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Sorry for the double-post, but to give a very basic answer to the question: Its a bullet. It is just fired much faster than a normal gun, and therefore has the ability to pierce tanks.

Im sure MSI gave a good answer, but I cant see it, so...

A railgun is a device that uses electromagnetic force to launch high velocity projectiles, by means of a sliding armature that is accelerated along a pair of conductive rails.

You can Google it to learn more.

I believe you can download the Tanki Client on steam, then log in using your normal password and nickname. It should save the info.

Are you saying that your mission chain is not advancing?

Are you making sure you complete them in standard, non-pro battles, and then claiming the reward afterwards?