bullets are normally made out of lead...are there bullets that are completely steel?? (not plated)
They will both knock it over, but it depends on how the bullets hit the block. If the steel bullet hits the block at any kind of an angle, it will knock it over because it will enter the block cross grain. If it hits it straight on, it will not have as much resistance as it goes with the grain of the wood. The rubber bullet will have more resistance because it does not pierce the block. The force from the rubber bullet would be the same no matter what angle it comes from.
Yes, mild steel has been used, especially in wartime when shortages of materials become problematic. The trouble with using them, though, is that they don't obturate the rifling well (plus the obvious thing that they're hard) so they tend not to shoot well, and they wear barrels quickly.
I personally know of no steel bullets. In order to be soft enough to engage the rifling in the barrel, it would have to have a jacket at least the thickness of the groove depth. Separate loading(those which a projectile is loaded followed with various sized powder charges) howitzer projectiles are cast steel with copper driving bands set in grooves to engage the rifling.
Not for small arms and not for conventional artillery. In both cases, the projectile has to engage the rifling, which requires a material soft enough to upset into the grooves yet tough enough to stand up to the friction and the pressure. Small arms projectiles, even if the core is steel, must have at least a jacket of softer material, usually a copper alloy, to engage the rifling and seal the bore. Artillery projectiles for rifled weapons use driving bands of bronze or a similar alloy for the same reason. The Germans during World War II and probably some other resource-strapped armed services have used sintered iron bullets for handgun rounds such as 9mm Luger (I have a box, but have never shot one because of their value to collectors), but sintered iron is too fragile for the higher velocities and energies of rifle rounds. Several companies do make sporting bullets entirely of copper, which is a way to solve the problem conventional bullets sometimes have with shedding the jacket on impact with a game animal. You'll also find lots of foreign military ammo with cartridge cases made of mild steel. Russian and Chinese 7.62x39 often has these steel cases. You'll find tens of thousands of them rusting on the ground at rifle ranges all over the country. The bullets have a softer jacket, usually a copper alloy. Armor-piercing ammo has a core of hardened steel, but a jacket of copper alloy.
