This doesn't look like the "where it happens" damage from small caliber guns.
I think it's cannon damage, but the reason it's not very detailed is that it's not damaged enough. I've seen it happen with other aircraft too. If you keep on pounding on them, then the internal structure starts to show. If you just score some hits on them however, you get superficial damage from shrapnel, just like in the screenshot.
The reason the Wellington is so keen to show off its entrails is that it usually burns easily
