I would like to set a block followed by two additional ones below the first block. However, the widths and margins differ (see screenshot below). I'd like

* the two narrower blocks to span across the same width as the wider block, and
* the margins to remain unchanged.

How can I obtain that? 

![enter image description here](https://i.stack.imgur.com/mrmUC.png)

```
\documentclass{beamer}
\usetheme{Boadilla}

\begin{document}

\begin{frame}

\begin{block}{Guguck}
    Hallo du da, im Radio!
\end{block}

\begin{columns}
\column{0.5\linewidth}
    \begin{block}{Guguck}
        Hallo du da, im Radio!
    \end{block}

\column{0.5\linewidth}
    \begin{block}{Guguck}
        Hallo du da, im Radio!
    \end{block}
\end{columns}

\end{frame}
\end{document}

```

I can adjust the factor `0.5` to get the desired result, but that is an annoying work to do and probable not independent in changes of margins, textwidth etc.

EDIT:
I used the option from samcarter, but it does not work properly. So I played around and find the option `\setbeamertemplate{blocks}[default]` which destroys the nice outlook. Here is the corresponding MWE. (I do not want to manipulate my first question.)

![enter image description here](https://i.stack.imgur.com/gMWzS.png)

```
\documentclass{beamer}
\usetheme{Boadilla}
\setbeamertemplate{blocks}[default]

\begin{document}

\begin{frame}

\begin{block}{Block 1: Guguck}
    Hallo du da, im Radio!
\end{block}

\begin{columns}[onlytextwidth, t]
\column{0.45\linewidth}
    \begin{block}{Block 2a: Guguck und eine laengere Ueberschrift}
        \begin{itemize}
            \item a
            \item b
            \item c
        \end{itemize}
    \end{block}

\column{0.45\linewidth}
    \begin{block}{Block 2b: Guguck}
        Hallo du da, im Radio!
    \end{block}
\end{columns}

\end{frame}
\end{document}

```