Dudedog
Contributing Member
OK Easy Way
Get the chrono
Shoot some of the 38 +P over it to get the vel from your gun
Load the .357 case at the starting .357 charge with powder X, vel will most likely will be higher that the .38 +P
If it is work down .2 at a time to bracket it, if its low work up. Fine tune to .1 if it suits you fancy.
You now have the charge you need to duplicate the velocity of the .38 +P load in a .357 case.
Powder forward/powder back may come in to play with the larger case.
Another way would be to ignore the vel, since you might be after feel only and load for feel,
either way a simple ladder is the solution.
POI/POA matter but at most distances SD happens with a pistol the difference 100-200fps makes won't matter
I like to load some practice ammo that is close to my SD ammo,
However I would say that more trigger time with softer ammo that is more pleasant to shoot lots of might be of more benefit than than ammo that is a very close match that may not be as pleasant to shoot a lot of....shoot lots!
Trigger time is more important, IMWO
When practicing calm I can usually feel about 30 fps in a 9mm round,
Running around in the middle of a match with my heart rate up and my brain (hopefully) on high speed makes a difference, hardly noticed when I went to a different
batch of ammo in the middle of a stage that was about 100fps hotter. (messed up and grabbed wrong ammo can on the way to the match......)
Might not have even noticed if it wasn't for the fact I knew that mag had different ammo.
Alliant does not give a spread like Hodgdon does but for fun
Hodgdon data
Bullet 158 XTP
Powder Universal Clays
.38 SP start 4.0 678 max 4.4 778 (no +P data)
.357 start 5.8 1026 max 6.3 1133
So just for fun lets say the vel increase/decrease per .1 is linear (which it isn't but for fun)
So lets call it about 21 FPS per .1 in a 357 case (using the start of 5.8 1026 and the max of 6.3 1133 .5gr =107fps) extrapolating down gives us
4.4 774
4.5 795
4.6 816
4.7 837
4.8 858
4.9 879
5 900
5.1 921
5.2 942
5.3 963
5.4 984
5.5 963
5.6 984
5.7 1005
5.8 1026
5.9 1047
6 1068
6.1 1089
6.2 1110
6.3 1133
So guesstimating 4.4 in the .357 case is a bit lower than 4.4 in the .38 case 774 vs 778
When I am retired and have time it might be fun to load the ladder and see how far off from a straight line the real data points would fall.
And yes I know extrapolation in this case is prone to error and not really valid until proven, but it was just for fun, but since we are going down pressure wise I would feel halfway comfortable with the numbers, going up from listed data would be a different story.
The .4gr in the .38 case gets use 100fps or 25fps per .1 (linear assumption)
so about 10% change more in the smaller case.
I am not saying any of this is valid or safe data other than the charges and charge ranges Hodgdon listed, just playing with numbers.
Get the chrono
Shoot some of the 38 +P over it to get the vel from your gun
Load the .357 case at the starting .357 charge with powder X, vel will most likely will be higher that the .38 +P
If it is work down .2 at a time to bracket it, if its low work up. Fine tune to .1 if it suits you fancy.
You now have the charge you need to duplicate the velocity of the .38 +P load in a .357 case.
Powder forward/powder back may come in to play with the larger case.
Another way would be to ignore the vel, since you might be after feel only and load for feel,
either way a simple ladder is the solution.
POI/POA matter but at most distances SD happens with a pistol the difference 100-200fps makes won't matter
I like to load some practice ammo that is close to my SD ammo,
However I would say that more trigger time with softer ammo that is more pleasant to shoot lots of might be of more benefit than than ammo that is a very close match that may not be as pleasant to shoot a lot of....shoot lots!
Trigger time is more important, IMWO
When practicing calm I can usually feel about 30 fps in a 9mm round,
Running around in the middle of a match with my heart rate up and my brain (hopefully) on high speed makes a difference, hardly noticed when I went to a different
batch of ammo in the middle of a stage that was about 100fps hotter. (messed up and grabbed wrong ammo can on the way to the match......)
Might not have even noticed if it wasn't for the fact I knew that mag had different ammo.
Alliant does not give a spread like Hodgdon does but for fun
Hodgdon data
Bullet 158 XTP
Powder Universal Clays
.38 SP start 4.0 678 max 4.4 778 (no +P data)
.357 start 5.8 1026 max 6.3 1133
So just for fun lets say the vel increase/decrease per .1 is linear (which it isn't but for fun)
So lets call it about 21 FPS per .1 in a 357 case (using the start of 5.8 1026 and the max of 6.3 1133 .5gr =107fps) extrapolating down gives us
4.4 774
4.5 795
4.6 816
4.7 837
4.8 858
4.9 879
5 900
5.1 921
5.2 942
5.3 963
5.4 984
5.5 963
5.6 984
5.7 1005
5.8 1026
5.9 1047
6 1068
6.1 1089
6.2 1110
6.3 1133
So guesstimating 4.4 in the .357 case is a bit lower than 4.4 in the .38 case 774 vs 778
When I am retired and have time it might be fun to load the ladder and see how far off from a straight line the real data points would fall.
And yes I know extrapolation in this case is prone to error and not really valid until proven, but it was just for fun, but since we are going down pressure wise I would feel halfway comfortable with the numbers, going up from listed data would be a different story.
The .4gr in the .38 case gets use 100fps or 25fps per .1 (linear assumption)
so about 10% change more in the smaller case.
I am not saying any of this is valid or safe data other than the charges and charge ranges Hodgdon listed, just playing with numbers.
Last edited: