Not sure what you are meaning when you say old school, but...
The biggest difference in the new inline guns versus a sidelock will be lock time and most weather reliablity.
I will try to answer your questions in order:
1. The newer guns will usually have a must faster twist rate in the rifling, meaning they can shoot more efficient projectiles. My .50 Hawken is still very accurate (minute of squirrel), but it has a slower twist, so it works best with a round ball. The range is not as far with a round ball, but accuracy is still accuracy, just not as far away. A smoothbore musket will be inaccurate at distance, but up close, may be just as deadly.
2. A smoothbore should work good with birdshot, as its basically a shotgun barrel. Its just a matter of having the right components.
3. A used gun is a used gun. You should always check them out thoroughly. To some people pitting is a deal breaker. My .50 Hawken has a pitted barrel, but it has made no difference in accuracy that I can see. It is more of a pain to clean.
4. No powder is suited to outdoor storage. If you are talking real blackpowder vs substitute powder like Pyrodex, I have yet to see any degredation of my powder stocks, but I use the heck out of it.
5. Real black powder is much less corrosive than many of the newer powders. American Pioneer and Pyrodex are very corrosive in my experience. All of them need cleaned and oiled when done.
6. As long as you keep the cap and lock area dry on an old gun, it should fire. The inlines will probably be a little better in this regard. You also need to keep water out of the barrel of course.
7. The inlines will be easier to clean, as the breech plug is usually removable, meaning you can push a rod clear thru the barrel. Not so in a traditional style sidelock. It becomes a bit more tedious on the older style sidelocks.
8. I think any gun in your hand when the law comes around can be taken into the wrong consideration. The ATF does not define muzzleloaders as firearms tho inline or not.
All this said, I still dont own an inline, but might some day. I will still use my sidelock tho as long as I can. In the broke college student days, I took a deer using a piece of notebook paper as a patch around a once fired cartridge bullet I found on the range. Had no money for round balls. Killed the deer just fine, but he wasnt too far away. The old and slow twist guns may be a little more versatile in this aspect than the fast twist guns of now. 1 in 66" twist was pretty popular, but now I have seen them with 1 in 9".
RipT
