50 minutes is a tough connection any time on any day at DCA if you have to switch concourses. You will have to make sure you know whether your connecting US flight departs from gates 23-34 or 35-45. It is a brisk 5-7' walk from the furthest gate in the 10-22 gate area to the security line at 35-45.

Saturday mornings are very slow at DCA. There are few passengers, but there are fewer TSA agents working. Security is usually pretty fast, but sometimes there are only 1 or 2 lanes open at slower times, so the least common denominator (Ma and Pa Clampitt) returning home to Omaha sometimes throws a monkey wrench into the operation.

I think you can make it. If you are delayed by more than 10 minutes, your luggage probably will not.