I assume you are referring to bullet weight, which is measured in grains. There are 7000 grains in a pound.

Typical .357 Magnum rounds have either a 158 or 125 grain bullet. The 158 is usually a little slower than the lighter 125 grain bullet. Recoil from both rounds is usually about the same, but possibly a bit less with the 125.

For your Model 66 either is fine, but I'd go with the 125.

__________________
USAF, 69-92
Vietnam, 72-73