It's really not a 45degree angle and most definitely not a cone, but a smoothly rounded entrance.
What this type of crown does, is allow the flow of patch and lead to happen at the muzzle as the patched ball
is seated into the bore. Once inside, with a wet patch it is an easy push down to the powder, which cleans the
fouling of the last shot. We load with ball compression in the bottom of the grooves, usually .005" sometimes
up to .010" compression of patch and ball in each and every groove.
A slow taper does not work as well, as the long taper increases friction & actually makes loading more difficult. We
have proven this on LB's .40 rifle. He was not able to load a combination in his long-coned rifle that loaded easily in
my gun. I tried it in his, and by jingy, it was hard loading in the long cone, not so in the short radiused and smooth crown.
This is a crown DaveC of this site, cut with his crowning tool (which he makes and sells)
Here are a few we've done by hand as below, or on the lathe. Most of mine are done by hand, with emery or wet/dry 320 paper.