Any time you reduce pressure, you reduce velocity as well. It then takes more powder to get it back to the same speed and pressure as before. It is a VERY fine juggling act that bounces back and forth depending on length of freebore(leade), "size" of freebore(leade), angle of throat and seating depth and so it goes.

Even in my little 9.3x57, which has a VERY long leade or freebore, seating the bullet .1" more deeply in the case, increased velocity over 100fps, and was equivalent to 2gr. of powder.(the longer seating needed 2gr. more to match the speed of the shorter length) Even with it's horifficly long leade (cannot be touched and still keep a 286gr. bullet in the case), this rifle shoots inch or better yet was made back in 1929. It does not care where the bullet is seated, accuracy wise, but velocity wise, it is touchy as noted.

Lee "Factory Crimp Dies" do not need a cannelure to crimp a bullet securely in a case as they are collet dies. Normal seating/crimping dies such as RCBS require a cannelure as the case is pushed into a constriction, and will buckle if no groove. With a collet, the case neck is constricted, being pressed in from the sides, not downward from the top.

In the .416, if powder capacity suffers with a certain powder, that powder is too slow burning and a different powder needs be chosen for testing.

Most modern rifles have magazine capacities shorter than the actual throat in the rifle will allow. In other words, the chamber's throat will normall allow longer seating than-will the magazine. I'd check that first, before altering the chamber.