It's important to specify what you define as the 158 gr 'FBI" load.
The FBI started using the X38SPD load in the mid 1970s. This was a .38 +P 158 gr LSWC-HP. Winchester still makes it and publishes a velocity of 890 fps in a 4" barrel.
Federal produced it's '38G load shortly after ward, and went through several iterations before they got it to work right, and it still didn't work right in revolvers that did not have a gas cylinder shield or ring as the lead and lube they used led to binding of the cylinder. Ruger incorporated a gas cylinder ring on law enforcement contract revolvers that were ordered in .38 Special. It required milling on the underside of the forcing cone for clearance and wasn't an option on the .357 Magnum as, like the Model 19, it made the forcing cone more prone to cracks with .357 Mag loads. In any event that load was discontinued in the late 1990s but seems to have resurfaced and Federal currently quotes 900 fps in a 4" barrel. I have no idea if they fixed there lead issues.
Remington also had the R38S12 load. I used this load for several years and it gave me around 825 fps in a 1 7/8" Model 36, about 860 fps in my 2 1/2" Model 66 and about 900 fps in my 3" Model 13. A few years go Remington "discontinued" it, which in Remington speak meant they renamed it as the RTP38S12 load in their "High Terminal Performance" series. The only change I noted was the price.
The Winchester and Remington loads perform well in ballistic gel tests in a 2" barrel, but there's no free lunch. Reliable expansion at 2" velocities requires a soft alloy which at +P pressures will start leaving significant lead fouling in the bore after 2-3 cylinders which starts impairing accuracy past that point.
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The other day I stumbled on a 300 round box of Hornady 158 gr LSCW-HPs at a really good price and figured I'd work on an FBI'esque low recoil load for my K frame short barrel .357s, but one that would give 4" velocities in a 2.5" barrel.
5.0 gr of Unique (.2 gr over the limit for standard pressure .38) gave an average velocity of 778 fps in a 2.5" Model 66. 5.5 grains gave me 889.5 fps, and 6.0 grains gave me an average of 956 fps, all in a 2.5" barrel.
The disclaimer here is that these are .357 Magnum loads that are well under 158 gr lead bullet maximums in the old Hornady 3rd edition loading manual (the last one I have that has loads for Unique), but are all over the maximum for standard pressure .38 and I'm pretty sure the last two exceed .38 +P pressures.
The first load is I suspect very close to the modern .38 +P "FBI" load velocity based on my experience with the Remington load. The second does a good job of producing 4" "FBI load" velocities in a 2.5" barrel, but fouling was noticeable after 12 shots. Not hard to remove, but definitely there.
In the end though I decided to just stick with my current 125 gr XTP load that produces 1250 fps in a 2 1/2" barrel with excellent accuracy and no concerns for lead fouling.
I might however revisit it for my 2.125" and 3" Model 60s. The recoil was pleasant by .357 standards and it would I suspect be effective. I may try it in some ballistic gel if it stops raining.
