Rail guns are almost ready for naval ship deployment, so tanks are probably possible within 20 yrs.
Not nearly the same thing (equiping warships and/or tanks with rail guns). Since Warships are BIG they have powerplants powerfull enough to "feed" a rail gun and these kind of (powerfull) powerplants aren't present in tanks for obvious reasons. I would say that it we would need to see a major leap in powerplant tecnology in order to see an effective rail gun equiping something as "small" as a tank!
When it comes to telling the future, it always boils down to people suck at it. The most brilliant scientists and philosophers always get it wrong. Arthur C Clarke, the man who did some of the best work in that area, thought that within 30 years of the first moon landing we'd have permanent moon bases and space stations with large populations and tourism, along with intelligent computers that still took up an entire room!
The Jedi Master
One more reason why I hate the ArmA3 setting! While it's possible to have a good aproximate guess to what will happen in the very near future (2 or 3 years max from now) it is IMPOSSIBLE to even have an aproximate guess to what will happen in 20 or 30 years from now - Trying to guess or to say what will happen in 20 to 30 years from now (which is what happens in ArmA3) falls in the realm of SCIENCE FICTION (SCI-FI). So no matter what some say or will say, the ArmA3 scenario/setting is SCI-FI, period! And for me ArmA is everthing but a SCI-FI game - Unfortunally BIS transformed the ArmA series into a SCI-FI shooter with ArmA3!
Your analogy to Arthur C Clarke (and his 2001 and 2010 work which I enjoy very much BTW) is a very good one inded.