By Tim Reid and Joseph Ax (Reuters) – More than 600 conservative gun rights supporters, including several major Republican donors, are urging Congress to pass gun control measures in the wake of mass shootings in Uvalde, Texas, and elsewhere in the United States. A group of mostly Texas residents organized by Todd Maclin, a former senior executive at J.P. Morgan Chase, signed an open letter in support of expanding background checks for gun purchases, passing “red flag” laws to deny firearms to people judged a risk to themselves or the public, and raising the minimum age to buy some guns to 21 …