After the luminosity upgrade of the Large Hadron Collider (HL-LHC), the rate capability of the muon drift tube detectors of the ATLAS experiment will be exceeded due to the higher particle background rates. In the regions between the inner barrel and endcap of the muon spectrometer the trigger selectivity is limited. Therefore 8 new small diameter (15 mm) Muon Drift Tube chambers (so-called sMDT BIS7A) are installed in the transition regions between the inner barrel and endcap of the muon spectrometer which provide a significant high background rate capability and free space for the installation of new RPC muon trigger chambers as required for operation at HL-LHC. This is a pilot project for the whole replacement of the MDT chambers in the small azimuthal sectors of the barrel inner layer (so-called BIS1-6) by new sMDT-RPC detectors in the long shutdown 3 (LS3). An overview of the validation and installation of the new sMDT BIS7A chambers, their cavern commissioning status and their performance in the ATLAS combined runs during Milestone weeks, will be presented.