I ended up doing an unexpected detour here after cycling around the whole island. I wasn't planning on spending long, just a quick look. There were 3 volunteer staff members who were coming to the end of their day. They each had an infectious enthusiasm for the Battery and its history, & I ended up with quite a personalised tour. If you make it there during opening hours (roughly 11am-3pm), then there's photo displays, but the gun itself is pretty impressive. Worth the visit.