Michael Barr barr at math.mcgill.ca
Sun Dec 29 14:57:04 CET 2013

I think this looks much better:

\documentclass{article}
\pdfoutput1
\makeatletter
\def\overrightarrow#1{\vbox{\m at th\ialign{##\crcr%
{\scriptsize\rightarrowfill}\crcr\noalign{\kern1pt\nointerlineskip}%
$\hfil\displaystyle{#1}\hfil$\crcr}}}
\makeatother
\begin{document}
$\overrightarrow{ABC}$
\end{document}

arrowhead smaller.  Less obviously, I changed the \kern-1pt to \kern1pt.
Could that have been an error?  You can play with that \kern if you like.

Michael

--
Every gun that is made, every warship launched, every rocket fired
signifies, in the final sense, a theft from those who hunger and are not
fed, those who are cold and are not clothed. -Dwight D. Eisenhower