With an Aero M5 upper is there a specific pattern of 308 barrels that are required? Since there isn't a mil spec to go off of. What are some recommendations for the best 20 inch AR10 barrel? Also, are there specific handguards that only fit with the Aero M5? Preferred to either get a MLOK or Keymod type of handguards. What are some recommendations for handguards to mate with that upper?