Yeah, thats why I went for a energy weapon. No fuss with reload, the fact is that there's no energy drop off with the Ion cannon, and to boot there's no accuracy loss due to the fact its a energy weapon. only downside is the fact he can't move when charging (thats walking, not pivoting to aim) also, you've got to consider bullet drop off from the rail gun, and wind speed, and travel time, and a every other factor that affects a projectile based sniper weapon (though I assume the mech would calibrate for some of that,)
while with the Ion cannon it can just point, click BOOM. but the two of them working as a team....