Well, you touch upon a rather complex problem: uniformity in military arms. It's very handy for an army to have the same musket; at least the same ammunition, each equipped with a workable bayonet. Every country was different, But I am afraid I can't tell you much about all of them. France built an armoury at Charleville and began producing fairly uniform muskets there in 1717. England contracted them out to London gunsmiths before 1689, and it was a real problem. Manufacturing capacity could only be increased by adding gunsmiths, but of course that increased expenses for the contractor, so there was a fairly constant hassle of the government wanting guns in a hurry, contractors not able to supplying them and so sub-contracting to other shops which added expense or forcing the government to buy sub-standard guns from abroad. Also, as officers would often be supplying their own units, they could return the issued muskets for the cash, and spend it on poor ones for their troops. Finally in 1722 the Board of Ordnance began cracking down, issuing pattern muskets to the shops, placing orders well in advance, which cut down on some of the worst abuses, and stockpiling parts, and using greater specialization. Birmingham shops supplied locks, barrels, and metal pieces like the triggerguards and buttplates ( called "furniture"). These parts could be bought by the government and stored at the Tower- especially in peacetime, when the prices could be lower. These parts were then distributed to contractors, for assembling into stocks (stocking) and making into muskets. Some of these shops doing stocking were pretty big. The Richard Waller shop had around 40 workmen and for the Seven Years War ( 1756-1763) made ( stocked) 263,000 muskets, and repaired 18,000, according to one source ( Blackmore)- I am pretty dubious about some of those numbers ( that's one man making more than 78 a month) but still, it seems evident that there were big shops turning out a lot of guns. The Charleville armoury had something of the same system, with specialized workmen , patterns, etc. but under one roof and, of course, with government supervisors. The 1717 model they produced is acknowledged to be the best of the time, for weight, durability, reliability- and they benefitted from being a slightly smaller caliber ( so with lighter ammunition) without sacrificing any effective range or power.

But though these shops in Birmingham and London ( and the Charleville armoury in France) were working to patterns, there was some variation. Lock parts could be rough forged with simple forging dies, filed to a pattern- but the pieces of the lock- tumbler, bridle, springs- had to be fairly closely fitted, polished and hardened, and were not interchangeable with parts of other locks. Still, each lock could be slightly different, for the gunstocker would be inletting, cutting away the wood to fit it. Buttplates and triggerguards the same. The wood would be sculpted around them, so they could vary somewhat. And the fact that all those metal parts were held together by wood meant that , if there was a failure, it was often in the wood- the stock could break through the wrist, the butt could shatter. In that case, the metal pieces could be sent back to the shop and re-stocked into another musket. If there was a failure of something else- the frizzen was broken off, or the cock- it was possible for a gunsmith back in the shop to perhaps fit up another frizzen or cock.

How did they manage repairs in the field? First, a musket was a fairly durable and simple thing, compared to a modern firearm. It was hard to drop it and break it. A key reason to drill the soldiers was so that they wouldn't damage their muskets by loading them wrong, and they were required to keep them clean and oiled. For spare muskets, any army actually fighting would likely end up with more guns than able soldiers. But field repairs were not easy. Perhaps if it was a lucky day another lock might be found that fit, or fit well enough to get by , out in the field. But almost never would a tumbler from one lock simply drop into place in another. A gunsmith was a smith: he had a forge, hammers, files, drill, a grindstone...not something that an army would carry with it.

So, inevitably there was a realization that military weapons would benefit a great deal from parts being interchangeable, because then more repairs would be possible in the field. The French had the idea first- and the Charleville armory would have been a better place to try it, than the smaller English shops. But it was never realized there. The idea was, however, communicated to Thomas Jefferson, who brought it back to the US and pushed it, and it slowly went from being a good idea to a goal. Eli Whitney, despite the legend, didn't achieve it. Nor did Samuel Colt for some time, despite his claims. A great deal of uniformity was achieved at the Harper's Ferry Armory in the John Hall shop, building his breechloading rifles. And then the Springfield Armory seized upon it by the mid 19th c., taking it as its main guiding light- pursuing it so doggedly that it resisted changing from its Trapdoor model long after the rest of the West had moved to more modern arms, because it had created so much equipment and tooling for making one gun.

But, in point of fact, even something like a 1911A1 pistol did not have totally interchangeable parts. And still today, many guns have to have the internals polished, slight changes made, in order to function well.

Hounsell: From the American System to Mass Production

Roe Smith: Harpers Ferry Armory and the New Technology

Blackmore: British Military Firearms 1650-1850