The FX Impact M4 is arguably the most advanced PCP bullpup on the market. With features like its Smooth Twist X Superior (STX) barrel system, improved Power Plenum 750 (increased post regulator volume), Adjustable Match Precision MKII (AMP) regulator, external micro and macro hammer spring tension adjustment, and external valve control, the customization options are almost endless. The M4 includes an improved reversible side lever and a new dual transfer port (.22 and .25 only) that lets you optimize performance for both pellets and slugs. Plus, the new Quick Tune system gives you an easy way to adjust your air rifle with very little need for tools. Of course, the M4 also features the rear Power Block. With multiple barrel lengths and calibers, there's a version for just about any shooter out there!