Directly quoting from RMC CL p 140, Base Movement rate is:
"... the number of feet the character can move at a “walking pace” in a ten second battle round"
But that's not the same as saying "X Foe moves 50' in the blink of an eye so X is now out of range of you". The throw of a spear at X is happening while X's running away is taking place. When you have larger amounts of movement allowed in a given portion of the round this problem is magnified (RMC/RMU).
RMSS you have three phases and movement is the only thing that is limited within those phases. Movement is the only part of the RMSS round that is largely non-abstract. Snap is 20% movement, Normal is 50% movement, and deliberate is 80% movement (note:
movement,
not action). So, if you have a 240' run allowed in a round you can move 48 feet in Snap. Let's assume foe X has a better initiative than his attacker and he runs away. In snap he can only run 20% of his full movement allowance. If his attacker is also in snap then he's not going to be able to run any father before the attack. If the attacker throws in Normal, then foe X would be capable of running 70% total (20% snap + 50% normal) of his full movement allowance before the attack is made. Now, if the attacker had the better initiative he could have attacked before even X's 20% movement in snap, or if the attacker attacked in normal, X would only be 20% of his movement away.
In any case, if that's far enough to get out of range, then you have to assume he fled far enough away to make the attack ineffective. You're going to have to have a line in the sand
somewhere. Even if you want to say combat is simultaneous then the line in the sand is simply offset by one round.
Now, you also have to consider that part of the reason for ranges on missile weapons being shorter than they can actually be fired is you're talking about
EFFECTIVE range. If you fire a bow at me from 200' away, unless I'm paying no attention, it's going to be pretty easy to sidestep that missile. Really, I don't even need to look and can just dodge and weave and you're going to have a much harder time hitting me. So you could actually argue even the longer ranges already in-game are possibly unrealistic. So the in-game mechanic tries to strike a balance between those things.
Too often people look at game mechanics and say "That's not realistic!" and they are right... because it's
really freakin rare you can make completely realistic rules and still have them be fun (i.e. manageable, balanced, etc).
BTW, this is why I like the BattleTech round. Movement, in order of worst to best, then Action, in order of best to worst. It pretty much completely eliminates the problem we're talking about there.