Kinetic energy from a projectile is equal to the mass of the projectile times the square of the velocity, but air resistance saps velocity off quite rapidly and independently of the mass. So a heavy projectile will deliver more kinetic energy to a target than a lighter one with the same initial velocity.

A lighter projectile can be driven faster by the same charge of propellant, but air resistance increases with the cube of the velocity so delivering more energy through increased velocity is a losing game. You can decrease air resistance by decreasing the cross section of the projectile, but that requires dense materials like lead.

tldr: the difference is absolutely not minuscule. There’s a reason why bullets are made of dense materials.

Lead from the bullet does actually come out of the barrel in a particularly impressive plume of particulate matter that you then breathe in. It’s caused by friction with the barrel and cuts from the rifling, but it’s very effectively reduced by using jacketed bullets. Just FYI.

Not very well documented, that one.