Generally, the FBI doesnt know which way is up with their ammo test protocols and limits.
Which is why we all worry about denim tests and min. penetration. The FBI 38 158 LSWCHP is one of the few times they got it right. Otherwise they missed the boat with their 9mm 147 gr and the 38 +P+ 147 grain.
I can understand your concern with penetrating
heavy clothes since you live in MN. However,
the FBI load was issued in Chicago and was extensively used by the Canadian Mounties, so it has been doing its job in conditions similar to your area. Some cops called them "38 splats" because of the splat noise the bullet made on impact.
Also, I did some "clothes" tests just last weekend. I balled up a bunch of t-shirts, socks, etc into a ball and taped them up.
There was at least 20 layers of heavy fabric.
A target 38 special at 625fps penetrated over 15 layers and the 185gr 45 autos- not known for great penetration- blew through the whole pile of clothes. Clothes dont impede bullet penetration much. The only factor with them is generally that the clothes plug up the hollowpoint. The 158 gr LSWCHP FBI 38 shouldnt have that much trouble getting plugged up.
Just so you know the FBI load is cataloged by Remington as R38S12.
Here is a source for it:
http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=548921
My 4" Model 28 Highway Patrolman rests beside me stuffed with Remington FBI 38s in it.
I trust them completely.