Once upon a time, about 6 years ago, I ran the same .38 Special load, 12.5 gr of Alliant 2400, over a chrony using 158 gr XTPs. I used three different guns to check the velocity and got: 1212 fps with a 6" Security Six, 1194 fps with a 4" M66-2 and 1198 fps with a 6" 686.

A couple of observations, 1) the gun has a lot to do with the actual velocity. The 4" M66 was only 4 fps slower than a 6" 686. 2) a .357 that's only 50-60 fps faster than a .38 Special with the same bullet isn't really a .357 Mag, regardless of what is being sold now for .357 Magnums.

A Lyman 358429 bullet at 168-170 gr will produce an average of 1270 fps MV with 622 ft lbs of energy (ME) out of .38 Special brass and not do any damage to a L frame or to yourself. If it's loaded as a .357 Magnum, you can stretch that to an average 1420 fps MV with 775 ft lbs of ME. That's getting close the original .357 Magnum load circa 1935.

If you want a little flatter trajectory, you'll probably have to go to 125 gr bullets and I recommend a Sierra JHC #8320 that will produce 1575 fps MV, or more, and 690 ME, depending on the load. At that velocity, it will kill whitetails. Even then, the drop at 200 yards is in the neighborhood of 20" with a 100 yard zero out of a rifle doing 2000 fps+.

I would also suggest checking with Tennessee Valley Bullets for your lead bullets and save some money.