Al's right!

But if you choose to use the shottie, you will need a true solid slug like the Brenneke or S-ball. I cannot stress this strongly enough!! Stay away from the 'hollow' slugs like the Winchester Foster slug, also the PMC slug, and on no account should scrub cattle be 'annoyed' with the RC4 offering!

Another member here, DarylS, has plenty of good things to say about shooting spherical-ball handloads in the smooth-bore, although my own attempts have not managed to duplicate his accuracy benchmarks. You could search for his posts, or simple stand by: I'm sure he'll chime in soon.

Not meaning to be negative, but you may not find rifled chokes as simple a solution as you hope. Most of the US offerings are designed for a single barrel (pump or auto): many extend slightly and a pair may not fit into your stack-barrel. Even if they do, regulation will be hit-and-miss! Perhaps a single rifled choke would work OK, with the second barrel for prodding distance if things go pear-shaped.

Personally I think you are taking an unnecessary risk if you plan to walk-up scrub cattle on foot and engage them at close range with a shotgun. But hey, some people hang-glide.....

On a more positive note, welcome to the NE forums. Let us know what you end up with, and photos of guns and vanquished quarry are pretty-much mandatory around here!