I use this same barrel now and use 50 grains of Goex with a 240 grain T/C cheap shot sabot and it shoots great ! 1 5/8" group at 40 yards.
2 777 pellets will feel like a 45/70 in a hand gun. 50 grains will let you know it is there.
I started mine out at 65 grains and I thought "holy crap" thats way too much recoil, one would start to get jumpy and lose your groups from holding too tight to the grips.....
I don't shoot past the 40-50 mark as all of my muzzleloader deer that I have taken in the past have been way less than 40 yards anyway so I don't expect to shoot out to 100 yards but if I need to I am sure that it would put a deer down with 50 grains.
BTW I question wether both pellets are even burning before it leaves the 15" barrel, (may explain the lighter felt recoil your buddy felt).
Have fun and let me know how it shoots for you.