Dave,
The 300 Weatherby (magnum?) uses a 30 caliber bullet (.308 in diameter) which is the same as the 308, 30-06 and the 30-30. The nib from a slimline will almost exactly match the outside diameter of the neck of your casing. The shell is 2.825 in long and will need a tube installed for the transmission. Here is one way that this can be accomplished:
1) Remove the spent primer using a suitable tool. A small diameter pin punch works well.
2) Drill out the primer pocket using a letter "I" drill (preferred) or 7 mm drill.
3) Clean and polish the casing. Apply powder coat or brass lacquer at this time, if desired.
4) Cut a piece of 7 mm tubing to a length of at least 2.9 in. You can trim the ends later after the tube is installed.
5) Cut a short length, 1/4 to 1/2 in long, of 8 mm PENCIL tubing if available. Slide this over one end of the 7 mm tube leaving the 7 mm tube sticking out about 1/8 in. Apply a drop on CA to firmly bond it in place.
Alternate 5) Wrap teflon tape, electical tape, masking tape or use shrink fit tubing over the end of the 7 mm tube. Leave about 1/8 in sticking out past the wrapped material. Test the fit inside the neck and adjust until you have a snug fit.
6) Insert the prepped tube into the casing from the neck and position it so that a small portion of it extends beyond the casing at both ends.
7) Apply 1 or more drops of thin or medium CA to the tube and let it wick into the joints at both ends of the casing. Repeat application if necessary.
8) After curing, remove any CA that made its way inside the tube and trim the tube so that it is flush with both ends of the casing.
9) Press the slimline nib into the neck end and press the transmission into the base end. Take care to not push the transmission in too far.
10) Make the upper portion like any other slimline except that the diameter of the center portion, adjacent to the base of the casing, should be about .532 in.
Alternately you could use a 30 caliber bullet instead of the slilmline nib. You can push the 7 mm tube in from the base and you should be able to center it on the tail of the bullet.
Hope this helps you get started.