This negatively affects modders trying to create accurate guns. 600 vs 700 rpm or 800 and 900 might not sound like a huge difference but the difference in feeling, handling and DPS is significant.
More details already reported: https://feedback.bistudio.com/T148987
It's a fairly significant issue that is also new to 1.00 or even more recent versions of the game.
Try setting the firerate of your modded weapon to anything between 300, 600 or 900 and it will round up or down to either of these values.
Thank you for posting this issue again, hope this gets sorted out soon.
This limits gun variety by making them all fire at same RPMs and negatively affects balance (guns firing slower than expected).
Maybe I forgot to clarify that this applies to all weapons, including every single vanilla weapon.