I'll be a little more straight forward....
I hand swage my own 20 caliber bullets and its impossible to get enough lead crammed into a bullet jacket short enough for the complete bullet to weigh 55 grains and for it to stabilize in a 12" twist barrel.
I typically use a fully drawn 0.224" X 0.705" jacket that measures ~0.760" after being drawn down to ~0.198" before swaging. It weighs ~14.3 grains.
The jacket is far too long for a 12" twist barrel, and yet even at that length it is impossible to cram an additional 40.7 grains of lead into it and form a point without the resulting bullet looking at best like an extremely long 38 Special round nose jacketed bullet in profile (not in diameter of course...).
To illustrate, here is a pic of a drawn jacket (0.761" in length and weighing 14.3 grains) and a lead core weighing 40.7 grains. They are almost the same length. Even if the point of the bullet is left mostly open, and the lead is allowed to protrude and to be formed into a point of some type, the resulting bullet will be even longer than the beginning jacket length. If the bullet point is closed up at all, the resulting bullet will weigh far less than 55 grains (maybe 50 grains at best) after the excess lead core is pushed out the nose of the bullet as the point is being formed.
So, for a .204" diameter 55 grain bullet to have a decent BC at all, it would take a much longer jacket than ~ 0.760" to start with, and thus you're moving even further away from a bullet that will stabilize in a 12" twist barrel.
Even if it were made with a longer jacket, the resulting bullet also is far from a good choice for deer where it’s legal, even if you had a fast twist 20 caliber barrel.
Buy a suitable deer rifle and use your 204 Ruger with its 12" twist barrel for its intended purpose.....which does not include deer.
Just for reference, I typically trim the ~0.760” jacket to 0.620” to form the 38 grain HP bullets that I normally make and shoot in my Tac 20’s and 204 Rugers. All of my 20 caliber rifles have 12” twist barrels.
-BCB