I would not worry very much about the harmonics on an octagon hunting rifle barrel. Not unless the barrel is very small for the caliber or bore diameter. The 8 "triangles" that are formed by those 8 flats make such barrels very rigid, compared to round barrels. Round barrels have to be very large in diameter, or have flutes cut in the barrel, or both, in order to be as stable as an octagon barrel. All barrel vibrate, but its the round ones that "whip" so badly that they need to be free-floated to get best accuracy.
If you are going to put a scope on any muzzle loader, you should consider a long eye relief " Scout " scope. Mount it over the forestock, so it can be mounted lower to the barrel, and does not interfere with your hammer or cock on a sidelock. The lower mounting allows you to use the same stock as you use for iron sights, without having to add height to the comb of the stock. The power of the scope can also be held down compared to the heavy power scopes put on modern rifles. With the scope forward, both your eyes can remain open and focus on " infinity" and the target, looking through the scope with one eye. You pick up the target quicker with a scout scope, see it clearer, and also see your cross-hairs faster. And, you don't get eye strain, of " fatigue" using a scout scope, like you often will using a rear mounted scope sight.
I also agree with using those Lee Shaver front sight inserts if you are going to mount peep sights. You cannot get better advice on that than what Idaho Ron has told you, and he has years of experience using these sights, and others, by comparison.
The first time I looked down a barrel at a Lee Shaver insert, I stopped to ask, " What is that, and where did you get it?" of the gun's owner! That is just how impressive, and well thought out, these sight inserts are.
Lee has a regular ad in the Single Shot Exchange, where more readers are shooting single shot breechloader cartridge guns than MLers, but most have MLers for target work, too. Like Idaho Ron, I got off the seat at the bench, and did some quick mounts of my Friend's rifle to see how easy it is to line up the sights using the Lee Shaver Inserts. No problem at all, and somewhat more easy than using the more traditional Lyman front sight. I would not hesitate to use them on a hunting rifle, particular when cloud cover can change several times during the day.
:hatsoff: