Pistol bullets harder than WW usually lead quite badly, as they do not obturate to seal, especially in softer (lower pressure) loads, ie: .38SPL.

Lane, I would use 2F rather than 3F. My 14 bore shows higher velocities with 2f than 3F. Seemingly, all 3F does is increase pressure in the larger bore rifles - at least with the GOEX I used.

100gr. powder would I expect perhaps reach 1,350fps - maybe 1,375fps in a 16 bore, with a good seal due to the wads used. I judged this by what 16 bore ML's & my own 14 bore ML will produces with the "light" charges.

Your load will suffice, however if you are wanting 1,650fps, you are going to have to use about 130gr. to 140gr. of Swiss 1 1/2F.

At some point, the velocity will diminish with more wads, due to increased weight being pushed out, exceeding the value of the better seal.

Also, never be sorry for asking questions. It is how we learn about subjects not covered in text.

Cast bullets from outfits like Laser-Cast, or The Bullet Barn, are usually quite hard. If they are oversize to the handgun groove diameter they are used in, they usually shoot fairly well, but a slightly softer bullet usually shoots more accurately and without leading.

For example, my M29 4" shoots WW bullets into quite small groups out to 100yards using full power loads. Some really hard bullets given to me, measuring .430" shot poorly & showed leading, although they shot a mere 3" wider group, however, that was 50% larger than the groups I was getting with my 275gr. SWC Ohaus mould, also casting .430". The softer bullets were overcome by the pressure to seal & prevent gas cutting, while the harder bullets did not.

Handgun range lead is a mixed bag. One must simply cast them up and see what you've got, using known alloys, like real lead alloy WW (not stick-ons or zinc). The corner of a screw driver can be used for a simple scratch test. This will usually show obvious difference in alloys.

As you do not have rifling, any ball hardness can be used, as long as it is sub-bore size or streak leading will likely appear in the tubes.