What will happen is that as the bullet base passes the ports, some of the powder gasses will escape through the ports and erode the base slightly in this region.
So you will get lead deposited in the ports (minor) and a bullet base that is no longer symmetrical (major) which will destroy all hopes of accuracy.
I'm not saying that it's impossible to make work, it's just going to be difficult. Depending on the bullet hardness, barrel length, pressures, etc., it might work in some cases but is known to cause problems.