Re: 9mm SBR ?
I am building a 9mm SBR with integrally suppressed 13" barrel (should be about the same length as your barrel with a can on the end of it).
If you buy standard ammo with the 147 grain bullet it will be sub-sonic, there is no need to special load or pay high dollar for "special" ammo. For plinking the 147 grain CCI Blazer is wonderfully reliable and very quiet with a suppressor. For "serious work" a good JHP load like the Winchester 147 JHP should be sufficient (but mine is just for fun!) But any of the commonly available 115 or 124 grain loadings will be supersonic and make a loud crack when suppressed. For unsuppressed carbines they all are loud but the sub-sonics do not have that sharp crack.
Also if you do load the ammo down and are shooting out of a 9mm AR15 (or other 9mm semi-auto carbine) you will most likely run into action cycling problems when you slow it down to sub-soninc, AND a sub-sonic 115 grain bullet is pretty weak. Just keep these things in mind. Remember sub-soning is below 1100 fps (typically). A standard full speed load for the 147 grain buillet in 9mm Luger is 900 to 1000 fps which nicely fits under the "speed limit".
Bolt ramping is needed if you run a standard AR15 hammer. If you use a "9mm hammer" a ramped bolt will not work. I believe a ramped bolt provided a slower reset of the fire control group and thus less impact on the hammer and hammer pivot pin. I ramped my bolt, I also use KNS pins for cheap insurance and supposedly less abuse to the lower. Once you jump through the hoops to NFA Register a lower you do not want to wear it out in a few years!
