var CoreData = new
    {
    Shoots =
   (from item in coresdb.Productions
    where item.Date >= StartShift && item.Date <= EndDate
   select item).Sum(x => Convert.ToInt32(x.ShootCount)
   )
};

var CoreData = coresdb.Productions
    .Where(item => item.Date >= StartShift && item.Date <= EndDate)
    .ToList()  // get the data into memory first.
    .Sum(x => Convert.ToInt32(x.ShootCount));

Shoots =
  (from item in coresdb.Productions
   where item.Date >= StartShift && item.Date <= EndDate select item)

.Sum(x => SqlFunctions.IsNumeric(x.ShootCount))