In our extensive testing of the HDD, we did find that it didn't work if the breech slots have been polished as part of a tune. The HDD requires some friction to work and the "factory finish" on these slots is ideal for the design.
So, if Roy has polished those slots, you won't be wanting an HDD. If you want the HDD, maybe he can omit that stage of his work.
Archer Airguns Inc.