Actually the question would be what the pressure of the load is, not if it is subsonic. How slow or fast a load is not the prefered way to decide it's safety in a given firearm. I'd be surprised if your load is running less than 25,000 psi considering the weight of the bullet, the bore diameter and the capacity of the case. Thats certainly less than the 30-30 which was chambered on like frames, but knowing that peice of information still leaves us in the dark as we don't know the actual pressures generated by your load.