I run a ciener kit in the ac556. I tested it with several round nose 40 grain rounds and they all ran. It runs very well, and very fast. I use the plastic black dog mags. I have seen posted elsewhere that the older kits didn't have a feed ramp on the chamber conversion, but both of mine do. So...
Mach 5 or above. I looked it up.
They even have a chart-
http://en.wikipedia.org/wiki/Hypersonic_speed
And just so my post isn't completely useless-
I have a 3-9 on one and a red dot on the other. Both work well. You can push the round far enough, but it really isn't the best choice for...
Ricuser, Mississippi Auto Arms have uppers on their site and listed on gunbroker as well.
http://www.mississippiautoarms.com/lwrc-545x39-upper-receiver-m6ak-in-545x39-p-3301.html
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=334044279