I have read that with a RLDBB rifle the strength of the recoil spring is more important and wears out faster than in a gas op rifle. I also know a weak spring can lead to a dimpled receiver. So what round count is recommended for spring replacement ? Early signs of a weak spring ? Is there a way to measure the weight, like putting a scale on the charging handle and pulling pack to the rear ? -Thanks

Should be replaced at 30,000 rounds per this article:

https://www.gun-tests.com/special_reports/long_guns/Troubleshooting-HK-MP5-9mm-machinegun-12312-1.html#.WV2Bz4jyuUk

Wow, that seems high for the weapon itself let alone the recoil spring. Oddly according to the article, the bolt and recoil spring both get replaced at 30,000 rds.

30K? That is 15K dollars if you are talking 308..  My HK was made in 1966 and I've never changed anything on it....I keep spares but not sure I'd worry about it too much.

Nope. It's gonna be a part that lasts a long time. Maybe once every 10,000 rounds if it makes you feel better.