On my AR-10 build, I have ran into a problem. My rear takedown pin's detent pin and spring enter from the rear of the lower receiver instead of from under the receiver next to the safety detent pin and spring. PROBLEM: spring is too short; therefore, it cannot keep the pin in place. Do they make a longer spring for the AR-10? If not, how do I resolve this...add another spring into the rear channel? Recommendations?