John Sukey is right, of course. John is ALWAYS right; you can take that to the bank.
Screwy things really do happen when you get into older Enfield-designed rifles.
REALLY screwy.
Enfield was the FIRST factory of any kind in the world to have 100% standardised screw threads WITH tolerances so TIGHT that the parts were guaranteed to be interchangeable. Previously to this, parts were made with a fair amount of slop so that they would go together with a minimum of hand-fitting.
The culprit in all of this was Sir Joseph Whtworth, who was NOT a gunsmith; he was a maker of specialised tools and measuring instruments. When he was appointed the Master at Enfield, Enfield already had a good rifle, the Pattern of 1853 .577`muzzle-loader. This was to become one of the common rifles in both the Union AND the Confederate armies during the Civil War. Whitworth's job was to tighten up manufacturing tolerances in order to make the rifles 100% machine-made and 100% interchangeable. He accomplished this to such a degree that the finished product was re-Sealed as the Pattern of 1858; these were the new, modern rifles which became the iron-barreled Snider-Enfields Mark I, I*, II and II** only a few years later. This is why Pattern 1858 rifles are much less common than the older 1853; the 1853 model was surplussed to make rack room for the newer rifles, thus were not at hand when the Snider conversion program started in late 1865. The new-made steel-barreled Mark III is properly referred to as a "Snider Rifle", not as a "Snider-Enfield".
In the process of tightening things up, Whitworth continued the use of some very old screw-threads but, whenever possible, changed things over to his preferred rounded 55-degree type. Once problems at Enfield were solved, Whitworth went on to design and standardise his two most famous systems, the coarse BSW threads and the fine BSF threads. These all have his 55-degree V with rounded tops and bottoms. The Bolt, Stock, Butt of the SMLE rifle, for example, is a BSW thread. But a FEW old threads continued in use on the SMLE rifle until the end of production and still are being made today in India and Pakistan, both of which manufacture all parts for the SMLE with the single exception of the Body...... and Ishapore even continues to make Bodies for their 8x50R Sporting Rifle on a Lee action.
Whitworth-designed threads became the world standard for a time and spin-offs from the Whitworth standard still are in common use. American NC threads, for example, follow the same pitch as Whitworth calculated for his BSW threads, but use the easily-cut American 60-degree sharp V threads rather than the stronger, less liable to stress-fracture Whitworth type. Bicycles and motorcycles used the CEI thread system (those d*mned English "26" threads), which is based (loosely) on Whitworth's BSF system.
The Number 4 Rifle is based almost entirely on 47-1/2-degree BA (British Association) threads which are halfway to a metric standard and interchange with nothing else.
But no matter how you look at things, they are still SCREWED, one way or another!
Yeah, I know: Whitworth was a screwy guy!
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