This spacer allows you to use a standard carbine buffer in you 9mm AR15 build. AR9 Bolt Carrier groups are shorter than standard 5.56 BCG's. This 9mm spacer allows you to use a standard carbine or heavy buffer with your Colt or Glock style 9mm BCG.This spacer was designed for the guy who already owns a 9mm buffer like the one offered by RRA or Colt and doesn’t want to replace it with another expensive $60-$90 custom buffer. The ST-9BS does basically three different things for you that we’ll explain; It limits the travel of your 9mm bolt, helps eliminate the chance of breaking bolt catches, and makes for a much smoother shooting gun. Of course knowing us, it’s made from solid CNC machined 6061 billet aluminum with a black anodized finish for extra strength and durability.The reason why it’s better to have less bolt travel on a 9mm upper is that the 9mm cartridge length isn’t near the full length of a .223/5.56 cartridge so it doesn’t need the same amount of travel. All of this extra travel is just wasted energy and thus creates excess wear and tear on your expensive 9mm AR-15. This spacer takes up the extra space that isn’t needed in a 9mm AR and shortens the distance the bolt travels to the amount that it actually needs to travel.Bolt catches break on 9mm ARs due to the bolt having a running head-start on the bolt catch after the last round is fired. This puts excess pressure onto the bolt catch when it pops up and can cause them to break time and time again. Our spacer solves this issue by shortening the distance that the bolt travels thus not letting it get a running head-start on the bolt catch again.Because there is less distance that the bolt has to travel, you are going to be able to have faster follow up shots due to a shorter reset. Also, due to there being more spring pressure there will be less gas getting shot up into your face and more of it shooting out the muzzle which is an obvious plus.To Install the ST-9BS spacer you first simply remove your buffer and spring from the buffer tube. Then install the smaller side of the ST-9BS on one end of your spring like you would with a buffer and then drop in the complete unit into the buffer tube and start shooting again (the ST-9BS goes in first followed by the carbine buffer spring and then the buffer itself). So you basically want the spacer to be inserted all the way at the back of the buffer tube.If you have any of these issues with your 9mm AR or just want to make sure you never do, buy one of these spacers today and save yourself a lot of headaches and extra money that you could be spending on ammo.