Last Fall, after cleaning my JP Sauer 20 gauge shotgun, I inserted 2 plastic Pachmayr snap caps in the shotgun. Last week I removed the gun from its case for a hunt. Upon opening the action, there was no visible snap cap in the right barrel. Fortunately the snap cap in the left barrel was still visible although it too had begun to slide down the barrel. I was able to get both snap caps out of the barrels. The plastic rim on each had disintegrated over the past year, allowing both the caps to slide down the barrels, obstructing the bores.
The snap caps are blue and marked Pachmayr, CAL.20 MADE IN ITALY.