So what is wrong with a light load for the 15", it will blow through a deer pretty easy.
I use 50grains of Goex 3FFF in my 209x50 15" with a Hornady 300 grain XTP and it has plenty of recoil for me.
I have also shot patched roundballs on that charge a couple times and the recoil is much reduced (177 grain ball) but have not tried on paper yet with prb's - you might want to try prb's since they are almost half the weight of most bullets your shoulder and arm might deal with the recoil a lot better. Also I have talked with a few people and they say patched round balls group very well in a 209x50 so it is worth thr try. PM me and I can help you with patch material ( I shoot a lot of prs in flinters and caplocks).
Also I am going to try the T/C cheap shot sabots to see how they do in it (they are 240 grains).
The longer barrel looks pretty front heavy but if you can shoot it off sticks it would be fun.