I don't know if there is any downside to solid copper bullets, other than requiring a faster barrel twist rate (though with only 55gr I can't imagine needing faster).  I saw some for sale (link below), which seem a bargain if there is any pros to solid copper.  Actually, unless there are any cons, they seem like they'd be great for inexpensive plinking.  Comments?

http://www.theamericanmarksman.com/223-Remington-55gr-Lead-Free-Solid-Copper-Bullets-Certified-for-use-in-Californias-non-lead-ammunition-zone-for-non-game-species-500-Rounds-_p_339.html

The main downside is they are expensive compared to copper coated lead.

the link you provided is reloaded/reman ammo, don't know if you saw that. I tend to avoid reman ammo due to higher kaboom possibility compared to factory ammo...but to each his own.

Solid copper work very well from what I have read about them. I don't know why the standard rules for bullet weight vs twist ratio should be any different than from lead.  55gr is 55gr.

Twist requirements are based on bullet length. Copper bullets are longer than lead bullets for a given weight.

OK I always thought it was bullet weight. bullet length and a longer copper bullet makes sense since copper is not as dense as lead. so 1/12 or 1/14 is probably out.