Just because you've not experienced something doesn't mean it can't happen. My primary skeet gun's breech face has erosion rings, the same size as primers, around the firing pins and the firing pin holes are enlarged. I'd tried to get some pics for you, but they come out too dark without the flash or overly bright with the flash -- my camera isn't happy making close-ups.
Big Jim questioned me about the same thing right I posted my remark. Here's an extract of my reply: When I had my Mirage last serviced, the Perazzi factory tech took one look at my gun and said, "You've been shooting reloads, haven't you?"
Haven't you had old hulls that have black around the primer pockets? It's from blow-by. I load with Double-A hulls in four gauges (the gun is tubed) and some Fed papers in 12-ga. If your BT-99, with all those rounds, shows no signs of breech face erosion, I'd like to know what hulls you load with.
Perhaps Browning (Big Jim shoots B-guns, too) uses a more robust alloy than Perazzi. The problem is common enough that Perazzi has a method of renewing the breech face. If the problem doesn't exist, perhaps I can send you the bill when I eventually get mine renewed.