How to make \rightarrowtail and \twoheadrightarrow longer?

Question

I am typing out short exact sequences and want to use \rightarrowtail and \twoheadrightarrow, but they have symbols above them so they need to be a lot longer, like \longrightarrow. Yet there does not seem to be quick and easy way to do this. How can I make these kinds of arrows longer? Here is what I currently have, with the output below.

\usepackage{amsmath}
\usepackage{amssymb}
\begin{document}
$$K\overset{\alpha}{\rightarrowtail}M\overset{\beta}{\twoheadrightarrow}N$$
\end{document}

Answer 1

Since you added the tag commutative diagram, I think you could use tikz-cd package.

Off-topic, see Why is \[ ... \] preferable to $$ ... $$?

\documentclass{article}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{tikz-cd}
\begin{document}
[
\begin{tikzcd}
K\ar[r, rightarrowtail, "\alpha"] & M\ar[r, twoheadrightarrow, "\beta"] & N
\end{tikzcd}
]
\end{document}