This is what a fbi armorer has to say about the "fbi" load.
I first saw the X38SPD ammo loaded in plain white boxes, nickle commercial Winchester case with red primer sealer, in 1974 at the FBI Academy at Quantico, VA. It was already being used by numerous local PDs, St. Louis, Chicago and Washington, DC among them. The bureau became interested when the FBI Laboratory started receiving requests to process and handle evidence from officer involved shootings around the country, where the ammunition was being used. They were impressed enough with it that samples were requested from Winchester for evaluation.
Once the bureau starting buying the ammunition, additional suppliers were needed to enable competitively bid contracts. Remington quickly jumped onto the bandwagon with its R38S12. Federal was late to the party and had problems getting the bullet cavity geometry, alloy hardness and lubrication right and has trouble meeting the functioning, accuracy and terminal performance requirements. Alot of Federal 38G was sold in normal commercial channels which failed contract acceptance and while some batches of Federal performed OK, it was not consistent from lot to lot. The Remington and Winchesterr stuff was always good in my testing.
At the time the FBI was moving away from the 4-inch Colt Official Police, maintaining those still being used by agents in the field offices, but issuing to new agent classes heavy barrel 3" or 4" S&W round butt Model 10s, (usually 10-6 or 10-8s) with Tyler T-grip adapter. Later they went to .357 Model 13s in similar configuration and held onto those well into the mid 1980s when they started transitioning new agent classes to 9mm autopistols.
It actually became known as the "fbi" load because the feds bought sooooooo much of it.
The fbi regularly checked their revolvers & their armorers would look for 3/1000th's to 6/1000th's (max) cylinder gap. They found that for every 1/1000th of cylinder gap it wound lower the velocity of the load by 10fps. The ww fbi loads would average 950fps from 4" bbl'd revolvers.
Most 158gr bullets and around 5.3gr of unique will give you 950fps from a 4" bbl'd revolver.
The fbi load wasn't extremely hot by any means, what it had was a superior bullet design. A hb bullet brings a lot to the table that most shooters wouldn't even know about, let alone test. A hb lead bullet does 3 main things:
Makes the bullet have a more consistent velocity when used in extremely large # of different firearms.
Makes the bullet longer
Makes a secondary mushroom.
What the bottom of the hb bullet looks like that is used in that 44spl load.
A side view of that same bullet. As you can see the hb/hp bullet mushrooms/compresses at both ends.
This is what the fbi saw when they processed evidence for other agencies with the ww hb/hpswc bullet along with impressive results. They decided to adopt that load/bullet for their standard issue ammo to their field agents. The rest is history.
If you really want to take your bullet testing to the next level you should take a hard look at hb bullets. I have over 15 hb bullet molds for 7 different calibers along with swaging my own lead & jacketed hb bullets. It gets interesting when you test a rh hp hb bullet vs a hbhpswc vs a fnhbhp bullet for the same caliber.
At the end of the day 950fps from a 38spl/158gr hp bullet is a proven performer. I like to use either large round hp's or penta point hp's for my 38spl/950fps snub nosed revolver loads.
The snub nosed 44spl is no different. It gets a steady diet of either:
A "fbi" hbhpswc bullet
A hbwc turned around to make a huge hp
A standard hpswc (200gr for 44spl/158gr for 38spl)
A hbhp fn bullet
Most people view the 38spl as an outdated/under powered cartridge along with the 44spl being a forgotten caliber.
Good luck with your testing