I want to build an AR 15 pistol for 5.56 and .300BO. I want to keep the .300BO to about 11" and have a pistol buffer tube. For the 5.56 - and I say that because I want to be able to shoot both the 5.56 and .223 - I want to keep this short, around 8" also with a pistol buffer tube. I'm told that the .300BO will have issues with three areas: Supersonic, Sub-sonic and Sub-sonic suppressed. For the 5.56 I hear that there may also be cycling issues dependent on length. In essence I would like to know which gas block for which build and which buffer. I know that JP has some nice stuff that's very adjustable but is also a pricy. Or am I even barking up the right tree?