How much leading you get will depend on a host of factors. In general, harder alloys will produce less leading, assuming the bullet fits your bore and your gun is set up right (throats right size, forcing cone cut correctly, timing right, throats perfectly aligned with bore, etc.). Pure lead is very soft and difficult to cast well. Adding things like tin and antimony improves castability and produces a harder alloy. "Hardcast" bullets are just bullets with relatively high amounts of tin and antimony compared to "softcast" (which are typically 20:1 or even 30:1 alloys of lead and tin).
Sometimes, however, a hard bullet will cause leading while a softer bullet will not. That's usually an indication that you are using too small of a bullet, because what is happening is that the softer bullet is obturating (expanding) to fill the grooves in the bore, while the harder bullet is not. The harder bullet is allowing hot gases to jet around it as it travels down the bore, and this jetting action is what causes the lead deposits.
I don't believe that Oregon Trail's bullets are any harder than any other manufacturer's "hardcast" bullets. They're also no harder than what you can produce at home. It's just a matter of using the right alloy.
If you are experiencing excessive leading, there are a couple of things you can do to reduce it. Slug your bore to make certain you are using the correct bullet size. Sometimes just going up .001 in bullet diameter solves all your problems. Switching to a harder alloy may significantly reduce or eliminate leading. Going to a flat-base bullet instead of a bevel base will often help. At velocities much above 1200 fps, gas checks are a good idea. If need be, get your gun checked out by a competent smith to make sure that bore/cone/throat/alignment issues aren't causing a lot of the problem. If all else fails, stick to jacketed bullets.
