Water dropping the alloys from the mould will make them too hard. It takes about 12 hours for maximum hardness to be reached, then over time of years, they will slowly soften back to the original WW alloy's brinel.
Due to the slightly differing temperatures when they hit the water, the brinel reached will normally be from about brinel 25 to 32 or 33 max.
This is way too hard.
The trick is going to be to use a bullet soft enough to obturate, but hard enough to 'take' the rifling.
I would suggest a mix of 50:50, pure lead and your WW alloy to get a brinel around 8.
This alloy worked VERY well in my .50 Alaskan with 550gr. bullets. They were very accurate and "took" the 24" ROT very well.
Straight WW alloy is likely too hard to obturate.
This would suggest an even softer alloy, likely to pure lead would also work in your 12 bore.
Paper cases might be too thick in the 'neck'.
Most compression formed plastic cases are VERY thin in the neck. The cheaper plastics, are thicker.