Below a few HARP gun pics (source internet for all).
HARP Guns at Yuma Arizona Proving Grounds:
The larger gun is 16+" with a 119 foot tube and I think the smaller one is 5".
Original 16" HARP install at Barbados (65 feet, before tube extension):
Here is some original video of the Barbados HARP gun being installed and fired:
I’ve not listened to the audio, but the video is good.
BTW, the HARP gun at Yuma still holds the altitude record for artillery at 180km (111 miles) high.
And below a report from C. H. Murphy and Gerald Bull on HARP:
Review of the High Altitude Research Program (HARP) BRL R 1327 (1966).pdf (6.3 MB)