Older pewter had a higher percentage of lead than modern stuff, some of which has little or no lead in it. Compared to pure lead, pewter is much lighter, and much harder, due to the higher percentage of tin in it. It was used to make musket balls during the AWI, where it worked OK in smoothbore muskets.
Some years back, a modern gunwriter (Massad Ayoob, I think) did a test with pewter bullets in a .357 revolver. The bullets were cast from a mold that threw a nominal 125gr. bullet. The pewter projectiles weighed around 80gr., and could be pushed much faster, (over 1800fps, if memory serves) and were devastating on blocks of ballistic gelatin. Don't recall what the disqualifying factors were, I'll see if I can dig up the article.
Joel