In a snubby the mentioned bullet often does not “classically” expand but rather puckers the nose a bit.
This would be erroneously seen by many as a failure but it is really not. In so doing adequate penetration is ensured, even if some impediment to penetration like a cross body shot occurs, intervening arm is present, or a larger person is involved. Penetration is usually about 10-20 percent less when this happens than a cast solid wadcutter shot at 780 fps from the same snubby but still considerably more than the minimum FBI standard, approximating the upper end of the acceptable range, which is actually more where I want to be.
A great influence on how well the bullet expands is where the powder is in the case when the round is fired. If it is bunched up near the bullet when the trigger is pulled expansion is less likely, but a moderately upset bullet performs as described in the previous paragraph, which is just fine.
It is possible with loads on the very top of what is claimed to be standard pressure (5.4 grains Power Pistol) to obtain 840-850 fps in a snubby. 6 grains is upper end Plus P but no fun in an aluminum 638 and controllability suffers so I avoid such loads. In truth, as others may agree here, I find them unnecessary.