Chapter mit einfachen Zeilenabstand

\documentclass[
		a4paper,
		12pt,
		twoside,
		BCOR12mm,
		DIV15,
		headinclude,
		listof=totoc,
		bibliography=totoc,
		captions=tableheading,
		english,
		pdftex
]{scrreprt}
 
\usepackage[american,english]{babel}
 
\usepackage{xcolor}
 
\usepackage{setspace}
\onehalfspacing
 
\usepackage[latin1]{inputenc}
 
\colorlet{chapter}{black!75}
\addtokomafont{chapter}{\color{chapter}}
 
\usepackage{lmodern}
\makeatletter% siehe De-TeX-FAQ
\renewcommand*{\chapterformat}{%
\begingroup% damit \unitlength-Änderung lokal bleibt
\setlength{\unitlength}{1mm}%
\begin{picture}(20,40)(0,5)%
\setlength{\fboxsep}{0pt}%
\put(23,15){\line(1,0){\dimexpr
\textwidth-20\unitlength\relax\@gobble}}%
\put(0,0){\makebox(20,20)[r]{%
\fontsize{28\unitlength}{28\unitlength}\selectfont\thechapter%
\kern-.04em% Ziffer in der Zeichenzelle nach rechts schieben
}}%
\put(20,15){\makebox(\dimexpr
\textwidth-20\unitlength\relax\@gobble,\ht\strutbox\@gobble)[l]{%
\ \normalsize\color{black}\chapapp~\thechapter\autodot
}}%
\end{picture} % <-- Leerzeichen ist hier beabsichtigt!
\endgroup%
}
 
\begin{document}
 
\chapter{Just for testing a long caption text to see if singlespacing is working}
 
\chapter{Just for testing a long caption text to see if singlespacing is working}
 
\end{document}