Ball and Patch???

Is there a modern-day muzzle-loader that shoots a patch and round ball accurate out to 75-100 yards?????

Probably not. Most modern inline muzzleloaders will have a 1:28 twist rate barrel which is good for sabots and maybe some conical bullets.
Spinning a round ball that fast will destroy it's accuracy and give real erratic hits.

Get one with a 1-72 twist and....HAVE FUN!

Use REAL BLACKPOWDER!!

Few years ago i experimented extensively with patched round balls fired from inline muzzleloaders.

There is a reason for the slow pitch used with patched round balls. Way back when hunters and shooters would often vary the powder charge. With slow pitch rifling nearly any well fitting load will put the ball in the same group.

There are two problems with fast pitched rifling and patched round ball. 1. Most fast pitched rifling is shallow; patched round balls like deeper rifling. 2. When patched round balls are used with fast pitched rifling the powder charge is critical.

My cheap old CVA StagHorn gun shoots patched round ball like a laser from it's 1:32" twist rifling using 70 grains of Pyrodex RS. The rifling in that gun is deeper than most other inline guns. i routinely shoot squirrels and rabbits in the head at 40 yards.

That gun will not shoot patched round balls worth a hoot with any other powder load, i've tried them all.

Don't take my word for it. Here is a good read on the subject from "Doc" White, maker of White muzzleloaders:

http://whitemuzzleloading.com/round-balls-in-fast-twist-rifles/

Thanks guys, great Info.