I bought Meprolight adjustable night sights for my 17 to use with a suppressor. They fit any glock, including the 21. Here are the front sight heights for a fixed front vs an adjustable front, both Meprolight. Fixed 0.172" adjustable 0.312" The fixed front sight clears a suppressor with a 1" tube, mine is actually 0.997" for the 22lr suppressor I use with my Glock 22lr conversion. The suppressor is not even seen at all when you look down the sights. The 9mm suppressor I will be getting has a 1.25" tube and so will most .45 suppressors. A 1.25" tube with the adjustable meprolights puts the tube just below the line of the sights. The adjustable Glock Meprolights look just like the HK Tacticals, the Tactical just comes without the tritium, the sights are still made by Meprolight and you can get the tall Meprolights from HK as an option. If you need a set I have one set left at our shop, I think they are $127. Email me if you are interested.