An unvowelled ra has no short vowel of its own. The immediately preceding sound therefore becomes the first consideration. Where the preceding consonant has fathah or dammah, the ra is heavy.
«بَرْقٌ» — ba before the still ra has fathah: heavy ra.
«قُرْآنٌ» — qaf before the still ra has dammah: heavy ra.
The stillness itself does not always create heaviness; the decisive immediate factor here is the earlier vowel. A preceding kasrah introduces additional detail, including the distinction between an original and a temporary vowel.