normal grouping

[mickey44]

I am shooting my Kentucky flint lock(45cal) with a 6" barrel at a target 20' away with 15 grain of 3f and getting about a 4 inch group is that about what this gun should be shooting

 

[AZbpBurner]

Keep practicing and, maybe work up an optimum load. In a little more time you should be able to punch one ragged hole at 20'... or at 50'

 

[Zonie]

That's pretty good shooting but it is impossible to say if it is what the gun really can do unless you were shooting it from a rest.

Because pistol shooters vary so greatly, the only good way I know of to determine the guns accuracy while shooting offhand is to compare the group size against the groups the shooter can make with a good modern pistol.
The muzzleloading pistol should be capable of shooting at least as well or better.

 

[crockett]

If the barrel is only 6" where is the rear sight? If mid-length on the barrel then you only have about a 3" sight radius. By 20' that means 7 yards? That's pretty close range. On the flint- there is more lag time. Just to test the accuracy versus shooting ability- I'd try sand bags. With sandbags the groups ought to be closer than 4" at 7 yards.
Patch thickness can make a big difference. On my 50 caliber percussion single shot I was getting 6" groups at 25 yards with a .010 patch and without changing anything other than the patch I used a .018 and the group went to 2". I was using 37 grains of fffg real black. You might want to try a slightly heavier powder charge. You might get better accuracy.

 

[M. De Land]

If all is right they will shoot right along with a well tuned cartridge gun.
Good semi-auto and revolver cartridge guns will do an inch at 25 yards from a Ransom rest but two inches is more the norm from sand bags and is considered good grouping at 25 yards.