As others have said it is an "Effective" range that you will get to use poison. You can March and shoot, and double tap.
Skirmishers start at 4's to hit with WS3
effective range 18" = 12 inch march, double tap. So that 4 to hit becomes a 6 to hit if your at short range which is 6".
effective range 24" = 12 inch march, long range. so again you need 6's to hit but to take advantage of poison in this instance you cannot double tap. If you double tap it is 7's to hit and you lose poison.