During extrusion process, rubber compounds frequently present volume defects which alter the quality of the product. In the present study, we have systematically studied the flow of such compounds in capillary rheometry. Compounds made with two types of rubber, butadiene (BR) or styrene butadiene (SBR) rubber, silica and two types of additives (coupling agent and covering agent) have been prepared in an internal mixer. The flow behavior of these various compounds has been characterized using a pre-shearing capillary rheometer, with dies of different lengths and diameters. BR compounds exhibit the classical behavior of linear polymers, with two stable branches separated by an oscillating area. SBR compounds show a transition above a certain shear rate, indicating the onset of strong wall slip. Except at very low shear rates, all the extrudates present volume defects. Even though these specific features are not modified when varying filler content or type of additive, it appears clearly that instabilities increase with the coupling agent. For each extrusion condition, samples have been collected and analyzed. The geometry of the extrudates has been quantified using a high-resolution scanner and image analysis. We show that the diameter distribution of the extrudates allows a global description of the severity of the volume defects.