Ok so I saw this situation in the middle of a game and it struck me as really odd. I never thought twice about it when I GMed.

A person entering melee with a rifle can he or can he not melee while holding his rifle?

The alternative at hand was to drop the weapon to use your hands or to spend a turn, turning it into a improvised weapon. Of course you couldn't parry ect,. I think I just ran it as if you were unarmed in my games.

It would seem to me a huge disadvantage to any ranged PC if the melee characters could all basically lock them down for one round while they ready up.

A RL rifle is basicaly a heavy length of material with a weighted stock. It can and will be used as a club or a small quarterstaff. I don't see why a 4 kg Lasgun can't be used as a mellee weapon. Use the rules for a club but add the chance that any parry with it will damage the weapon's firing mechanism/sights

Used in melee, a rifle counts as an Improvised Weapon (as per the stats in the melee weapon section of the Rulebook).

Unless you add a bayonet. Then it counts as a short spear.