Seems the biggest reason t convert an M&P from 9mm to .40 is ammo costs but with the cost of a 9mm barrel and mag near $100 and a case of 9mm another $250 to $300 consider 1000 rounds of 22 lr is $50 and a .22lr M&P about $350 ,
Would it not be more cost effective to purchase an M&P 22 for cheap plinking and recoup the cost of the gun with every 1000 rounds fired ?

Just sayin...