In about 1963 the BMOU for the 39 calibre barrel didn?t include FR. These nations also agreed some characteristics of a 155mm shell, notably a weight of about 45 kg and the length and shape of the US rocket assisted HE shell, but not necessarily with the rocket or other form of assistance.

None of this was a ?NATO? agreement in formal terms. There is actually no such thing as NATO standard artillery ammo.

Artillery propelling charges (cartridges) consist of charge bags filled with propellant, they may or may not be in a metal cartridge case, 105mm are but the 155mm BMOU and JBMOU are based on carts without metal cases. There may be 2 or 3 separate carts covering different range bands, and the highest charge may be just the one big bag. The charge bags are numbered 1 to whatever, up to 10 in 203mm (M110A2) but 8 is usually the max for modern 155mm. With the carts holding several bags then bags may be removed to bring the set down to the required size. In these conventional carts each bag is usually a different size. Ie if you have a cart like L8 which covers charges 3 ? 7, then if charge 5 is ordered bags 7 and 6 are removed.

US uses single base propellant and they have green and white bag carts, with charges 3 -5 existing in both. Europeans designs use triple base propellant and don?t have the green and white bags (although they may use a standard colour for each bag number). There several synonyms when talking about charges bags, sometimes they are called increments, zones, portions and others names.

Modular charges are different, the basic concept is to a have a base charge and then identical size modules, the JBMOU seems to have envisaged up to 5 of these identical modules to give reasonable range overlap at each charge. Archer probably needs the half modules because it has a bigger chamber and without the halves the overlap at short ranges is insufficient.

Rotating drum mags are one way of holding shells and charges in the turret and presenting them to the loading system. Obviously big drums aren?t a very efficient use of space, other modern SPs use other arrangements.

The purpose of FPF and its predecessors (DF(SOS) and SOS) was to provide an instant response, the guns were loaded and layed on the SOS tgt and a gun sentry stood by the gun and fired it when order (which helpfully woke the detachment to continue firing). However, loading on the FPF generally stopped awhile back, and in the modern age when most shells are fitted with multi-function fuzes is not the best idea because the next tgt is most likely not to be the FPF. Of course with big guns (anything above L118) then leaving the gun pointing towards the most likely target area is always a good idea. Computers producing instant data have chopped that bit of time out of the response time, even better if there?s data to the gun. All in all the response time for an FPF is unlikely to be much different to any other fire mission, although its worth pointing out that some western armies are much slower than others and may benefit more from an FPF. It?s possible the real benefit of an FPF is if it means the guns can fire without waiting for an air clearance,  although I?d be pleasantly surprised if the USAF had agreed to this.