Two photos of the 25-pounder gun on display at the Imperial War Museum in London - the detail of the breech mechanism (right) and a rear view of the gun (bottom right, note the round firing platform).
The 25-pounder
Field Gun
Jan Hyrman

The First World War and defence budget cuts during the inter-war period left the Royal Artillery with outdated, obsolete equipment. Indeed, the 18-pounder field gun had a very favourable muzzle velocity, while the 4.5-inch howitzer had exactly what the other lacked - high angle of fire. But these two had to be combined to supply the artillery with a perfect replacement. And, in the end, it got one, which turned into the most satisfactory field gun of the WWII British and Commonwealth armies - the legendary 25-pounder field gun.

As all artillery designs, this one started from the shell. After considerations for  various calibers and weights of the projectile, the gun was planned as a
Modern anti-tank warfare, dealing with fast moving targets, required a gun which could be traversed rapidly and easily. Therefore, a 1918 design by Hogg and Paul was adopted. A round firing platform could be lowered from the trail, on which the gun could be pulled by the tractor. While on this platform, the gun had the advantage of 360-degree movement. During the war, there were several improvements on the carriage and platform, as well as a few special versions for the glider- and airborne troops and for the jungle.
3.7-inch gun/howitzer, firing a 20- to 25-pounder shell. However, due to large numbers of existing equipment, it was decided not to design a completely new weapon, but rather utilize as much as possible from what was at hand. Therefore, many of the existing 18-pounder guns were relined to accept the larger shell. Also, 18-pounder carriages were adapted to pneumatic road wheels, making it possible to allow towing at higher speeds. This "budget version" of the gun was known as Ordnance QF 25-pdr Mk. I. Nevertheless, most of these, also known as the 18/25-pounder, were lost during the battle for France.

Mk. II was a completely different
The ammunition used was, contrary to the WWI 18-pdr. design, loaded in two sequences. First, the shell was placed into the breech, followed by propellant charges in a separate metal cartridge case. This practice gave the gun a possibility to adjust the charge to specific kinds of targets. Also, various kinds of ammunition were developed, including high-explosive, armour-piercing, smoke, flare or star shells. Carrier projectiles containing propaganda leaflets were also in use.

Two main kinds of ammunition trailers were used with the guns, the larger and lower No. 24 and the No. 27 design, newer and more practical to use, adapted for carrying the firing platform. 25-pounder guns were towed by Morris C8 and Chevrolet/Ford (Canadian Military Pattern) 4x4 field artillery tractors or Quads, but there were also self-propelled designs, such as the Bishop (a slow and unsuccessful design on the Valentine tank chassis) and the Sexton (a design by the Canadian Army Engineering Branch, quite successful and a replacement of the American Priest SP gun).


Sources:

Henry, C., The 25-pounder Field Gun, New Vanguard 48, Osprey Publishing, Oxford 2002

Hogg, Ian V., Allied Artillery of World War II, Crowood Press, 1998
design. It had a new barrel, different from the previous design, giving the shell high muzzle velocity. New carriage was designed for this gun, with an old invention put back into practice.