For anyone wanting to use heavy bullets in .45 ACP wheelguns, I heartilly recommend the 275 grain truncated cone design available from Hunter's Supply, (I get mine from Midway).
It weighs an actual 273 grains. A lot of the weight is in the nose of the bullet, which leaves more room for powder in the case. Other bullets share this feature also.
Propelled by a suitable charge of Power Pistol, it gives good performance.
I have even fired it in Gov't models, by seating it just slightly deeper. It feeds well.
I would not, however, push the envelope in a 1917 model. These guns deserve an honorable semi-retirement, and IMO, should not be fed anything warmer than standard .45 ACP.