My most recent purchase was a 380 Auto of the Walther PPK design.
I decided to see if I had some cast bullets that would work in the 380. Understanding the bullet weights for the 380 run from 85 grains to a maximum of a 110 grain bullet by Nosler. Most manufacturers top out at 100 to 105 grains.
I purchased some 95 grain FMJ ammo made by S & B. I shot very well and the first twelve rounds I fired at fifty feet were all in a 3" group with a six shot cluster measuring only 1".
My search of my cast bullets came up with only two possibilities. One 111 grain wad cutter and one 115 grain SWC. Both are a bit heavier than the typical 380 bullet but are not too long or so I thought.
Now the trouble starts. I found with new Winchester brass and the once fired S & B brass the cast bullets could not be seated any deeper into the shell case than .200" without entering into the part of the brass where the case wall starts to thicken. I can actually feel the additional force required to seat the bullet past the .200" depth. Seating beyond the .200" depth causes a bulge ring at the base of the bullet and the pulled bullet can measure .002" to .005" smaller at the base than it started out at. Bullets with .355" to .357" diameter all do the same thing.
This undersizing of the bullets base causes poor accuracy and leading of the bore. This problem is present in the 9mm, 38 Super, 40mm and even the 45 ACP if lead or cast bullets are long enough or otherwise require being seated deep enough to control over all length that the base of the bullet starts into the area where the brass starts to thicken. I shoot all of the mentioned calibers and have run into the undersized bullet base in all of them. The 9mm was more of a problem than the rest, but it can happen in all of them.
If you have any doubt about what I am saying and want to see for yourself, seat a lead or cast bullet .250" or more into a 380 Auto case, then pull the bullet. You will be shocked by what you see.
Bullets with a BHN of 14.0 still were a problem. I am guessing a BHN of 20.0 or more may resist the undersizing but will still bulge the shell case. I don't have any cast bullets that hard.
I have ordered some 95 grain FMJ bullets and hard cast bullets. These are short enough and will not require the bullets to be seated deeper than .200".
If any of you are loading the 380 Auto with cast bullets and I see a lot of you are using the Lee 105 grain bullet, I suggest you pull a couple of bullets to see what the base looks like. If it shows a pronounced taper it was undersized in the thicker part of the brass. If you have had less than good accuracy and or leading, this could be the cause.