TunnelRat said:
A foot low at 10 yds as a result of the sights would require a comically large front sight.
I don't think it would be comically tall , but it would higher than needed. But a too-low rear sight wouldn't be all that noticeable -- and that is a possibility. And, as TunnelRat notes, the sight picture IS important. See "Proper Sight Picture for VP9", below.
SIGHT ADJUSTMENTS
To compensate for a foot LOW point of impact at 10 yards, you'd need a SHORTER front sight (to raise the barrel end), or a taller rear sight (to lower the back end).
How much shorter front or taller rear? It depends on the sight radius (which the H&K website says is 6.38").
http://www.brownells.com/.aspx/lid=13093/GunTechdetail/Sight_Correction_Calculator
Using the tool above (click on the first image to open a working model)
you'd enter amount of error = 12"
sight radius = 6.38"
distance to target = 360" (30 yds = 36" x 10)
and the result would be a .213" lower front sight or .213" higher rear.
(Note: front sight moves in opposite direct of the desired point of impact, while rear sight moves in the direction of desired the point of impact.)
If it turns out to actually be the sight, H&K will probably send you a new, higher rear sight for free. (They may not have a front sight that is THAT MUCH lower.)
PROPER SIGHT PICTURE FOR THE VP9:
The VP9 is a service pistol, and when used, you should have the dots aligned to assure that front and rear sights are even,
with the TOP of the sights centered on the center of the desired point of impact, and not the dots. You should not be using something like the 6 O'clock hold (so you can see the entire target area, as it the case with some types of shooting.) You'll see this in the image, below.
Not all service pistols call for the same sight picture, although many seem to be the same: the devil is in the details. Glock, which use a white outline around the rear notch, calls for the front sight to be half a dot above the top of the rear sights. Some guns call for the dots to be aligned on the MIDDLE of the desired point of impact -- which would give a higher point of impact if used with the H&K VP9. If you're doing exactly as recommended by the Operator's Manual and still hitting low, you probably do need to get a higher rear sight.
If it turns out to be the sights, you may choose to change out sights on the VP9 so that all of your guns use the same sight pictures, if the VP9 uses one different from what you're used to...
From the manual V9 Operator's manual:
