Have to agree with you on the gelatin. You mention mud; I wonder how clay would react. It is a bit denser than just plain mud. Any pig would give you a fair indication of what your load would do in a defensive situation.
Being the old soldier that I am, I am not too fond of the AR platform in any caliber. That is just me and, yes, there is a long story behind it. But it's like OC vs CC; if it works for you, great!
All those have been tired and proved to be inferior to 10 percent gelatin there is a reason it has become the testing standard.
Plain old water is the next best but that well show a bit more expansion then gelatin. As far as game animals go the whitetail is most likely the closest.
Personally I shot close to a couple hundred deer, seen a few hundred more shot, dozens of hogs, some bears and thousands of smaller critters with every thing from 22lr to 458 DGRs.
I been interested in bullet performance for over 50 years and tested them in various medias. I have recover hundreds of bullets from animals and other testing media.
I'll take gelatin and water for the best testing results.
Neither of them can be a true indication of how a bullet well perform on a live animal at any one time just because of the many variables involved , shot placement, distance velocity and body structure hit.
Animals are a very complex structure that are very hard to duplicate in a non living testing media.
One can shoot the Sh-t if they want.
But for bullet testing I prefer more reliable medias.
I'll leave shooting the Sh-t for story time.