To get the most out of your markers a TechT MRT Spring Kit is essential! The kit provides players with three distinct levels of resistance which will allow for lower operating pressure, as well as reduced dwell times. This means better efficiency = more time on the field, rather than the fill station. The MRT Spring Kit is a perfect match when paired with the TechT MRT Bolt for the Invert Mini & BT TM-7 or TM-15. For those of you who prefer a different bolt, The TechT MRT Spring Kit works with ALL Mini Bolts, including the stock bolt.