Well, it didn't do so good calculating a .50 caliber roundball @ 177 grains weight loaded in a 32 inch barrel with a 1:60 twist* over 90 grains of GOEX FFFg powder.
Lyman found that combination to give 1844 fps velocity.
The link above said it would be 1349 fps, about 500 fps slower.
Giving it the benefit of the doubt I tried a .50 caliber roundball @ 177 grains weight loaded in a 28 inch barrel with a 1:48 twist* over 80 grains of GOEX FFg powder.
Lyman found that barrel and load would give about 1573 fps muzzle velocity.
The links calculator said it would be 1079 fps, about 500 fps slower.
Now, I'm not saying Lymans data is right but I've fired a lot of shots out of a .50 caliber TC using a 80 grain 2Fg powder load and they always "crack" when their shot.
That indicates to me the ball is exceeding the sound barrier at the muzzle and a velocity of only 1079 isn't going that fast.
It would be interesting to see what our members have been getting with a cronometer with their .50's and an 80 or 90 grain powder load.
* I thought it strange that the rate of twist was not a variable in the calculator. There is no place to insert the value.