well a couple things here .
a long barrel does not make a rifle muzzle heavy . if built correctly it will balance correctly .
Even a rifle that’s in the 10 -15 lb range , when built correctly and balanced well , is a joy to shoot . While they are heavy , they do not become cumbersome .
there also is a reason for building a rifle that is balanced farther forward.. It hangs much steadier on the target then a rifle balance back . But that’s for another discussion .
Originally barrels were made of iron . Better barrels were also very soft .
I have read where Hacker Martian” granted still a 20 century smith “ would when making a barrel , make it so soft that the flats could be drawn with a knife blade . Now that’s pretty soft .
American rifleman used to do a whole section on muzzleloading in it printing back in the 1930-40’s .
In many of those articles , especially when speaking of target rifles folks would use large barrels bored for a small caliber . There are many references also referring to softer barrels being most desirable .
A couple weekends ago I actually found an original long distance rifle that was probably built in the lat 1890- early 1900 time frame. It was in very good condition and if I could have afforded the asking price which was very reasonable , I would have snapped it up .
The rifle was bored to 40 cal but the barrel was 1 3/4 across the flats .
I bet the rifle weighed in at around 15lbs give or take . But the balance was perfect. When I sighted down the barrel on a target the sights were rock solid.
What a wonderful piece ..
If we move back in our time line where iron barrels become the norm . IE the era of the plains rifle . We notice that these barrels are very heavy .
Once of the reasons is IMO that this was a mind set of the time . Things were built strong , and robust . They were built to last a lifetime with the intent of items being capable of being refurbished . Barrels produced in calibers 50 and greater are commonly found with measurements greater then 1 inch across the flat . they needed to be this way to hold the charges they were using . Not to mention the amount of shooting that many of these rifle were doing
Further back where rifles were built lighter and IMO more graceful we find barrels that are also very large across the flats when it comes to larger calibers . Remember that back then a 50 was a large caliber and as that caliber grew , so did the barrel thickness .
This holds true especially for muskets where the breach section is often very large but only for a leaght , then its thinned down . The reason for this is based on the characteristics of BP itself .
This very thick area is the location where the pressure spike of ignition takes place . It takes the most beating . Forward of that , the peak pressure within the barrel is actually dropping . As such the barrels needed to be thick to withstand the charges used in those weapons .
So way are the barrels in reproductions , today so thick ?
Well one has to understand that even though modern steals are used over iron , those rifles are still made with an eye to being along the lines of examples of the day . Even if most times very loosely.
Basically because its whats expected .
With the modern designs , we see the barrels getting smaller , thinner . But really IMO this is because , again , its whats to be expected . The guns are designed and built to be more like a center fire in shape , feel and heft .
Another reason for this also could be said that because the weapon is a muzzleloading weapon . The manufacture has no way of knowing just how the rifle will be used . Just how much powder will be dumped down the barrel . Will it be cleaned and taken care of . Will the barrel hold if short started or an obstruction accurse. As such they try to look at the worst end to some existent. But even then we still see barrels fail .
But then also , depending on the rifle your looking at as a compression, the barrels today can actualy be much smaller . But again that depends on what your looking at as a compression as well as what calibers your comparing it to .
There is no doubt that barrels could become even smaller do to modern technologies concerning steels
But I think most folks just associate traditional muzzleloaders having thick barrels . Its whats expected and thus whats provided .
In closing , I would like to say this .
We should keep in mind that BP can produce VERY high pressures when all the variables align. Because of the nature of how our rifles are designed and the way we load , the chances of that happening can be seen to be greater then those same things happening with say a cartridge rifle .
That should always be taken into account in the manufacturing of barrels for mussleloading use