Thread: A conundrum

I'm trying to get a 38/55 going before the big bore and can't quite figure out what's going on.
250 grain cast flat base bullet with 6 grains of trailboss shoots nice 2 inches groups at 50 yards and is only doing 890 fps and sounds like a FART.
15 grains of 4227 gives me 1475 fps but the bullets are tumbling giving large oval holes and no group.
There is no leading in the bore. The rifling twist is 1: 25 the Greenhill formula shows that this bullet requires a 1:22 twist but I don't think that's the problem.
I'm stumped, in my experience as speed decreases instability increases not the other way around as there's less gyroscopic spin to stableise the bullet.
I could always stick with the mouse fart load and accept a bit of ridicule but a bit faster should in theory be better.
Anyone got any ideas.