Have you flushed your permalinks?
If not, visit your permalinks page in your dashboard, then, refresh the page you are trying to view.
Let us know if that helps.
Edit:
Upon re-reading your question, actually, no – just because you create a single-post_type.php file does not mean WordPress will use that file with regards to your front-page.php template file.
Your front-page.php file is what is rendering your post. This is your issue.
If you want your single-post_type.php file to render the display of your post type within front-page.php then you can do the following;
<?php
$portfolio_query = new WP_Query(array(
'post_type' => 'headerhome',
'showposts' => 1
));
?>
<?php while ($portfolio_query->have_posts()) : $portfolio_query->the_post(); ?>
<?php get_template_part('single-post_type.php'); ?>
<?php endwhile; wp_reset_postdata();?>
But obviously make sure that single-post_type.php does not contain a loop/query within itself for if it does then you should remove the outer while/endwhile statements contained within front-page.php