\bar below symbol

The accents package has the feature you're looking for:

\usepackage{accents}
\newcommand{\ubar}[1]{\underaccent{\bar}{#1}}

Why the \newcommand? Because you might change your mind or find a new way to underline a symbol, so you can just modify the definition instead of hunting through the document for occurrences of \underaccent{\bar}.

My solution is \newcommand{\ubar}[1]{\text{\b{$#1$}}} since I get a conflict of accents and amsmath packages concerning \vec command.

You can play with the three parameters (1.2pt underset gap, .8ex rule length, .075ex rule width

\documentclass{article}
\usepackage{stackengine}
\begin{document}
\newcommand\barbelow[1]{\stackunder[1.2pt]{$#1$}{\rule{.8ex}{.075ex}}}
\( \overline{A} \bar{A} \barbelow{A} \)
\end{document}