WELCOME Mike!
SORRY to read of the problems!
Let me first say I agree with the guys posting so far, BUT let me back up a bit and say CLEAN THE BARREL FIRST! Its likely quite leaded!
Driving a soft lead slug down the bore will be very difficult in a leaded bore! Becareful of your crown! Either use a pull thru system or clean carefully from the breech.
To reiterate Tturner53's comments. Shooting lead is not as simple a medium as many think it is... You need the correct size and composition for the task at hand...
Spendsome time over on the cast bullets part of the board and you will get a idea. BUT as a broad statement and just quickly, you have a number of different "cast"bullets aval to you.
1) SWADGED are avail from the big makers and are nearly dead soft and suitable for light loadings only.
2) Cast bullets are generaly harder but vel shoulc still be limited.
3) Hard cast is suitable for more vel but should nto go too much past mach 1.
4) GAS CHECKED Hard cast are suitable with the fastest velocities atainable form jaketed bullets.
Once you now what you want to shoot/do with the bullet,THEN you need the propper size! A swadged bullet can be as much as .003 larger than your bore (Rem its dead soft lead) as you go harder (Hard cast) the diameter needed (obutrate, this means the bullet actually expands in the bore to seal gases) gets closer to a jacketed diameter.
REMEMBER, also as stated by all, you need to know the bore diameter!
Slugging OR a chamber cast will tell you this information.
Back to your issue at hand, Tumbling bullets are caused by a number of things. A under sized bullet is top that list of probabilities! But, bad barrels, badly fouled barrels and incorrect rifling twist rates are also candidates too...
"Most important, as frustrating as things can be, don't give up, also remember, there are no stupid questions! You will get this one, we can help thru the forum here. Let us know what you find.
Good luck,
CW