This one is an HX, ser # 339xxx
The bullet is an as cast lead .380-.3805
Cases current mfr WW
A dummy of one of these went .398OD and is a tight no go into the throat. Another at .396OD was just barely able to be chambered but would take some force to get the extractor to get it out. Once fired it popped right out. I think a .395 MAX OD would be about the limit for ease of chambering/extracting. FWIW, keep your bullet/brass combination at that , or less, and you may do well.
If I had any Starline I would give that a go, but sadly, no.
The throating reamer of .400 would solve this for sure, as does neck turning, or using fire-formed 30-30 (shorter) brass, though I was hoping the '07 chamber would take the combination.
Im torn between buying, or making, an outside neck turner, or renting the reamer and do both; I think the cost would be about the same. I am very particular about the bore spud fit when neck reaming to keep as concentric a chamber as possible (the neck/throat is the critical area). Wobbly reamers just make me shudder (and them too).
update: I thinned a WW case neck thickness from .010 to .007/.008, with my .380s it chambers up only 'snuggish' with an OD of .395, and will extract with only a tiny bit of firmness. Fired case pops out nicely. With this combination I dont resize, just decap, recap, charge (full case to bullet base) of Trail Boss, and seat in the bullet with the seater die. It has enough neck tension to not be in danger of falling, or being pulled out and the very slight bell I roll in with a small pair of needlenose pliers if need be (to ease the bullet base in) is flattened to the bullet body only, no real crimp.
So far I dont have a need to try .381s as rifle #1 has been accurate enough for some friendly 'schuetzen' competition, silhouettes or plinkin'/playin' or hunting (and the mould drops these at .380 of WWts and a dash of tin. Thus far rifle #2 is looking to perform as well as #1.