Considering a metallic cartridge resolves the gas seal issues...isn't that the main thing that is needed for a practical breech loader?
The reason is a lack of machine tools and precise measurement technology.
As you yourself have already noted, gas sealing is critical for breechloading guns and that seal cannot be achieved without improvements to manufacturing tolerances. Until machine tools and precise measurements become more common, the ability to manufacture large numbers of working breechloading guns is extremely difficult.
IIRC, usertron happens to be a CNC machinist. Hopefully he'll chime in with a more detailed explanation.
that and the IDEA of interchangeable parts isn't there yet. That's what's needed for the firearms revolution of the 1830s-1840s to happen.
The Kammerlader was one of the best early breeckloaders with a good gas seal.
This person has created a breech loading flintlock pistol using a similar mechanism. He reckons it could be built by a 18th century gunsmith.
Step back, take a deep breath, and think for a moment...
... just what does a metallic cartridge fit into?
The trick isn't to manufacture metallic cartridges. The trick isn't to manufacture breech loading guns. The trick is to repeatedly manufacture metallic cartridges of known dimensions with fixed tolerances which can then fit into a repeatedly manufactured breech of known dimensions with fixed tolerances.
That's the difference here. Breechloaders have been around ever since firearms were first developed, while large quantities of useful breechloaders had to wait for several technical advances.
Considering that metallic cartridge expand to create the seal...
Let's step back, take a deep breath, and think again, okay?
The metallic cartridge expands to help create the seal. Tell us, how much does it expand? Tenths of an inch or thousandths of an inch?
And which to you think expands further? The metallic cartridge or the Minie ball?
We've a cartridge which expands a few thousandths of an inch to form the gas seal against the breech. That means both the cartridge and the breech need to be repeatedly manufactured to known dimensions with fixed tolerances. Too small a cartridge or too big a breech and the seal doesn't form. Too big a cartridge or too small a breech and the cartridge can't be loaded.
Manufacturing capabilities are the key in this question.
What heck is with the "step back and take a breath?"
well, the idea had been around since the late 18th century, in fact Eli Whitney demonstrated the ability to assemble a bunch of muskets from a bag of jumbled up parts which impressed them very well. He did this in 1798, however the process of manufacturing those parts was very difficult and expensive, which is something he struggled with for some time. However if you can figure out a way to have someone (preferrably Whitney himself) figure out the American system shortly after that and you've got yourself a hell of a lot closer to developing a practical breech loading rifle well ahead of OTL. Since now you have a workable manufacturing system married to interchangeable parts, all somebody has to say is "hey I bet I could make a lot of money by making a breech loading rifle a hell of a lot cheaper than anybody else in the business." And there you have it.