Since Crosman is based in WNY, it seems like there should be some M-Rod mechanics in the area. I am following the 22 slug thread, but have no desire to dive into my M-Rod to change power and etc. I am willing to drive a few miles to do face to face.
You bought a pellet rifle with a barrel designed to shoot pellets accurately. If you want a slug rifle it will need a new barrel without a choke. You might talk with Air Gun Revisions:
Airgun sales, service. All your airgun needs, master airgunsmith, airgun tuning
www.airgunrevisions.com
And then you will have a slug rifle that will likely not shoot pellets as accurately as it once did. Frankly, IMO, there is just not enough reservoir capacity to support the fpe needed to push heavy slugs at a good velocity and have any shot string. So then you need a bottle.