Someone stopped by searching for “killing range of a .38 special from a rifle.”
That becomes a matter of how far you can maintain a “killing aim” more than a matter of the capability of the cartridge.
Add to the problem of the inherent accuracy o of the gun and the rifleman, I suspect the safest route is to take the advice on the box the cartridges came in. Usually some variation of…
“WARNING, this product capable of inflicting serious or fatal wounds at up to three miles. Know your target and what is behind it.”
If you are determined to fore the combination with a possible human or valuable animal within three miles, exercise or diary caution. Keep the muzzle low, make sure of a good backstop that is steep enough to prevent ricochets, and always look before you put your finger on the trigger.
And, as with every sporting firearm, never point a gun in the general direction of the moon and let er’ rip. That is a really good way to earn time for manslaughter.