“Far better it is to dare mighty things, to win glorious triumphs, even though checkered by failure, than to take rank with those poor spirits who neither enjoy much nor suffer much, because they live in the gray twilight that knows neither victory nor defeat.”
― Theodore Roosevelt
Might as well add my measurements, in case anyone cares:
Magtech .357 158gr SJSP, Ruger Speed Six with 2 3/4" barrel, ambient temperature about 1°C. Muzzle velocities were about 350m/s (my notes are in my truck and I am not, but it was pretty closely thereabouts) fairly consistently. That's about 1150 fps at a temperature of about 34°F. This Magtech load has a bit more felt recoil than other similar loads from Fiocchi, PPU, S&B.
Magtech .38 Special +P 125gr SJSP, same gun, same conditions. Only one out of six shots registered on the chrono (I have to try that trick of leaving the shades off the next time), and that one was just a hair over 290m/s or 950 fps. Recoil felt very mild. Muzzle flash was hilarious - about twice that of the .357 round.
IDPA SSP classification: Sharpshooter
F.A.S.T. classification: Intermediate
I guess I may as well add some more data to compare and contrast different barrel lengths.
M19-3 4"
140gr xtp over 9gr unique
1168 fps
424 ft lbs
125gr Remington sjhp
1360 fps
509 ft lbs
M64-5 4"
158gr tc hardcast over 5.2gr unique
924 fps
299 ft lbs
130gr jsp over 6gr unique
938 fps
254 ft lbs
Interesting that the Remington sjhp shot faster in my 2.75" gun than the 4".
The first thing that comes to mind is the cylinder gap. I've been using that Remington 125gr SJHP as my carry load for over 30 years now, and the only time I had a 4" chronograph that slow was my original 1986, blue GP100. It had a cylinder gap of .10, and chronographed about the same velocity as you show. I set the barrel back and it ran over 1400 fps. Every other 4" has been well over that. My current long term average with that load, four different 4" guns over a period of 25 years, 5 or 6 rounds per session, is 1442 fps ave over 94 rounds.
M66-8 2.75"
158gr over 11.5gr 2400 in .38 brass
994 fps
347 ft lbs
M64-5 4"
158gr over 11.5gr 2400
1089 fps
416 ft lbs
The most recent 2400 data I found in my pile of loading manuals is in Sierra's 5th edition with a 2003 copyright. It shows 10.3 grains as max with a 158-grain JHP. You're not going to blow the gun up, but you are stressing it.
That load might have been used a lot back in the day, but those folks also thought 50 rounds was a big range session. Bullseye shooters went through a lot more ammo but it was lightly loaded.