Not sure if this is a specific issue to the "Speedy Heavy Marksman Rifle" as opposed to a handmodded one, but that version still only has a mag capacity of 45, identical to the unmodded Heavy Marksman Rifle. 

Update: The error isn't quite what I thought it was initially. So here is a more detailed explanation of the glitch. While having the speedy version equipped, the regular version shows 45 as it's capacity in inventory. In order for it to show correctly, I need to to equip the regular version. So it seems what is happening is that it's applying the capacity of the currently equipped version to all versions in inventory (at least visually).