I was running a carbine stock with AAC rate reducing buffer but I want to go alot slower.. So I changed out the stock to a rifle length stock and now am looking for a heavier buffer to slow it down.. I was thinking somewhere around 600 to 700 RPM..
Thanks
Also where to get the buffer..

I don't know about a rate reducing buffer but have you looked into using a Lage upper?  This upper reduces the cyclic rate to 600 RPM. Check out this website.

http://www.max-11.com/

HTH.

I believe AAC makes a couple of different versions of their rate-reducing buffer.  I have the one for a rifle-length buffer tube.  By its self, I've timed it as slow as 665 rpm.  To me this feels really slow.  It also had problems, as the bolt would sometimes not hit the primers hard enough to set them off.  

I ended up putting a cut down section of the factory recoil spring back in the gun, along with the AAC buffer.  Now I'm running around 750 rpm.  The gun is reliable.  The ROF is just about right.  It's slow enough that I can get singles on demand, and runs pretty smooth overall.  

I also have a 9.7 oz. steel buffer that I had turned on a lathe.  It ran between 700 & 775 rpm, depending on the ammo.  To me, the AAC buffer has a better "feel" to it.  To gain more weight, people have also had buffers made up out of brass.  

The AAC "super slow" rate reducer will give you the slowest cyclic rate, but as noted sometimes the reliability suffers with the factory spring, which is pretty weak.  You can substitute a GI spring and the cyclic rate will go up a bit, but the gun will work fine.

I have a 12.8oz stainless steel buffer in mine.  


Set the blue diameter to a complement the ID of your spring, and increase the green OD to still freely allow spring to slide, I think mine is .750" (adds a slight bit more weight...

Go with the MAX11 upper - it works great and no drilling in the back of the receiver.
www.max-11.com/

www.uzitalk.com/forums/showthread.php?t=20885

also just received my lage upper and tried it out this weekend. one word: outstanding. if there are any other guys who have M-11s and want to slow them down, this thing is the real deal. worth every penny.

ron