While fooling with the 358156, loading it in 357 magnum brass and into 38 Special brass, it seemed to me that most online explanations on the 2 separate crimping grooves is a bit too vague to get the real picture.
In a nut shell, the upper crimp groove is used for 357 and the lower crimp groove is intended to be used for 38 Special.
357 Magnum: In 357 Magnum brass, if 358156 is seated and crimped in the top crimp groove, the overall length will be just less than the maximum cartridge length of 1.59". I trim my brass on the min side, and my LOAL is 1.57". Individual molds differ somewhat, but the LOAL will be 1.59" or a bit less in a trimmed case. IOW in a case that is no longer than 1.29" (Cartridge Max.).
38 Special: In 38 Special brass, if 358156 is seated and crimped in the bottom crimp groove, the overall length will be just less than the maximum cartridge length of 1.55". I trim my brass on the min side, and my LOAL is 1.53". Individual molds differ somewhat, but the LOAL will be 1.55" or a bit less in a trimmed case. IOW in a case that is no longer than 1.155" (Cartridge Max.).
It's all in the numbers:
- A 357 Mag case is 1.29 - 1.155 = 0.135" longer than a 38 Special case. I'm using SAAMI max #'s for each case.
- However a maximum length 357 cartridge is only 1.590 - 1.55 = 0.040" (40 thousandths of an inch) longer than a maximum length 38 special cartridge.
The two crimp grooves of a 358156 are about 0.135" - 0.040" = 0.095" (95 thousandths of an inch) apart.
The real conundrum is why Lyman, in all the reloading manuals I own, seat to the top crimp groove for both the 38 Special and the 357 Magnum.
And if you load a 358156 into a 38 Special brass, you can crimp it in either crimping groove, it will not be too short or too long. The SAAMI spec for the cartridge OAL is 1.275" min to 1.550" max. In either groove, it is still 38 Special ammunition. And if you choose, you could exceed 38 Special pressures, ignore the load data and overload it, in either crimp groove, and it may be safe to shoot in a 357 Magnum firearm, but would fit and be unsafe to shoot in a 38 Special firearm.
Seating a 358156 into a 38 Special case and crimping it in the bottom crimp groove does not make it 357 Magnum ammunition. It will still chamber in any 38 Special, and IMNSHO if it says 38 Special on the headstamp, it should be 38 Special ammunition. If you want to load to + P, it should be in a case marked + P. If you want 357 Magnum ammunition, use 357 Magnum brass.
But; I don't have any business preaching, I load ammo for my 357 Cowboy guns in 38 Long Colt brass, and it does exceed 38 LC pressures. So, I suppose that makes me a hypocrite.
If you load 358156 into 357 magnum brass you will exceed the max cartridge OAL if you crimp in the lower crimp groove. The SAAMI spec is 1.405" -> 1.590".
Note: Just to confuse the issue, 1.590" was the longest cartridge that would fit into the rather short cylinders used in the first 357 magnum guns. Most 357 magnum cylinders today will accommodate ammunition a bit longer than 1.590". A 358156 can be crimped into the bottom crimp groove in 357 mag brass and it will fit in a Ruger Blackhawk, but it stick out of the cylinder of a Ruger New Vaquero.