I have a couple of questions. I'm interested in buying one of the RRA complete rifles when they come out, most likely the flat top.

1. I currently have a regular AR with a 512 Eotech; could I get the GG&G Accucam and switch the Eotech from the rail of the 5.56 to the  rail of 9mm? (I'm trying to avoid spending another $350 for a 2nd Eotech)

2. How much would the accuracy be off from one rifle to the other?

3. Say at 50 yards, will the 9mm hit further down because of a slower FPS rate? What is the average FPS out of a 9mm AR?

Any help would be appreciated.

If you use both guns with flat tops the Eotech should be close. Most of my guns have Arms SIR's on them and I will take off the Aimpoint of my RRA 9mm set up and stick it on my AR10 set up and the POI is very close.

It depends on the 9mm load - 115gr, 124gr, and 147gr are pretty popular 9mm "factory loads".  Rule of thumb is about an additional 10 fps per each additonal inch of barrel.  Since most 9mm ammo ballistics are shown fired out of a 4" barrel, I think it's safe to say that with a 16" carbine upper barrel - you will pick up an additional 120 fps with the extra 12" of barrel.  

I'm looking to make the investment in a 9mm upper, mag block, mags, buffer, etc...  Good luck and wish me the same!!

Thanks for the help in answering my questions...it makes me want to get one even more!

You could take both to the range and determine the elevation change. It would then just be a matter of dialing it in when you move it.

Thought I read a study somewhere online that said the optimum 9mm bbl length was close to 10", and anything extra did very little for velocity.  I'm sure some different powders and handloads would alter this equation, though.  I think SBR for 9mm is a good thing.  9mm simply looks better with a 10" bbl.