Pronounced HM AIR instead of HMR. Its the 17 HMR or the airgun world. This is the 1st .172 caliber air rifle that I know of. YES, it shoots .172 cal cast lead slugs and NOT .177 caliber Diabolo pellets. That's right, the same bore diameter of the world famous .17 HMR. The barrel even has the same twist rate as the HMR which is 1-9" twist.

This video showcases Doug Nobles machine work and some Glycerine Clear soap testing. This is part 1 and the upcoming video will be part 2 and contain actual field results and specs.
Cannot show the rest of his tests on the open forum but you can check out the rest of his channel here:

.172 HMaiR - Tofazfou