Surprisingly offhand shooting can be done at quite long ranges offhand if practiced.

The following example, but not with DRs. At a deer club shoot we had a number of deer targets placed out at different ranges. One was maybe 200 metres or so. Shoot at it using any sort of field rest. One member insisted it be shot at free hand. The next year, most members placed every shot in a killing zone on the target freehand.

Now I don't advise this in real life, especially when a wounded animal can escape, especially a DG animal.

I think the main things about shooting free hand is practice, practice, practice. Practice with other rifles, .22s, air rifles, medium bores plus your chosen double. Practice a lot with your double without shooting it, just practice aiming it at the TV (unloaded!) or other objects (safely of course). Take it for walks. Use snap caps. Try some reduced loads and shoot a lot of them. The point isn't to "practice for recoil", it is practice aiming and firing it. Start and finish with full power loads.

Practice snap shooting. This is offhand training. Bring the rifle to the shoulder and aim to fire it as soon as the target is acquired. Don't hesitate or stuff around.

I don't practice enough myself BTW, so not preaching at all.

Take it out hunting other game. If you can't hunt a deer with it, how will you hunt a buffalo?