How many rounds have your fired through it? Each of my VP9s (I've had 4, which 1 was only unfired) started out with a little grittiness. After about 500 plus rounds, it smoothed out considerably.
First I've heard of one getting gritty after use, none of mine have done that. Trigger work suggestion is:
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I had Rick do the HK-VP9/40 Master Reduced Reset Action Package (HK-MRAP) along with Electroless Nickel Refinishing on Internal Steel Action Parts on my FDE, and it is very sweet.