The immediate problem is that the relatively immense power source involved for powering the weapon would be immediately detectable by sensors, negating the sniper's stealth advantage. That's already becoming a problem in sniperville with conventional small arms, hence the proliferation of suppressor/silencer mounts on sniper rifles.
Will we return to the crossbow as a sniper tool? A kinetin-energy launched silent weapon with a terminally guided fire-and-forget projectile, with the effect of a 40mm M203 or GP-30 grenade once it gets there? Stay tuned.
My weapons of choice were an will always be a M79 and a TRW built M1A . Those were my eras fire an forget tools. Albeit a love for the M82A1 Barrett an raufaus recipe fodder did exist the last few years of my career.....
Rail guns ...... Phft....