Re: sks with tapco 20rnd mag
what you've seen is either a modified gun or an sks that takes ak mags. any regular sks with any detachable mag will require you to hold the bolt back to remove the mag if there are still rounds left in the gun
i suppose you could remove the portion of the bolt that holds on the magazine feed lips. it crossed my mind to do this if i ever started using after market mags in my sks. you wouldnt need to remove the whole section only about the back 2/3s of each offending rail. leaving them intact in the front would keep as much strength in the bolt as possible and as much weight.