I have heard that real mirror doesn't work because it eats away from behind. I use stained glass that has crinkled foil behind it so it looks like mirror. i get it at the stained glass store. I use GE II for windows and doors. It can be bought anywhere like home depot or walmart. I have made a bunch of bowling balls and this has worked for me. It is real tacky so it stays in place. sometimes it slides and i slide it back while working on it. I have had my ball for a few years and it is doing well. I take it indoors in the winter.

Its actually the adhesive that can eat the mirror backing off and make it spotty. I personally like that effect-it looks antique. Solutions to that are to either seal the mirror with miror sealant before gluing or use proper adhesive. (Loctite silicone sealant and adhesive for example.)
The e 6000-I used that once upon a time before I met silicone and it held up very well thru all kinds of weather so it will probably be fine. Next time-get silicone, its cheaper than e 6000 anyway.
Grouting--I like to mix my grout dry-nice and thick, not runny. Dryer grout is easier to apply and clean. Smoosh it in all the spaces, wait a few minutes-and I mean a few, especially if its hot, and with a slightly damp spong or crumpled newspaper remove the extra. wait till it sets up a bit and clean it better. (so you dont remove it from the spaces). Its not hard and everyone kind of develops their own way to do it. Good luck!

finished gluing the mirror on, now i have to grout it, i think next time i will put my mirror closer together. Any thoughts?

another pic of my bb with the mirror glued on.

It is a matter of personal preference but I agree that the next one you do you should glue your pieces closer together so your grout lines aren't so big. However, as long as you haven't grouted it yet, and if you still have mirror left you could still add more pieces to fill in the areas between the mirror you already have glued down.

I feel that way too!!