For what is worth, which is not much I expect-A 45-70 with a long barrel, BC for example, with a heavy cast bullet and a small charge of Trailboss is vewwy vewwy quiet. The trajectory, on the other hand, rules out anything but 25 yd or so shots. A whelen and cast also can be loaded way down to cat sneeze territory. Speed is needed for a flat trajectory, unwanted for the noise it produces. Without a can, you can't easily get there from here. I've been trying for a few years to do just what you are looking to accomplish. The closest I've come is with a 30-30, 200+ grain lead bullets, and tiny charges of powder. The rainbow effect is the problem, the slow, heavy projectiles retain a lot of energy, but hitting a target at long and unknown ranges is problematic to say the least.