The problem with your question is that the time span is too great. 1750's would see much larger calibers being more common. By the 1770's the bore sizes were in general , tending to the .45 to 50 range . There was an article some years back which summarized a great many rifles and their calibers through the mid 1700's and then through the revolution period . If I recall correctly, an average of .54 for the earlier rifles and an average of .47 cal for the later rifles was the result . So, it's hard to go wrong if you choose a .50 . That said, you will still need to decide on a style for your stock, barrel, furniture etc. Look at the difference between Chamber's Edward Marshall, Lancaster, and others. These span the time frame you're looking at , and are very different styles. Caliber is only one small part.