Shotguns are great for indoors. They can be tricky however, as it is a bit different of a play dynamic. Sorta like sniping compared to a rifleman role. Others will have the semi auto advantage, but I always have fun with and appreciate the challenge of using a shotgun.
I've become a little bit of a shotgun collector as well. I can attest to the UTG/Double Eagle/Matrix/etc trishot spring shotguns. Decent for the price, and always great back up guns to have and bring along. I strongly recommend the CYMA metal 870 line though, well worth the money. It is essentially upgraded compared to most other spring trishots on the market, being a little stronger, and having brass barrels. Also, their internals are more durable than the plastic versions.
I was able to run my HPA tapped TM Breacher a couple weekends ago, and while it started out pretty slow for me, still getting used to the controls, remembering to start the game with a loaded shell(too many games/times was I shooting nothing haha), and managing the heft for such a small gun, I had a ton of fun running it. As I had used trishots numerous times before, I went for the 6 shot option to best utilize the platform. Didnt have the best for range, but it sent a decent spread at the ranges I was engaging, which was perfect.
TL;DR
Everyone should have a trishot; cheap, reliable, great backups, and highly recommend the CYMA metals.