I shot a case of +P Zero 38 Special loads with a 158 LSWC Hollow point bullet through my 357 M66. All those soft lead bullets left lead rings in the chambers that were very hard to remove. I had to remove them though, because a 357 magnum case would form around the lead ring, and be locked in,making extraction very difficult.
I decided to develop 38 Special equivalent velocities using 357 cases with a 158. My standard 158 38 Special load is 3.5 grains Bullseye and that chronographs just at 760 fps in a four inch barrel.
At least with Bullseye pistol powder, about a half grain more powder gave me the 38 Special velocities I was looking for. I was surprised to see AA#5 shot very well, too bad I am using up the one keg I have, and don't have any plans to buy more.
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
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
158 gr Valiant LRN 4.5 grs Unique lot 364 3/9/1992 Midway 357 cases WSP
13-Sep-18 T = 72 ° F
Ave Vel = 815
Std Dev = 26
ES = 106.7
High = 853.5
Low = 746.8
N = 18
Low recoil, about 2" above point aim, accurate, low noise
158 gr Valiant LRN 5.0 grs Unique lot 364 3/9/1992 R-P 357 cases WSP
13-Sep-18 T = 72 ° F
Ave Vel = 852
Std Dev = 22
ES = 85.04
High = 891.4
Low = 806.4
N = 24
increased recoil, about 2" above point aim, accurate, louder
158 gr Valiant LRN 5.5grs Unique lot 364 3/9/1992 R-P 357 cases WSP
13-Sep-18 T = 72 ° F
Ave Vel = 919
Std Dev = 32
ES = 117.2
High = 972.1
Low = 854.9
N = 24
shot close to point of aim, much louder, more recoil, some leading
158 gr Valiant LRN 12.0 grs AA#9 R-P 357 cases CCI500
13-Sep-18 T = 90 ° F
Ave Vel = 1129
Std Dev = 42
ES = 156.2
High = 1230
Low = 1074
N = 22
loud and accurate
158 gr JHP (W/W) 13.5 grs AA#9 R-P nickled 357 cases WSP
13-Sep-18 T = 90 ° F
Ave Vel = 1127
Std Dev = 61
ES = 187.2
High = 1225
Low = 1037
N = 11