Has anyone used Gamo ball in the Webley?

We used two Webleys in a speed shoot at the club last night and experienced a couple of malfunctions, in that balls bounced off balloons without breaking them. This only happened a couple of times and in each case a relatively fresh Crosman Powerlet had been inserted into the pistol. I had reservations about using Gamo ball as it is a little bigger than the 4.4mm lead ball I usually use. However one of the pistol's owners assured me the Gamo worked fine for him in the past. The Gamo were a tight fit in each cartridge and I wonder if anyone else has ever had a similar experience?

John