Those look mighty fine for a beginner! Great job! As far as cutting the sprue, the best time is when the sprue "frosts over". You will notice that right after you pour the lead into your mold, the puddle of lead on the sprue plate will change color/texture and go from a shiny (molten lead) look to a frosted (semi-hardened lead). When you see this change, you are OK to cut the sprue. As the mold and sprue plate gets hotter and hotter, it will take a little longer for the sprue to harden...thus you have to wait longer to cut it. What I do is I have a damp sponge on hand and when the sprue is taking too long to harden (more than 5 seconds or so) I will touch the bottom of the mold to the sponge for 2-3 seconds (with the bullets still in the mold) to take some of the heat from the mold. This helps keep the mold temperature in the "sweet spot". Keep up the great work!
BTW....try Bullplate lube on the mold....it makes casting a dream!
http://bullshop.gunloads.com/