[quote=“DocAV”]The only time there was a problem with .303 ammo NOT chambering suitably was in 1915-16, when certain makers in Britain (“New factories”) made .303 ammo whose shoulder dimensions were not properly set and checked, and as a result, these cartridges Jammed (failed to chamber or failed to extract) from Canadian Ross Rifles (which rifles had pre-war “sporting rifle” toleranced chambers, and were only suited the the finer toleranced Canadian-made .303 ammo of peacetime)
The British and Canadians soon recognised this fact (soldiers in the field were actively “trying” different lots of ammo, and selected out the worst before going into battle (“M”., “N” and “J” were the worst offenders in this respect…).
The problem was the incorrect setting of the neck and shoulder die in the filling assembly machine, allowing for an over long head to shoulder length, causing the cartridge case to "jam " into the chamber. (not allowing the bolt to close). BY 1917, this problem with Ammo Factory QA had been overcome, but it ruined the reputation of the Ross Rifle for ever and they were withdrawn from trench duty by late 1916 (and replaced by SMLE )
It was so bad with Ross rifles that instructions to “Lengthen Chambers” with a reamer were passed out, as a temporary “fix” and “LC” was stamped on the chambers of all Ross Rifles so converted. At the same time, a reamer was also authorised for SMLE chambers, taking the head end of the chamber to .464; thus allowing for the Mud of the trenches and better extraction.
Otherwise, .303 cartridges are pretty well interchangeable with all SMLE type rifles, and the British cartridge was also interchangeable with the Japanese Type 87/T92 Naval MG cartridge (for Lewis and Vickers Guns, Boat and Aircraft) which the IJN copied from samples sent out by Kynoch in the 1920s.
After the fall of Singapore, the IJN used a lot of captured British and Australian and Indian made .303 in their own MGs.
This is not quite the full story. Although there were problems with some British ammo with regard to gauging, particularly that from Birmingham Metals and Munitions, the real problem lay with the manufacture of the Ross rifle.
The original Ross Mark III rifles were chambered with a head diameter of .460" (and some were found to be .458") which was lower that the British low limit of .462". Canadian ammunition was also manufactured to this lower tolerance. Quite how the Canadians came to manufacture rifles and thd ammunition that were different from the Imperial standard is not known.
Another problem was that Ross gauged the chambers before assembly and not after breeching up. If the barrels were torqued too tightly in the receivers this compressed the chambers even further.
When this was discovered in 1915 the chambers were reamed to .462" and the chambers stamped with an “N”. This was done from 9th July to 15th August 1915 when the chamber was again changed to .464". Rifles with the larger chamber were stamped with an “E” for enlarged chamber.
The rifles that had come to the UK with the CEF still had the smaller chamber so Lt.Col. Harkom (an ex-Enfield apprentice) was sent from canada to the UK to sort matters out. He set up the Canadian Ordnance Depot at Ashford in Kent and reamed the chambers to .464", stamping them with “LC” for Large Chamber, not Long Chamber.
He also hardened the bolts and bolt stops, but that is another story.
Pictures show the three different chamber stamps.