"Deriving technology from Izanagi Corporation's Micro-spy military line, the minelayer is a deadly combination of cutting-edge high explosives, A.I., and robotics. Carrying thermite encased within it’s fragmenting Duranium shell these drones will sit quietly until a vehicle is within range, then charge and detonate on contact. If an on-foot enemy is within range, the mines will chase them down and leap onto them, detonating. Alt-fire activates a guidance laser from the Minelayer itself, causing all launched drones to flock toward the painted point. Although each clip can only hold four drones, the minelayer is deadly for both infantry and vehicles alike."
The Mine layer is a spidermine deploying weapon. It occupies weapon slot 3 together with the Bio Rifle, which is usually not found in Onslaught maps.
The primary fire launches Spider Mines onto the ground. The Mine Layer is able to control up to eight mines at the same time and will stop firing when this limit is reached. New spider mines can be layed when at least one of the existing mines is destroyed. Every player can have 8 mines in play at a time. If they launch another mine, the oldest mine explodes harmlessly.
The secondary fire is a laser that allows you to "herd" your spider mines into strategic positions.
If a player dies while some of their Spider Mines are deployed, then all Mines will explode harmlessly, and if a teammate is nearby and were to die from the Spider Mine, then it will count as a suicide.
There is a hidden, unused animation for the Mine Layer in UT2004's files that is UT3's "Death by Spider Mine" animation. Instead of exploding, these mines would latch onto the opponent and drill their head. The alt-fire, meanwhile, was going to be simply a grenade launcher, but it eventually made it into a different weapon.
"You may recall Quake 3: Team Arena'sproximity mine gun, and UT 2004 introduces something similar ... albeit a bit more sinister. The new "spider mines" are proximity as well, but when triggered, they attach to and then climb up the poor player. Once on the head, the player hears a ticking sound before his head is released from his now-defunct body. The alt-fire on that weapon (well, I think that was the same weapon) is a grenade launcher, but for a nice twist, the grenades are magnetic. Therefore, you can launch three or four grenades onto a vehicle and watch the fireworks begin."
"Of course, with all these vehicles racing around, you're going to want some new weapons to defend yourself with if you find yourself on foot, and Epic isn't planning to disappoint. Sticky mines provide a fairly obvious way to target vehicles in the game, but the new spider mines are far more inventive and promise to be a lot more fun. Once fired, these eight-legged explosives land on the ground and will scamper toward any enemy in their range or, if no enemies are nearby, will sit tight and wait for one to get close enough. One potentially devastating tactic demonstrated to us this morning is to leave a whole bunch of them in areas of tall grass, in which they're practically invisible."
"As players will be using armored vehicles, there are several additional weapons for this mode to combat these while on foot. There’s a Spider Mine, which when fired sprouts legs and runs towards the closest enemy. If you’re on foot, it will actually climb your body, drill into your head and jump off after depositing a mine in your forehead (which explodes shortly afterwards). It’s a concept reminiscent of the Cerebral Bore from Turok 2, although not quite as over-the-top as that. (...) The reason we were given [for their absence outside Onslaught] is that Epic worked very hard to get UT2k3 respect with companies like The CPL, and they don’t want to start heading towards the gimmicky style of deathmatch shown in other games. At the very least, they’ll be available as a DM mutator, allowing server operators to add them if they wish."