A strengthening United States dollar is a mixed bag for U.S. defense contractors as it makes U.S. exports appear more expensive, though a large majority of federal government-approved major arms sales were to oil-exporting nations shielded from economic volatility.The dollar has been gaining strength since early 2014 against the Euro and British pound, home to two large U.S. trading markets. Since May 3, 2014, the exchange rate has risen from 0.72 Euros per dollar to 0.94 Euros per dollar on…