Here are the ballistic gelatin results for DoubleTap 9mm and .40. Note the similarities in penetration and final expanded size of the bullet. This data certainly calls into question whether there was ever really a "need" for the .40 to be developed.
First measruement is penetration into the "four layers of demin and some cloth" FBI test and the last measurement is how much the recovered bullet had expanded to in inches.
115gr. Gold Dot JHP @ 1415fps - 12.00" / .70"
124gr. Gold Dot JHP @ 1310fps - 13.25" / .70"
147gr. Gold Dot JHP @ 1125fps - 14.00" / .66"
135gr. Nosler JHP @ 1375fps - 12.10" / .72"
155gr Gold Dot JHP @ 1275fps - 13.00" / .76"
165gr Gold Dot JHP @ 1200fps - 14.0" / .70"
180gr Gold Dot JHP @ 1100fps - 14.75" / .68"
200gr XTP @ 1050fps - 17.75" / .59"
The general "word on the street" is that the Double Tap ammo is quality stuff at a fair price.
I've liked what I have purhcased so far (he is the best source of "hot" 10mm hand-phaser ammo). Delivery is pretty quick too - if I order on Monday I generally have it before the end of the week.
Now, supposedly, Winchester has come out with a new second-generation Ranger-T bullet that also expands impressively with those wonderfuly sinister starfish "talons" extended (rather than curled back along the side of the bullet). Just don't call them "talons". That spooks the sheeple into stampede mode. Winchester calls them "petals" .