I reckon it's a feature.
IMTU, the standard missile can pull 6Gs for a good long while (4 weeks or so in theory, but I'm toying with ten turns as a practical maximum). However, to keep it interesting, I've imported the rule from Mayday that vessels running Maneuver/Evade (or Auto/Evade) must devote 1G of their own acceleration to the "Evading" business. This means if the target also intends to be dodging laser fire, the missiles will eventually catch them, since missiles don't mess about with evading. (They're still -3 To Hit when targeted outside Anti-Missle Fire range, though -- due to their small size.)
In practice, it can sometimes be possible to turn perpendicular to the vector of an incoming missile and watch it shoot on by due to the excessive length of its own vector, but this requires a Pretty Fast Ship and a long range of engagement which enables the missile to build up a big ponderous vector (and remember the "cone" it can change its vector by gets narrower the higher its velocity), and it only buys you a little time since the thing will turn after you once it misses on the initial approach.
I was also toying with a 180-degree-turn safety to prevent "own kills", since dogfighting can get hairy sometimes: once you execute a break turn and the missile swings around to reverse course and pursue you back in the general direction from whence it came, you could theoretically cut your powerplant (doing a Dishkili Maneuver and running cold on a stored charge in your j-drive capacitors for a few turns perhaps), and just amble across the line-of-sight between your enemy and the missile, and it should lock on to him instead and whizz right past you on its way back home...
Well, it works that way in the movies...
