This topic comes up every so often:
Most AR15 hammers look like this:
[img]http://www.biggerhammer.net/ar15/hammers/ar15_std.gif[/img]
however, the last ones that I got from RRA looked like this:
[img]http://www.biggerhammer.net/ar15/hammers/ar15_early.gif[/img]
If you have a hammer that looks like that latter, you don't need a special hammer. The "stepped" face on the top illustration will hang up on the bolt GOOD, and will never function. You can grind the step off though. Alternately, you can ramp your 9mm bolt (joeblack does it, check this [url=http://www.ar15.com/forums/topic.html?b=3&f=15&t=163203&w=searchPop]thread[/url].)
As far as the buffer spring and buffer go, just run with what you have. I have a RRA lower, with an A2 stock, .223 buffer and .223 buffer spring, and a hammer like the one pictured in the bottom picture (and my bolt's not ramped), and it runs absolutely 100%. Any more question, you can feel free to email me!
-Gloftoe