Well, he sure didn't make that the priority of the discussion which for intents and purposes is far more important.
Not when the entire discussion is about guns it's not. His memory of his parents killer is absolute, but a given, and Batgirl knows that, the fact the gun looms LARGER than that in his memory should make her appreciate the gravity of his comments.
Heck, he even says "But the image that's more clear to me is his gun." I mean, can you imagine what Bruce would have said if the murderer used a pencil?