Well, let me show my ignorance here, but, nose pour moulds have never made sense to me! My thinking is that base fill/ square base can be seen when I open a bottom pour mould. I immediately return any bullet I can see when I cast, to my spruce pan, including any other in that mould. When I get consistent bases throughout( visually) I start keeping bullets, but also keep the first 50 or so “good” bullets separated from the majority I cast that session. Those fifty are usually returned to the pot after comparing them to what I cast later in that session, and those cast while I have got my “rythem established. Doing so, has helped me dramatically with bullet numbers I need cull later.
To my way of thinking, the turbulence within a mould , while filling, displaces the air in the cavities. Too fast a pour, too cold the mould, too cold the alloy, alloy blend, and casting cadence, all effect the rate that air is displaced. The shape of the bullet can also effect displacement of that air, if you think about the way we induce/ pour molten alloy, in relation to venting lines.
In my mind, nose pour mould, if there is any taper to bullet nose, is attempting to displace that air to a constant tapering surface( the nose) which slows that displacement???? Yes I realize the denser metal is also displacing that air, or should, if was entering the mould cavity in a uniform pattern/ level, but we/ I am not.
If anyone is trying to picture what I’m questioning here, fill a clear bottle with oil, put the cap on, and turn it upside down. Watch where the air bubbles spend the longest amount of time. Use a “Great Value” water bottle from Wal-mart that has multiple ribs in the sides, it kind of resembles the sides of a Loverin style bullet. Reverse the bottle next to simulate nose pour/ base pour.