Was .50 Common in 18th Century America?

I think the takeaway and what I tried to convey was .50 wasn't a standard caliber during the period as it is today. There were no "standard calibers" per say. Variations existed from each and every bore due to the nature of their manufacturing process.

A swamped barrel is the only way to go.
There is a reason these barrels were swamped. If you are spending the $$$ and sparing no expenses that should be a number one priority next to wood selection.

Likely get ate up for saying there were no "standard calibers" but please show me refrences of an exact rifle cut to .50 repeatedly during the period. In respect, none of our rifles are HC in that matter. Sorry gents, get tired of beating a dead horse at times. We do our best at emulating what we have and I understand the OP's question, but frankly 50 nor any of our modern calibers are HC, the variations and differences existed. Only way to get a truly HC caliber is to have it forged on a swage over a specific mandrel, forge welded, cut for rifling then finished by hand.

However, we have to get close and MAKE DO with what we have available today and that is acceptable. Should have said it from the start and I didn't but yes, obviously. 50 will be adequate. However I would absolutely get that .50 swamped. You won't regret the decision once you shoulder her and feel just how well she balances. A straight .50 barrel is heavy and cumbersome. Swamp it, very pleasant to carry in terms of weight.

Bingo, in terms of doing it right no maker that I know of will steer someone away from a swamped barrel to a straight, respectably it's the opposite. Bring me a straight barrel and I'll do it for you, but I'm going to put two examples in your hand to feel and examine. 99% positive you'll select the swamped profile.

I built a straight barreled genaric golden aged rifle some years ago in 54. Excellent shooting rifle but still it was very awkwardly balanced and that will be more so the case with a 50.

If your maker won't build a swamped rifle, go elsewhere. They really are not that hard to do right. I personally have only worked four swamped barrel builds so I'm no expert but the proof indeed comes down to comparison and if it was my money, wouldn't even be having this conversation. I highly, highly suggest you get yourself a quality swamped barrel, you'll certainly be glad you did.

For what it's worth, the swamped verus straight profile is more important than the caliber IMO.

In a way this is much like a blacksmithing topic I had with a friend on anvils yesterday, he loved his until he came out and worked on my anvil. People must have an object shaped like a true pattern anvil... Well railroad track configurations are everywhere. Simply put, railroad track anvils are absolutely terrible for serious metal moving. Why? Mass, they have absolutely zero mass unless turned on end. Real anvils have an iron base that makes up approximately 90% of the body which = mass under the working surface. The next 10% is your hardened work surface. Bla, bla, bla... I'll stop.

Long story short, this anvil topic is much like a straight verus swamped barrel. Proof is in the function and that will wins everytime.

Absolutely! As I said before, go with swamped. After all, you’re saying you want this gun to be as close to an original as possible.
If that’s the case, put the straight barrel away until you want a mid-nineteenth century gun, and save up for a swamped barrel.

Don’t kill the correctness of this project from the start.