i am building my first AR. I have several rifles and handguns, all more traditional calibers (45, 45LC, 38, 30.06 etc) my AR is 5.56 NATO. I have been shopping for ammo and have questions. I understand different grain weights, but I have found 5.56x45 62 grain. I have found just 5.56 62 grain. What is the difference? Is the x45 the length of the bullet and if so what is recommended for a 5.56 NATO with a16in barrel with 1/7 twist? Any advise is appreciated

"5.56" is common-speak for 5.56x45; it's all the same stuff. The "45" is the length of the case (in mm), not the bullet length.

Typically, the faster the twist, the heavier bullet you want, or at least the heavier the 'optimum' bullet will tend to be. For a 1:7, you're probably ok with bullets from 55 grains on up, but the "best" bullet choice will vary by rifle. If it were me trying loads for a 1:7, I'd start with trying out various loads from 62-77 grains to see what my particular gun prefers.

Afterthought - check out this thread for a lot of common ammunition Q&A:

http://www.ar15.com/forums/t_3_16/503947_AR15_Ammo_Forum_FAQ.html

Thanks for the advice. I'm sure I'll be back for more.