There is a Jerry Miculek vid from a few years ago where he is shooting my with a couple of guys and giving pointers.
I can’t remember about holding steady but for recoil control and rapid firing the key was a death grip and squeezing the hands together at arms length.
This suggests to me, grip strengthening exercises but more importantly a pectoral exercise such as a dumbell fly or pec deck that mimics squeezing the hands together out in front of the body.
Unfortunately, this would only be the beginning. You would also need to do shoulder exercises. Particularly ones that targeted the posterior deltoid. This would allow a powerful co-isometric contraction through the pectorals but also with the three deltoid heads assisting on the shooting arms keeping them stationary.
That is still not the end of it. Trunk support through the abdominals would be necessary from a standing or kneeling position. Leg support would also be important. Stronger quadriceps, hamstrings, glutes, and calves are your base of support. You have heard how a golf swing, baseball swing or throw comes from the power in your legs. So to does a shooting stance and firearm stability. A strong and developed posterior chain is a characteristic of all higher level athletes. Shooting may not seem like an athletic event but the same principles still apply.
Short story is that you will need a full body workout to get the maximum results for shooting stability. I am sure there are crutches like the hammer methods mentioned, which I am sure is still highly beneficial but it only targets one portion of the entire support system.