Depends on the rifle, caliber and what you consider decent groups...

Both of my flintlocks are deep cut rifling, not the shallow button rifling found on factory rifles...The .40 is a 1-48 twist and the .54 is a 1-72 twist...Both will keep all shots inside an inch at 50 yards...Both are very tolerant of changes in powder charges, elevation may vary a bit but not windage...That's where the deep rifling helps...

The Hawken Brothers used a 1-48 twist, but their's was cut as well...

I have worked up loads for some of my buddies in the past with factory rifles and the 1-48 twist...I was able to attain hunting accuracy but I wouldn't classify them as having great target or competition accuracy...