For one a .17 cal HMR is a .172" projectile and is TOO SMALL for a Air gun barrel.
Also HMR bullets at JACKETED over a lead core. The friction a jacket bullet has over bare lead is a massive !! By design Powder Buring guns shooting Jacketed bullet are sized in such a way the the Mien & Max diameters of barrel & bullet are opposed so a good seal can be made with minimal blowby. Because the pressure of a burning propellant is so high the friction is easily overcome and the bullet easily go't out the barrel. ( The .22 WMR in which the HMR is based upon ) runs at 19,000 PSI and while unable to find the data, likely the HMR is above that a bit.
There have been some specialty made .172" PCP bullet shooters, but the rifling depth, Mien/max of fit has been modified to shoot a LEAD bullet able to do so at our typical PCP operating pressures.
just some FYI and thoughts shared ... nothing more.
Scott S