I probably mentioned this, Jim - the 14 bore is probably as large a size as you are going to get 1,500fps with a load that is comfortable, in a 30" to 31" bl. You've seen the recoil picture of my rifle - that's the 1,500fps load. 69fpe recoil and easily absorbed by the over 2" wide butt plate - and an almost supple body.
My Musketoon has a 24" tight bore of .574". Shooting a .565" pure lead ball (Lee.562" mould) along with a .021" ticking patch(calipers) with 75gr. 2F, I get 1,308fps. I was surprised. For stricktly hunting, I'd certainly use up to 120gr. of 2F & pure lead ball and I suspect the speed might climb upwards to 1,700fps or thereabouts - or maybe 1,600fps. Due to the small diameter of the bore, I'd not try WW balls unless a smaller mould was available, or an attempt with paper ctgs. I'd be concerned the deep grooves in the breech would promote blow by with the paper and alight it producing a fire hazard.
A 21" .50 cal I had, had no difficulty making 1,800fps at 90gr. 3F - or maybe it was 100gr.
Ed Rayle will make you any length, any calibre, any twist, etc. The 30" length is great in my opinion as all-round length.
James Forsyth would prefer 24" to 26", in 14 bore (2 barrels of course, however 1 would suffice for us). The lands would be narrow, groove depth about .008" and a 104" twist.
I'd personally go with an 80" to 85" twist max. 28" to 30". The powder charge to achieve descent hunting speed could run 120gr. to 140gr. easily. The gun should weigh 9 pounds to 9 1/2 max. A straight 1 1/8" bl. will be necessary, with decently dense wood to make 9 to 9 and a tich pounds at 26"- 28" bl. length. Just a guess. Mine weighs 9 1/2 pounds with 31" bl. and English Claro stock.