Powder bridging in dispensers can cause overloads as well a squibs.
The problem, as mp45acp10+1 wrote, the powder that does not get through to one case can be added to the next case's normal charge when the "bridge" breaks, creating a double charge. That is a possibility when the powder is bridging AFTER the metering cavity. So, it is wise to look for overcharges if you have found under charges.
On the other hand, if the powder is bridging somewhere BEFORE the metering cavity, then it only produces light loads. A somewhat related issue is the changing weight of the powder above the metering apparatus as the hopper is drawn down during reloading of multiple rounds. If the metering cavity was set with the hopper full, then reducing the hopper level may cause the powder throws to become progressively lighter. On the other hand, if you set the metering cavity with the hopper too low, then add more powder, you can get higher charge weights than you were intending.
One way this is addressed is to add baffles to the hopper, so that there is an intentional type of bridging well above the metering cavity, which tends to make the powder weight at the cavity entrance stay more constant.
Another way to address bridging is to intentionally bump the powder dispenser THE SAME WAY EACH TIME. On a progressive, the normal operation may give the appropriate bumping when things are running smoothly, but get somewhat off when there is a stoppage, and the bumping becomes non-typical for a few strokes. On a stand alone measure, there are sometimes even little weights hinged to the device that are intended to be raised and dropped to thump the measure in a contolled way. But, you can do the same thing by the way you raise and lower the handle or tap the device with a knuckle.
Last edited by SL1; December 9, 2012 at 01:51 PM.