Ok I'm having a senior brain fart moment.

My situation is I built two 380PDKs I had orginally planned on using 280 brass necked up to 375 like the first one I built 20 years ago.

As I was doing the first one for my buddy he decided he wanted to use 380Howell brass so we cut the neck long enough to handle the 2.6" brass. He then fireformed some brass and sent it to Hornady and had dies made. In the mean time I played with my 400 version. After thinking about it I elected to go with Howell basic brass since my shoulder is forward of the howell it saves a forming step. Since Hornady already had the specs and the same reamer cut both I had them send me a set of dies made per his brass.

After running some basic brass into the sizer the bullet falls into the case. The basic brass is a couple .001" thiner per side as I measure. The expander is the same dia as my 375H&H dies.

If I drive a basic case into and out of my 375H&H die it the brass will hold the bullet just fine. I don't know the exact neck Hornady cut for his brass maybe my thiner neck makes that much difference. I have no way of measuring it either. I have not tried my die with his brass. Which is a next step.

Isn't there a bushing neck sizer that would allow me to size my necks? I did a search at Midway and found bushing but no die. Someone point me in the right direction. I hate to toss a $hitload of brass or start over waiting for a new sizer. Using a ram to push the brass into my H&H die then knocking it back out with a punch works but is a pain in the a$$ as well.

I would definitely check your brass in his dies to be sure the problem is with your brass. 2 thou can be enough difference. One option is to send $50 and 2 fired cases and two bullets to LEE Precision and let them make you a custom collet die.

Maybe a Lee collet type neck sizing die would squeeze the too loose brass necks tighter against it's .375" internal mandrill.
Also this internal mandrill could be polished one thousands of a inch smaller if necessary.

Thanks for the advice. After another hour or two of looking I've decided the "Generic" neck sizer with bushing was faulty memory on my part. Found one of the 380Howell cases expanded the neck and ran it in my die. Works perfect. Measuring as close as I could it appears the neck on the 380Howell brass is .015 per side. The neck on a formed basic case only measured .011. So that is a total of .008" difference in OD of the case neck.

Appears that a factory 375H&H die which is designed for a normal .015 think neck has enough slop allowing for expander control to handle the thinner brass.

I will be curious to see how the thinner brass holds up. I bet they start with the same brass coin to make both. One stopping at 2.6" with the basic being drain out to over 2.7" Thus the thinner walls.

Since a H&H die works guess I'll try and locate one and cut it off to allow me to use a normal shell holder. Then just use that die to neck size and the FL to set the shoulder back if needed. Have never used a Lee Collet but that might work as well.

At least now I have a plan.

Thanks greenjoy your suggestion worked perfectly.

Got on line with Midway Sunday the 13th ordered a Lee collet die for a 375 it arrived last night. Today I shorten the enternals by .2" to adjust for my 2.65" case vs the 2.85 H&H. It works perfect. So for a little of my time and a $24 set of dies I got away not having to scrap a bunch of basic brass nor did I have to have new dies made. All is good.

Anyone need a Lee 375 seater?