Pletch's test are wonderful. Sort of like Consumer Reports for longrifles. :-)
Oddly, I had no issue with switching to a muzzleloader offhand. I was shooting a BPCR, so maybe I had already accommodated for the long dwell time in the barrel? My personal opinion is that my long swamped barrel's stability seems to offset the long dwell time. In fact, my flinter is perhaps my best offhand rifle, equaling or surpassing my best modern rifle.
I took a class with three Marines on the USMC Pistol Team. They gave great advice directly applicable to longrifles offhand.
1) Just because you lift a gun up to shoot, doesn't mean you have to touch off a shot. If you haven't broken the shot in something like 3-5 seconds, you start to wobble. Lower the gun, take a breath, and start again. No harm, no foul.
2) When you can clearly see the shape of the front sight, you are concentrating on it enough. I have to use mild reading glasses to see the front sight that well. It works though. Front sight sharp. Rear sight and target fuzzy.
3) Ignore "shake". Some days you have jitters or shakes. Apparently it has zero effect on your group and score. It will shake your confidence, until you remind yourself, "not an issue". One of the Marines won a match with the gun shaking the whole time.
4) "Wobble" is an issue. Practice and conditioning help this. Everyone wobbles to some degree. If you yank the trigger "as it goes past", you'll miss most times. Accept your wobble on any given day, as there is no way to stop it. That is, unless you can get a rest, which is always best in the field. A handy tree is a great friend.
5) When lining up to shoot, close you eyes, mount the gun, then open your eyes. Instead of twisting your upper body, move your feet instead. That way recoil will be all up-and-down, rather than the added yaw to one side or the other. The long dwell time for a longrifle exaggerates that issue.
I hope these tips help someone. They sure helped me.
One last suggestion on grip. I grip my longrifle tight. I don't want lax muscles to allow the barrel to move any more than it has to in recoil. Also, on a bench, I never let the rifle touch a bag or surface. I lay my hand on the bag, with the rifle in my tight grip. In the rear I use a big soft bag for my elbow, and brace my chest against the bench. Same as for BPCR. A gun bouncing and whipping off of rests with a slow roundball going down the barrel doesn't work for me.
Best wishes, and God Bless, Marc