The Break Barrel Tune Up Kit provides everything you need to custom tune your gun, and gain significant fps, within minutes; AND without the need for a chronograph or any special tools. The kit requires no irreversible changes or modifications to your airgun, and it's fun and simple to do. Gain maximum performance at a low cost! The kit was developed by an exhaustive analysis of the barrel/breech seal/compression tube interface; a major cause of muzzle velocity loss in standard factory guns. The kit allows you to minimize air loss and thus maximize muzzle velocity. TheTune Up kit fits many Ruger air rifles, including the Ruger Targis, Yukon, Ruger Air Magnum, Explorer, Blackhawk, Blackhawk Elite, Air Hawk and Air Hawk Elite. The kit for Ruger Air Rifles provides a significant increase in muzzle velocity of 80fps - or even more - compared to an average "out of the box" gun in .177 caliber, an increase of over 20% in muzzle energy using the same pellets! For professional installation only. Requires special tools for installation. Kit Includes: 3 Precision Machined Steel Shims, 3 High-Quality, High Durometer Breech Seals (2 are spares) Comprehensive Illustrated Instructions Kits work with both .177 and .22 caliber air rifles.