While these are meant for the MK23, these will pair great with a 5.1 outer barrel and CowCow Thread Adapter. Some fitment work may be needed on certain hopup units.
Nine Ball tight bore barrels are our favorite on the market. The 6.00 inner barrel diameter is perfect for heavily increasing your FPS as well as accuracy. These our are go to barrels, and will give you a great overall performance boost.