Bullet Length

Ok, finally got moved and casting equipment set up, somewhat.
What bullet length is working best in the 48 twist TC for long range?

It’s a .50 cal, BTW.

Acceptable bullet length is not only dependent on rifling twist rate.  The bullet balance point has much to do with in flight stability.  A bullet can be stabilized in two ways . The most commonly referred to is  spin.   Spin or rotational velocity must increase as the projectile length increases to keep a bullet stable in flight.   More bullet = more spin .
Ther is also another way to stabilize a bullet which is known as form stability.   Form stability addresses the balance point of the bullet putting most of the weight forward  such as in a Foster slug fired in a smooth bore.  The nose heavy balance    
keeps the bullet flying nose forward with no spin/rotational velocity.
If you combine spin with form you can get quite good accuracy with a bullet length far longer than what the Greenhill formula might predict.   This is not new news as form and spin was combined in rifles and projectile put to quite accurate long range use in our first American uncivil war.

Thanks for that response.
I got an adjustable mold, so lots of experimentation forth coming.

There is another form a stability I didnt mention because it really will not apply to our purpose and that is fin stability as with an arrow. I guess if you were to load a big bore smooth bore with a load of fletchetts or an arrow it could apply. I just mention it for those that just knew I was wrong when I said there were two ways.

Bullshop, could you fire a foster slug out of a round ball slow twist barrel and maintain accuracy???

My 48 twists seem to like big lead like my 54 especially. the 50 as well. I have a 50 cal i bullet mold from accurate molds. The 50 bullet drops 450gr and the one for the 54 weighs 515gr both shoot great for me. I ordered mine .002” over bore diameter and don’t have any issues loading and I don’t have to size them either, good luck, these are just what mine like, my 54 doesn’t shoot round balls worth a darn no matter what I do, can’t say if the 50 does I’ll play with that some more but it likes the 450gr bullet in the 48 twist barrel

Using The Greenhill method, the longest length for a 1:48 twist in a .50 is .78" long, so very slightly over 3/4 of an inch.  The Greenhill method is still used for lead and lead alloys.  There is another method (Miller Twist Rule) used for jacketed bullets and allowing for different bullet shapes.
This only works for pure lead, and everything is done in inches.  For alloys, there is another part of the equation required to find the density ratio of lead to the alloy, taking the result and finding its square root.

Also, the amount of bearing surface engaged into the grooves makes a difference. If a bullet is mostly grease grooves, it may plow through the rifling. There isn't enough lead making full contact in the grooves.

Bullet size = .50 squared
Multiply the answer above by 150
Take that answer, and divide it by the bullet length.

.50 x .50 = .250 (same as .50 squared)
.250 x 150 = 37.5
37.5 / .78 (bullet length) = 48" (twist rate of 1:48)