I have 4 Mexican 1910/36 actions. A small ring mauser bolt lug lap tool screws into 2 of the 4 receivers. A military take-off barrel screws into all 4 receivers. Is this a common problem? Would a small ring receiver tap be the solution or is it likely to be the bolt lap tool threads?

Common to have threads slightly tight or oval. The solution is to custom fit the barrel threads to each receiver. If they are oval, then a tap might be good. And all are probably within manufacturing tolerance.