I don't own a chronograph (seems like there's always something else demanding my time, money, and attention), so I can't test this myself. However, I can't help but wonder if the velocity issues reported with Double Tap and others (it seems there are several 10mm loadings which don't live up to their advertised velocities) is due at least in part to pistols which are slightly undersprung by the manufacturers to ensure reliability with the watered-down FBI-spec 10mm ammo.
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The velocity issues with Double Tap ammo is mostly guys using chronographs that don't understand how to interpret the results. Double Tap tests their ammo in a standard factory G-20. I'm sure it gets the advertised velocity from THAT gun. Most revolver loads in real guns are WAY slower than advertised because they are tested in 8" test barrels with no cylinder gap. Shoot a load advertised at 1400 fps in a 4" revolver and you might not get 1200 fps.
The other problem is that no 2 guns are exactly alike. I've done most of my chronograph work with rifles, but the same things I've noted will apply to handguns as well.
Some guns just shoot faster even though they appear the same. I've seen 2 different 30-06 rifles with 130 fps difference with ammo from the same box even though both rifles had the same 22" barrels.
Altitude and temperature can make a difference. I've shot the same loads through the same rifle and seen 100 fps faster in August when temps are near 100 degrees than when fired in January when it is 20 degrees.
Double Tap advertises their 200 gr hardcast at 1300 fps. My G-20 got 1315 fps when I chronographed it. Someone else's 10mm might be 50 fps or more slower than mine with ammo from the same box. If I were to take my gun out and shoot it in sub-zero temps it may well be much slower than the 1315 I initially got.