While oversimplified, bare gelatin gives information about best case performance, while 4 layer denim provides data on worst case performance--in reality, the actual performance may be somewhere in between. The four layer denim test is NOT designed to simulate any type of clothing--it is simply an engineering test to assess the ability of a projectile to resist plugging and robustly expand. FWIW, one of the senior engineers at a very respected handgun ammunition manufacturer recently commented that bullets that do well in 4 layer denim testing have invariably worked well in actual officer involved shooting incidents.
With few exceptions, the vast majority of .38 Sp JHP's fail to expand when fired from 2" barrels in the 4 layer denim test. Many of the lighter JHP's demonstrate overexpansion and insufficient penetration in bare gel testing. Also, the harsher recoil of the +P loads in lightweight J-frames tends to minimize practice efforts and decrease accuracy for many officers. The 158 gr +P LSWCHP offers adequate penetration, however in a 2" revolver the 158gr +P LSWCHP does not reliably expand. If it fails to expand, it will produce less wound trauma than a WC. Target wadcutters offer good penetration, cut tissue efficiently, and have relatively mild recoil. With wadcutters harder alloys and sharper leading edges are the way to go. Wadcutters perform exactly the same in both bare and 4 layer denim covered gel when fired from a 2" J-frame.
Currently, the Speer Gold Dot 135 gr +P JHP, Winchester 130 gr bonded +P JHP (RA38B), and Barnes 110 gr XPB all copper JHP (for ex. in the Corbon DPX loading) offer the most reliable expansion we have seen from a .38 sp 2" BUG; Hornady 110 gr standard pressure and +P Critical Defense loads also offer good performance out of 2" barrel revolvers.
