Hi Folks, Gun ammo tends to be of the QF (quick firing) type, ie having a cartridge case that contains the propellant and the means of igniting the propellant (normally a Primer), while Howitzer ammo tends to be of the BL (breech loading) type, ie not having a cartridge case, where the propelling charge is in a bag and for that reason there is a separate means of ignition, normally a Tube, that fits into the Howitzer's breech. It's all about rearward obturation, the term for gas sealing in the firing piece (Gun or Howitzer). It is quick and easy to eject a cartridge case after firing and insert a fresh one, while it may be slower to reload in respect of a Howitzer - think of burning debris in the barrel that is left over from the previous firing.
There were three types of QF ammo:
1. QF Fixed - where the cartridge case and projectile are permanatly attached to each other until the round is fired.
2. QF Semi-Fixed - where the base of the shell is inserted into the top of the cartridge case before insertion into the gun.
3. QF Separate - where the propelling charge - although in a cartridge case - and the shell are loaded separately into the gun.
I hope this helps.