Center ring for manual SVD

<p>Center ring on A a K SVD rifles determine exact axis position of the spring and piston. But if you want to use stronger springs, more than M150, ring doesn`t fit on it. Strong springs has small inner diameter. You can use these springs without center ring, but  when you charge the rifle, piston lifts up. It is not fully hold by catch.</p>
<p>So this is why we develop new brass ring. Allows to use any strong spring and keep piston in the right position.</p>