Fuhrmann - thank you very much for posting the notes from Zimmer's book. I don't know where I've heard of him, but his name is familiar to me.
: Some of the ideas are great, others, of course are in error, as normal. Even Forsyth was 'out to lunch' on a few of his statements, but not many.
: Zimmer's 'faster twists for ball' was a common error of the mid 1800's period. We know the longer the projectile, the faster the twist is required, not as he's mentioned. Many English makers also made the same error, much to the chagrine of Forsyth. One of his doubles, a 13 bore, stripped with anything over 1 1/2 drams of powder, and had a trajectory of 13" for 100 yards - useless for hunting, and with it's tiny powder charge, wounded more game than it killed. He states in the English and Scottish highlands, hounds are jused to bring to bay, wounded Stags, so none were lost, therefore, to wound is to kill. Such nonsense will get a man killed when hunting game that bites.
: Your rifle is lovely indeed. I'm guessinf the 'breech' shows a "Delvingne" breech, common in military rifles of the 1840 to 1850 period in Europe. This 'guess' on my part is due to the low powder charges Zimmer notes for that area of the world, and the rifling twists. The Delvingne system was used with, then replaced part way through by some makers with the "Tige" system that had an internal projection in the middle of the bore at the breech. Later, but not much, the minnie system came into use, which had hollowed projectiles with clay, box-wood, wax or steel plugs in the base which casued expansion upon ignition of the powder. It was the American's who changed this 'plugged' bullet 'minnie' to a thinner skirted 'minnie' bullet wihtout the plug, which also relied upon ignition to expand. In your breech, the powder charge was dropped in to level with the 'step'. An undersided elongated bullet was pushed down with the rod to the 'step' of the plug, then rammed a few times with the steel rod to upset the slug into the rifling. The rifle was capped and fired. The only real negative in this system, was the very small powder charge which gave a terrible point blank range as did all military 'rifles' of the period. The number of strokes to 'slug' up the bullet had to be precise in number and strength, to ensure accuracy. Too, as the powder chamber fouled, it would hold less powder, but when the same charge as before was used, the powder would rise above the 'shelf' and prevent the slug from contacting the metal, resulting in badly compressed powder cahrges as the rammer was pounded down, which in turn effected the burning rate of the powder and also prevent the slug from expanding properly into the rifling. As European military and military-type muzzleloading rifles using small powder charges make poor hunting rifles due to their looping trajectories, por accuracy and lousey killing/stopping power, it's easy to see why modern governments there, restrict them from hunting.(my own opinion given the information I have at hand)
: High sights will give the illusion of a flatter trajectory, but any descent range zero of zero (90 to 110 yards) makes for very high shooting at close range, as in missing a charging animals vitals due to high hitting. The high trajectories, of coruse, being caused by small powder charges which are demaned due to too-fast twists - a viscious circle. The faster the twist, the deeper the grooves have to be to hold the bullet. The deeper the grooves, the more upsetage of the bullet causing innacuracy, so the faster the twist has to be to give accuracy. The faster the twist, the deepr the grooves have to be and you end up with grooves .0276" deep in a rifle designed to shoot a short slug. Unfortunately, your rifle, beazutiful as it is, was caught by this viscious circle. The system of loading you are using is probably the best for it. I would be trying an American-type picket bullet with cloth patch. It was usually tapered on the front with a rounded base and sides to allow the patch to fold around it easily - sort of a water-droplet shape, with a small flat nose.
: Another error of Zimmer's, is his - faster twist for the shorter barrels. For a hunting rifle of bore size, Forsyth's recommendation is a 1/4 turn in the length of the barrels (double) usually 24" to 26" long. We know this will give reasonable hunting accuracy with good heavy hunting loads, and there is no charge you could shoot from your shoulder that will cause innacuracy. The heavier the charge, the more accurately it shoots. Today, we feel that 80 to 85" is about perfect for round ball hunting guns from .55 to .75 cal. Rifling depths of .010" to as shallow as .006" is all that's needed. The deeper rifling is for the higher velocity, small calibres of .50 to .60. Larger calibres can use shallower rifling, due to their lower velocities. Indeed, .004" may be all that's required for a 14 to 10 bore as ball speeds do not achieve more than 1,550fps muzzle velocity.