I've noticed that if you are farming bolters for the Pilgrim Outpost bounties that it doesn't register if you kill the same bolter over and over. To negate this issue, you can run back and forth between two different bolters. Or play through the bozak hoard, as there is a constant flow of bolters thrown at yiu throughout it. Otherwise, this technique is still good for the bolter tissue needed for blueprints. It is also a good way to gain experience on nightmare difficulty with minimal risk.