Not too sure whats happened here but I just master worked this hand cannon I just got and it didn't change the stat, this a known bug?
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This isn't a bug, you have a perk that subtracts reload speed on an already low reload stat gun- to the point that it is either negative or close to negative. To avoid this the game makes it so that if your reload stat is anything below 10 it is bumped up to 10. Your reload masterwork adds +9 reload speed, but you had 1 or less reload speed so it is still 10 or less- thus it is still displayed at the same level with or without the masterwork.