I don't know a darn thing about Citadel shotguns I sure hope it works out and you know this run a couple hundred shells you are going to use to ensure the bugs are worked out. My steel plate play shotgun an older custom Rem 1100 with higher capacity 2 3/4 " with speed chute , briley ported on the front, large charging handle, speed feed stock, smoothed up, etc. it will unload shots faster then you can imagine, but it's heavy, if I personally were to do what you are talking about it would be a simple matter for me to replace the barrel with a 21 " and screw on a shorter mag extension. The gun is just reliable and soft on the shoulder. Benelli M-4 is another fine gun. I would look for an older rem 11-87 which handles 3" shells in a 21 " turkey barrel, add some appropriate sights and I do use red dots, more than likely it would be an Aimpoint (rugged & reliable), keep an iron sight/bead on the front just in case, install a Briley ported choke tube rifled if you want to occasionally shoot rifled slugs or stick with a short range smooth bore choke. That would be something a person would need to think about when selecting slugs or shot after testing to make sure it works to your expectations. You could alternate slug, buckshot slug buckshot in the mag after testing. In my case I would ditch the speed chute and large charging handle and leave it stock, get a speed feed II stock, add a shorter mag extension tube and barrel / tube brace , and smooth out the transition to the extended mag tube, and add sling, it will still be approx 8 pounds or just buy a Benelli already set up. Remington did make some short barreled combat semi-autos at one time, might be something to look at.