RE: Need help from those more experienced.
1. Your 870 with a 18" barrel should be able to shoot rifled slugs.....and I suspect you' d like the groups too.
2. if it' s legal in your state, the .223 will do the job on deer. I' ve used a 16 1/4" .222 on deer with total success. Shots must be well in range and placement must be good...you' ll do fine. I suggest a 55 grain or 60 grain bullet.