My 71 Mauser chambers and shoots .446 grease groove bullets with smokeless. Guess the German designers were looking to the future.

Lots of good suggestions. I see suggestions to turn the brass. I have turned hundreds if not thousands of necks over the years on modern cartridges, but they have something to actually turn. One would need a custom turned spud, etc which I have made  several spuds over the years, for my custom projects.
   Nor do I wish to have a custom reamer made either with a larger neck, not at the price of reamers, not too mention the wait time. 
   I am not seeing why with patience, and care to keep away from the throat, that the neck couldn't be lapped open a couple thou. When I opened the die, about thirty minutes with 400 wet or dry on a piece of drill rod accomplished the task.
    I think the best bet as this point, make a sizer myself, or as suggested, get a mold made, cut to specs for the lead mix. I had cast some softer bullets, sized to .379 shot the gun yesterday, would get a couple shots an inch apart, and the others barley on paper. A lever gun is not a match gun but a small jackrabbit would be pretty safe. Planning to trying some even softer mix, maybe a hotter load.
Waters suggested 20/WW680/245 with good results at 1450. I didn't have my chrono set up but he might have been shooting softer lead. I tried to match his suggested mix, but never been very good at that. He talks about the bullet diameter headache.
Opening the chamber likely is the best answer but I want to try to other things first.
Thanks to all the suggestions. Ron

Again the mystery to me, is why Marlin would not join the 20th century and continue with the bore size of .381, in their cowboy version? I haven't shot but three or 4 rounds of factory in this gun, but seems to me they would rattle down the bore big time.
I sure who ever bought them brings up the quality a bunch.
I have one of the cowboy 1894s, 38 special, and it shoots like a house afire. Not certain when they made it, but for sure before the blind monkey crews started there.
Ron

Thanks, the fact that the 38 55 predated Sammi by about 150 years, maybe Sammi  just copied the original? Still doesn't explain to my hard head, why not all the gunmakers were on board with the larger bore.
  Maybe it was based on cartridge popularity, the 38 55 being phased out, and tooling for a bore change too expensive? The bean counters get wrapped around the axle when it comes to pennies spent I guess.
   The heel bullet aspect is intriguing. I knew what they were, but never really put a study on them, or if I did, I had forgot.
   A quick skim this morning enlightened me a bit, but still having a hard time visualizing how the heck a smaller base seals an already large bore.
On the other hand, the bullet base  would only need maybe .0025 smaller to cure the  chambering issue. 
   A real soft mix so they will seal?
An aside, the original Marlin 1893, has a  short leade compared to the modern 93 cowboy.Along with case length varying by brand, I need to begin keeping better records

I want to thank all for their suggestions. This is a followup of what I accomplished to make this a shootable gun.
   Someone suggested a heel bullet. Accurate mold has made me several molds in the past, and I contacted Tom, I believe, and ran the notion of a heel bullet by him.
    I ordered a similar already standard 38 55, about 245 grains. But he cut the base to .375, and the nose to .380, to marry up with the throat on my modern 336 "cowboy", Marlin.
   I requested a softer mix, and we guessed really close, cause the bullet drops, with the base not being touched, and the nose just touching the .378 sizer die from RCBS, special order, to sort of match the bore on my rifle, .380
   After casting, the bullet is a thumb push fit into the fully sized case. The sizer puts lube into the groove, and just kisses the upper .380 bullet nose section.
   Had a little adventure, they wouldn't chamber using the crimp groove length. After some farm boy tricks, I realized the bullets were too long, for the throat/leade of this particular gun.
   I trimmed the cases, back to 2.075, and seated the bullets to match, and everything is kosher.
    With the fires, and record heat for northern NV, I hadn't been to the range in several weeks, un til this morning, to give it the acid test.
    I shoot unproven rifles, at 50 yards, as if a gun does not come close to cloverleafs at 50, they sure wont be bragging groups at 100, and a guy can save time and ammo by testing at 50.
   Some will disagree, but with a new unknown rifle, I will often zero at 25, which near guarantees hits at 100.
   Anyway, with a tang peep, and less than ideal bench, I was close to cloverleaf 3 shot groups at 50. I had reworked the OSHA approved trigger down to around two lbs, soon after I got the gun, and those outside the groups, I called out. My 65 year old eyes are the best excuse I have.
   I considered all the options suggested, but am happy as can be, that I simply got the special mold. 
    Accurate Mold makes a darn fine product.
Thanks again, Ron

Bernie Rowles at Old West made me two special sizing dies.
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Mick McPherson and his book has my 336C shooting 2" groups at 100 yards. For a lever gun, I would think the groups are satisfactory.

For those who want to fix the Marlin, be aware that Manson has a reamer that will fix the problem. The reamer is the result of McPherson and Manson. If you have a 38-55 Marlin that has been throated another way, the reamer will not solve the problem.