I have a new RRA M4A3 upper.  I put it on a lower with a post ban M4 stock that uses the CAR style tub and buffer.  
Took it to the range and had my shoulder beat to crap.  I have the bruises to prove it!
The recoil was very harsh.  You could hear the bolt hitting the stop.
I next changed the stock to a A2 style with the regular A2 style Buffer.  Same thing but not as bad.  Still bruised the shoulder.  
The Upper has the faux flash hider and is in new condition.  I never had a upper that beat me up this bad...Hell, I Never had a upper that beat me up at all!  I want to fix it somehow as this upper is very accurate.  Anyone have any suggestions?  

BikerScout
Sunny AZ

Not familiar with the pigtail or fatboy, who sells them?
Found the regulated one that Fulton Armory sells.  $60 for the carbine lenght! I'll have to wait till payday.  
In the mean time I was hoping their was something else I could try.  How about swaping bolt and bolt carrier?  I have some head space gauges by Bushmaster and will check headspace today.

BikerScout
Sunny AZ
[8D]

I have seen this with more than a few carbines.  Replace the buffer spring with a Wolff Extra Power spring.  You can get them in rifle length and carbine length.  Works like a charm.  A $15 fix.  From Wolff's site:

http://www.gunsprings.com/RifleShotgun/Colt_RsNF.html#AR

ACTION SPRING- extra power. Improves reliability and functioning.
Stock No. 16500 - AR-15, M-16 and Sporter.....$ 14.99 ea  
Stock No. 16501 - CAR 15 and M4....................$ 14.99 ea

Well tried the Bushy headspace gauges and everyhing checks out fine.
I'll try some of those extra power springs.  How about the speed lock springs?  My local gunshop has some of those.
I'll let you know what happens.

BikerScout
Sunny AZ
[8D]