Most that shoot a lot of conicals say that the 1:48 shallow groove TC's prefer one that is fairly short in length, no more than 1" or so.
You don't say what caliber you have but if it's a .50, Hornady Great Plains Bullets generally shoot really well in a T.C. Hornady stopped making them in all other calibers, .50 is the only one they still make.
I have shot a deer with one from my .54. The shot was 55 yards with a 100 grain charge of 2f, hit a rib on entry and exit hole was about twice the size of the entry. They are a hollow base designed to flare out and seal the bore, so I shot them without a wad. They did lead the barrel fairly quickly, but I got great accuracy with them.