It's not going to damage it as long as you clean it. It is no different than shooting lead bullets out of a muzzle loader, or a pistol or a rifle. Many hand gun ranges force you to use non jacketed bullets and .22 rimfires use all lead bullets. Lead is a fairly soft material and barrels are made of hardened steel. Rifles shoot copper jacketed bullets at much higher pressures without damaging the barrel and the copper alloy is harder than lead.

And don't believe that sabots don't foul out a barrel either, you still need to clean out all the plastic that will get in the grooves as well. I would say more rifles are ruined by improper cleaning than what was shot through them.

The saboted bullets biggest advantages are they have better ballistic coefficients and normally perform better on game than a large chunk of lead. It also lets you shoot a lighter projectile with more stability if you want to.