For full disclosure, my 8ga is not 3 3/4 inch. I ended up making a chamber adaptor 8 to 12ga. Quite a home machinist project for me. I have not used that adaptor. It is another angle to try out, maybe BP cowboy loads? That is beside the point. I ended up with 3 3/8 long adapter and just barely fit at that with a slightly beveled end. Given the level of fitment, I guess now, that the gun is really 3 1/4 with a little extra and a very gentle taper. I decided to post because, I dont want to mislead anyone with wrong information. I will go back and add a foot note to my inaccurate post.

As for the loading, I was able to swage down the industrial brass. I load up 20 rounds with black powder. we will see how the plastic holds up for one shot with a light load. I have been distracted and not been able to fire the gun. And, waiting to get together with my buddy and his skeet machine and the pattern board at the club. Then, I will see where I am at. I used double felt wads to fill the case. If I do order brass, I will get 3" and have less unused space to deal with.