Back in 2007, I was chasing a Winchester Lee Navy rifle.  I came across an internet listing of both the Lee Navy and a High Wall in 6mm Lee Navy that were listed by the seller as "used by Winchester for ammunition testing".  The seller said the bores in both rifles were "very dark".  Seller was asking $4,000 for each.  

Beyond my financial capability, so I quit looking.

There may well have been Oberndorf Mausers made in that caliber.  DWM had a catalog listing for the cartridges.

There were also Mexican Mausers and the smokeless powder Remington Rolling Blocks made up in that caliber, but probably as after-market rebarrel jobs.  Rifles in 6 mm Lee Navy are rather rare, but they do exist.

I have a High Wall that was customized to 6 mm Womack.

6 mm Womack is a .30-40 Krag necked down to 6 mm.

It has sharp shoulders like a Weatherby case, and never had many firings before they would split.
I rechambered it to 6 mm Remington.

Yup Rimless. I moved the Womack barrel over to  a Mauser 98 and converted the High Wall to .25 RKS with a new Ron Smith barrel.

I have a set of 6 mm Womack dies if anyone needs them.

Jerry Simmons had a lovely lovely little High Wall elegantly factory stocked in rosewood and factory chambered in one of the 6.5 Mannlicher cartridges----possibly the Dutch version if my memory is correct. 
The back story was that it was a factory custom sporting rifle made for a member of the family that had the London Win. distribution franchise.
I photographed it and have the images somewhere.   As I recall it had all the London marks. To me irt was an "Audry Hepburn" of a rifle.  Classy beyond classification.  Sadly it was also astronomically out of my price range when I asked him about it.   I wonder how it faired in his estate.

The Winchester-Lee barrels all had a very fast twist, 7" IIRC.  I played with one of the Lee Sporters for a bit.  It had conventional Winchester-type rifling.  I have read that the first production run of the Winchester Lee Navy Muskets had Metford rifling, but again with a quick twist.  Never seen one up close, so cannot verify.  I pursued a Lee Navy, thinking to try both modern jacketed bullets and modern cast bullets.  Prices for the Lee kept going up.  Then there was a story making the rounds about a guy who was killed when the striker blew out of the bolt.  I listened to enough stories to think there was some truth to the tale. End of search.

In any event, my adventures with cast bullets and quick twists have not been positive.  I have a 6.5 mm Dutch Mannlicher carbine with about an 8" twist.  It shoots pretty well with RN jacketed bullets.  Bullets cast out of Linotype or HS Babbitt at about 1600 fps will produce smaller-than-pie plate groups at 100 yards. Softer bullets lead like crazy.

I blundered into a reference to 6 m/m Lee Highwalls in a old May 1, 1920 number of Arms And The Man.  Someone should see if there is an online file.

If someone needs some long 220 swift brass to make 6mm Lee Navy brass with I have some, it's not finished trimmed and no flash hole in it, primer pocket is formed.  It probably was a lonch box requisition.
LD1