I would try to get in touch with someone on here that lives close and is familiar with 870 to run thru it with you. They are not difficult but it never hurts to speed up the learning curve. The bolt and rails are a little tricky to get back in but other than that it is pretty straight foward. Like they said that nut should be tight and there are tons of useful info on YouTube.
It isnt hard to get the bolt back in, you just have to grow a third arm and shrink your hand to that the size of a 3 year olds all while having the strength of a 25 year old farm boy! Actually you just have to have that strength if some jack wagon before you tried to force the bolt into place and you must now free it to do it right.