Bob,
The SAAMI drawing says a .380 Auto case may be 0.670" to .680" long within the standard, and 0.675" will be the trim-to length, based on that. So your long cases are actually the ones that are the ideal trimmed size.
What is most likely causing the buckling is you have the expander adjustment on the M-die screwed too far in. It is also possible the adjustment shifted. The expander plug screws into the adjuster screw with the knurled head that you turn, and if it gets loose, it will try to push too deeply into the case. Loctite will put a stop to that problem if you have it. But either way, whether you need to set the die up again or you have a loose expander plug, the first step is to unscrew the adjuster all the way and check out the plug.
The setup on the Lyman M-die is slightly different from other expanders. Back the die body out of the press a couple of turns. Run the ram of your press up (handle all the way down) with just your shell holder in place. Turn the die body down until it just kisses the shell holder. Unlike a resizing die, it is turned no further in. Set the lock ring to keep the die body in this position. Now start the adjustment screw into the die. Put your
longest resized case into the shell holder and run it all the way up. Turn the adjuster in until you just feel it touch the case mouth. Lower the ram and case.
At this point, the Lyman instructions have you nudge the adjuster in with partial turns until you feel the light resistance of the expander plug just starting into the case. You back the case out and turn the adjuster in another fraction and you just repeat those steps until you feel a significant increase in the resistance to the case going in. At this point you are at the first step. You then turn the adjuster in another 1/16 of an inch. Well, the adjuster threads are 18 TPI. If they had made them 16 TPI, you would just go in one more full turn. But with 18 TPI you need 1⅛ turns. But however you get there, that is all the expansion you are supposed to need.
For the .380 Auto SPECIFICALLY, I have a different adjusting method. Go back up one paragraph to where you just feel the expander plug kiss the case mouth as you turn it in. Then, lower the case. Put a pencil or Sharpie mark on the adjuster and turn it in exactly 4 full turns. With 18 TPI, 4 turns is 0.22 inches. Assuming your M-die profile is the same as mine, the first expanding part is 0.2" long and 0.22" will just get you onto the step a short distance. Now you can turn the adjuster in gradually to increase the depth of the step BUT DO NOT GO PAST 4½ TURNS (0.25 inches into the case mouth)!
That 4½ turns is a hard limit in the .380 Auto. Why? All pistol cartridges start getting thicker inside as you get close to the head. This is because the makers anticipate the case having to fire in a partially supported chamber, and the thicker brass prevents a blowout in the unsupported area. When I went through a bag of about 300 miscellaneous .380 cases, I used a pin gauge to determine where they start to thicken, and in all of them, it was just barely past ¼ inch into the case mouth. You do not want to let your expander go any deeper or it will start to expand the thicker part of the case. This will create a bulged ring around the case that prevents chambering.
Other case calibers start thickening at different places. .45 Auto typically starts at about 0.340" to 0.350" inches into the case. Shorter cases start sooner. This thickening also determines the maximum seating depth a flat base bullet can be inserted to.
So, what do you do if you follow the above instructions and still don't have enough expansion? First, chamfer your case mouths to see if that solves the problem. Then try calling Lyman. They may have slightly oversize expander plugs available. If not, you can always go for an extra operation by getting one of the inexpensive Lee Universal expander dies. This can be adjusted to add a flare to any case mouth if you need it. You could make the step first, then add the flare, or vice versa. You still want the step to keep bullets straight, but a small additional flare at the mouth will help them seat. This is no issue in rifle because there is no thickening to encounter, It should also be true for revolver cartridges, but they may the expander plug noses longer for them, so keep that in mind, and proceed slowly and carefully.
My own approach, if I were having an issue getting enough expansion would be to chuck the Lyman expander nose in my lathe and use a toolpost grinder to shorten it maybe 0.025" or so to let the flaring portion get deeper into the case.
