The 5.56 sufferers greatlty from velocity loss from such barrel lengths at the 7.5" SBR but with the right ammunition such as the Hornady 55gr VMax the 7.5" 5.56 still serves as an excellent defensive carbine but it would be wise to add a silencer as the 5.56 SBR is extremely loud, even with ears,.
This is my Barnes Precision Machine 5.56 7.5" SBR with the Liberty Suppressors Triumph silencer. I have reviewed both the SBR and silencer as well as 7.5" vs 11.5" vs 16" 5.56 ballistic tests if any of you are interested.