Welcome aboard and congrats on your new 100ATR.
I don't have any experience specific to the 100ATR, but for accuracy I get my absolute best results out of my .308 Remy 700 from Federal Premium Gold Medal 168gr Sierra Matchking HP. I shoot a lot of the cheapo Wally World Fedaral Power-Shok 150gr SP when I just feel like put'n some rounds down range and still manage pretty good results on paper. I don't hunt though, so depending on your purpose I'm not sure how useful my information would be to you.
We've got a great group of folks here on Mossberg Owners and the atmosphere is very laid back. The only thing we like better than discussing each other's guns is lookin at 'em, so be sure to post up some pics of any firearms you own the various sub-forums. Let us know if you have any questions.
Enjoy the site!