this may be dumb, but what's "in line" mean in terms of muzzleloaders?
edit: I googled it, looks like bolt guns
Since you asked, "inline" is a type of muzzle loading design where the nipple is "inline" with the barrel. That means that the spark from the percussion cap doesn't need to travel around a bend, or 90 degree angle to reach the powder chamber in the breech portion of the barrel after it's loaded with powder. It promotes more certain and stronger ignition of the powder.
Modern inlines have many different types of actions, to set off the #11 percussion cap or #209 shotgun primer. Some are bolt action, break open, exposed hammer, plunger bolt, pivot block, electronic ignition, and maybe even a rolling block.
They usually have fast twist barrels for shooting conicals and saboted bullets, just like some barrels for traditional sidelock guns have.
Their fast twist can usually stabilize longer and heavier bullets for hunting at longer distances, versus shooting patched round balls or conicals out of slow or medium twist barrels.
They often can handle more powder, or even smokeless powder in the case of the Savage smokeless muzzle loader.
Traditional guns are much easier to load and shoot, especially for plinking, and they are made in much larger round ball calibers, up to .75 caliber is common, and even 10, 8 and 4 gauge smoothbores too.
Inlines have a removable breechplug for easier cleaning, while sidelock barrels don't. They must be cleaned from the muzzle end.
BTW, here's my Traditions .50 Hawken Woodsman with the discontinued 1 in 66" twist:
Hope this helps somewhat.