Barrage

[rah]

From the reference sheet http://apolyton.net/civ4/units.php

Machine gun
Type: Siege
Domain: Land

[wodan11]

"Siege" is a game term and like any game term can mean whatever the designers want it to mean. I personally see no contradiction by having the definition include a unit primarily used to defend during siege conditions.

[Brael]

MG's are great defenders until marines start showing up.

Their siege tag seems to be more a necessity of game mechanics than what they actually are. If you remember back to earlier versions of Civ4 they were gunpowder. With the siege tag they're not only immune to collateral damage, but in order to get an attacking bonus against them you must use mounted units promoted combat 1/charge (nullifies their first strike, drill promotions, and gunpowder bonuses that way too). But even with that, MG's are stronger than cavalry which is the closest they have to a counter for a long time.

Also, for some added fun I'm almost positive you can give units combat promotions and then promote them to MG's, which is more effective than drill for them.