>>64103184
In case you are not trolling.
20" has greater velocity and the whole point of 5.56 is velocity.
rifle length gas systems are softer recoiling and more reliable.
16-24" are the acceptable lengths for 5.56
If you want 11.5" ju8st get a 9" .300blackout or 11.5 7.62x39mm.
.30cal rounds are better suited for sbr where the loss in velocity doesn't matter as much and shorter gas systems retain their reliability.
Honestly get a 9" .300 blackout sbr if ease of use, home defense is the main concern. You can then get a suppressor and subsonic rounds which will save your ears indoors.
or be cool like me and use a 20" rifle with bayonet for home defense.