Attachable pouches for plate carrier use to take up 4 slots wide by themselves when placing them into a backpack in 0.55 giving you 2 additional slots inside of the pouches. Now in 0.57 stable they take up 5 slots wide.

not sure if this has been changed but you got more available slots then they took up in inventory. Also the 300 round can only has 4 slots which doesn't add up to 300 rounds