roblem is it consistently manages only about 10" of penetration in ballistics gell
http://www.brassfetcher.com/Speer ... Barrel.html. While that isn't bad (10" is my bare minimum), there are deeper penetrating rounds out there that meet or exceed the FBI's recommended 12" minimum. I have seen it posted quite a bit lately that the human skin is more elastic/resistant to bullet impact than other tissues. The rule of thumb I've seen is that just to penetrate the skin is equal to 2" of penetration in ballistics gell. If that's true, then the Speers have 8" to spare.
The brass fetcher reference is good, but I'd also point out that sometimes his block isn't fully within spec, he doesn't always get multiple shots, etc.
As far as process goes, I trust DocGKR to get more accurate results from his gel tests. He reports
.38 Sp Speer 135 gr +P JHP Gold Dot (53921), ave vel=856f/s
BG: pen=13.1”, RD=0.56”, RW=134.5gr
4 layer denim: pen=13.6”, RD=0.53”, RW=134.1gr
auto windshield: pen=9.4”, RD=0.51”, RW=129.6gr
I'd also point out that out of the 2" barrel guns the 158gr lswchp usually fails to expand through fabric. Lighter jhps that do expand usually underpenetrate. A soft shooting round like a 148gr wadcutter might be worth trying as well. They'll have sufficient pentation, be controllable and not bad to practice with in light weight guns. The sharp shoulder of the wadcutter will help with stopping your attacker too.