If you like to keep spare parts on hand for maintenance or replacement, add the appropriate size Plunger to your kit. Plunger service life varies from 10,000 to 25,000 rounds, depending on your firing schedule. When you replace a worn-out barrel or bolt or when the working end of your Plunger has been worn away by half, then it's time for a new plunger. You can also select shorter or longer plungers to fine tune the range of adjustments used for your specific firearm. This provides a tremendous amount of flexibility thanks to the modular design of the RIFLESPEED™ line of Gas Controls. For example; you've completed the setup instructions found on riflespeed.com and determined that with Plunger 937 installed your carbine uses setting number 1-6 for suppressed fire with milspec ammo and setting number 1-11 for unsuppressed fire with commercial 223 .ammo. You determined these settings on your local range during the early summer when temperatures were in the mid 80s. Since more gas will be required during extreme cold winter temperatures you can select a shorter plunger like Plunger 917 to shift the position of your ideal settings to the lower portion of the adjustment range. Since every plunger number represents a shift of 2 positions on your control knob, instead of using settings number 1-6 and 1-11 you'll now be using settings number 1-2 and 1-7 for suppressed and unsuppressed fire, respectively. So what's the purpose of doing this? Shifting the range of adjustments to lower numbers allows you to have more room to increase gas later when temperatures drop without having to dial past the Rotation Stop to use Rotation Range Two. Since cold guns need more pressure to run and cold ammo produces less pressure, it's important to be able to increase gas flow when temperatures plummet. Selecting the specific plunger that puts your preferred settings in the lower third of the adjustment range gives you the most potential adjustment range. The opposite holds true if you've determined your firearm's settings during cold conditions. You may want to select a longer plunger to insure that you can reduce gas when summer heat arrives. Again, this process is not a necessary step as the plunger provided with your Gas Control has been selected for general-purpose use in most configurations, but the ability to easily fine tune the system is one of many benefits of the modular design of RIFLESPEED™ Gas Controls. Plunger 937 comes installed in every RIFLESPEED™ Gas Control. This is the general-purpose size that covers a broad range of firearms. Plunger 937 measures 0.937"" long and is suitable for most configurations. Plunger 917 measures 0.917"" and provides less restriction for configurations where more gas is needed to ensure reliable function. Plunger 917 also allows the use of the lower-numbered settings on your Gas Control, giving you maximum adjustment range in cold conditions where more gas is needed. Plungers 917 and 937 are included in the retail package with each RIFLESPEED™ Gas Control. These sizes provide coverage of the full range of potential settings needed for nearly every configuration we've identified during development of the RIFLESPEED™ Gas Control and allow the shooter to select the best Plunger for a specific firearm. Additional sizes are available for those who choose to precisely tune their Gas Control's settings. Keep your RIFLESPEED™Gas Control functioning like new with genuine RIFLESPEED™ replacement parts. Made in the USA.