", and look for what you're missing; click on something on the left to see what it needs, and keep digging until you find what you're missing.
Once you hit Promotion II your armament options will open up fairly significantly in general. Prior to that the best guns you're going to get are via
Contact: Cult Arms Dealers and various levels of research into each cult, most likely. Dagon/EXALT/Red Dawn will get you assault carbines (M16/L85/FAMAS/AKSU), Black Lotus can get you a sniper rifle.
And finally, I got an exobiological contamination missions that turned out to be swarms of insects, and I had to abort because I didn't have any flamethrowers or anything else that could do much damage. Will that missions come up again at some point?
Yes, there are multiple types of exobiological contamination. They're one of the rarer mission types though I think.
