Getting a COA registered

[MacMillan of Rathdown]

The problem is this - derivative works do not share the same protections under US copyright law as original pictoral, graphic, and sculptural works. My assumed arms might be unique...maybe. Your granted arms surely are not (and of course they should be derivative of the MacMillan chief's arms since you are a MacMillan).

Kenneth, I am afraid you are in error. Provided you are the owner of the copyright of the original piece, you also hold an equal copyright to any other pieces derived from the original. With regards to arms based on, or "derived" from, arms already in use (ie: chiefly arms) you need to remember that the letters patent issued by the granting authority are prima facie evidence that the arms depicted on the document are, in fact, uniquely the sole property of the individual named on the document. Thus, each act of a substantive granting authority creates a new and unique piece that can be copyrighted if the recipient so chooses.

EDITED TO ADD: I neglected to mention that the standards for copyrighted works varies for different media (visual arts, performing arts, etc.), which is why your mention of a book and its sequels is irrelevant.

No, it's not irrelevant, it is 100% on point. Unless I have misunderstood you, you had suggested that copyright covered the exact image, without variation. With a book it isn't the size (hard back, trade edition, or mass market paperback) but rather the intellectual content which is copyright. The same applies to the arms; it is the intellectual content of the blazon which is copyright, not the size and shape of the shield, or the manner in which it is depicted on the letters patent.

Additionally, I missed a point you were trying making I think. Unlike filing a business with your Secretary of State which can be done and then ignored to some extent, in the application for a trademark, you have to actually prove it's use in commerce (and perhaps even interstate commerce).

Rather than go into a really looooooong description of tradmarking in the USA (which really is off topic), I would suggest that anyone who wants to do this first read the Wikipedia article on trademark (which is a very good overview), and take away the understanding that (1) you can file for your trademark before you go into commerce, and (2) that virtually anything can (and probably has been) trademarked. The basic requirement is that the trademark is intended be used in commerce; merely placing your arms on your calling card qualifies for a trademarking.