I wanted to duplicate 38 Special velocities in 357 Magnum cases, primarily to get the noise and recoil down in my 357's without having to deal with a lead ring in the cylinder. I shot a case of +P 38 Special in a M66 and that left a lead ring that prevented 357 cases from ejecting. Removing the lead ring in each cartridge hole was time consuming and frustrating.
My standard load is a 158 gr LSWC or LRN with 3.5 grains Bullseye in a 38 Special case. This is a standard pressure load, very accurate in every 38 Special it has been used in. I suppose 3.5 grains Titegroup would provide the same velocity, the two powders are very close in terms of charges, but I have been using Bullseye for decades and am very familiar with the powder. Bullseye is outstanding in light to medium loads in 38 Special cases, it was a staple for the PPC crowd. For a powder that has been on the market since the late 1890's/early 1900's, Bullseye is still doing great. I think it is either the first smokeless pistol powder, or the oldest pistol powder still around.
After testing, I decided that in a 357 case, 4.0 grs gave the best overall accuracy, and was just a little faster than my standard 38 Special load.
Code:
Colt Trooper MKIII 6" Barrel
158 LSWC Factory Zero 38 Special ammunition
13-Sep-18 T = 72 ° F
Ave Vel = 739
Std Dev = 38
ES = 152.4
High= 804.1
Low = 651.6
Number rounds = 18
158 LSWC Factory Hornady 38 Special ammunition, box dated 12-1982
13-Sep-18 T = 90 ° F
Ave Vel = 734
Std Dev = 16
ES = 50.56
High = 755.8
Low = 705.3
N = 16
158 gr LRN 3.5 grs Bullseye 38 SPL cases WSP
13-Sep-18 T = 72 ° F
Ave Vel =764
Std Dev =24
ES = 87.45
High = 804.1
Low = 716.6
N = 21
158 gr Valiant LRN 3.8 grs Bullseye lot 907 6/20/05
once fired mixed 357 cases, WSP
19-Sep-18 T = 80 ° F
Ave Vel = 788
Std Dev = 38
ES =147.4
High =830.3
Low =682.8
N = 25
accurate no leading
Code:
158 gr Valiant LRN 4.0 grs Bullseye lot 907 6/20/05
once fired mixed 357 cases, WSP
19-Sep-18 T = 80 ° F
Ave Vel = 800
Std Dev = 26
ES = 102
High = 846.2
Low = 744.2
N = 24
very accurate, no leading
158 gr Valiant LRN 4.0 grs Bullseye lot 907 6/20/05
357 Midway cases, WSP
11-Oct-18 T = 71 ° F
Ave Vel =817
Std Dev =23
ES = 72.55
High = 853.6
Low = 781.1
N = 12
Based on this, if you want to fire 148 LSWC's in 357 Magnum cases, I am going to claim that all you need to do it bump up the classic 148 LSWC load of 2.7 grains Bullseye in a 38 Special case by one half grain. So a load of 3.2 grs Bullseye with a 148 LSWC in a 357 case should give about 700 fps.
Colt Python Stainless Steel, 6" Barrel
148 gr LBBWC 2.7 grs Bullseye 38 SPL cases CCI500
24 Sept 1999 T = 78 ° F
Ave Vel =710
Std Dev = 18
ES = 72.02
High =746.1
Low = 674
N = 32
shot fine at 25 yards, no leading, light recoil