I have a PTR91 SC that has the welded rail on top. I was wondering if it was a picatinny type or a weaver type? Also what type are the handguard rails? And how can anyone tell the differance between a weaver type and a picatinny rail? And Ive heard a weaver will fit picatinny but picatinny will not fit weaver, true? Reason being Im ordering some optics and I wanted an Aimpoint with the A.R.M.S. mount....