Yeah, but the water used to cut steel is also loaded with abrasives. I wasn't recommending putting several hundred jacketed rounds through an old timer just suggesting that the OP needn't be fearful of harming his rifle by cranking a few jacketed rounds down the pipe. Factory loads are very mild and unless he is a handloader trying out a family heirloom shouldn't have to require getting lead bullet ammunition produced for it. These old rifles have been fed jacketed ammo since shortly after the turn of the century.
Water doesn't need to be loaded with abrasives to wear through steel. It just needs to be travelling at sufficient speed to generate the friction/heat required to cut steel. The same basic principles apply with bullets. The actual hardness is a contributary factor to producing increased friction/heat but it is not the only factor involved in the equation.
I personally wouldn't worry about a box or 2 of factory jacketed ammo through this rifle. Modern 44-40 factory ammo is still thin jacketed low pressure, low velocity stuff.


















































