joshiwa220,
I made a necked down 375 Ruger without any other changes. The neck IS short but it works.
Talked to Dave Kiff about not moving shoulder. He said several guys did it when the 375 first came out and they had no issues with the short neck.
I wanted to do this with dies I had on hand and not buy custom dies till I knew it would shoot.
Besides having a hornady 338 RCM dieset I have a Whidden custom FL/bushing die and seater for the 338 RCM. The 375 shares the same shoulder angle and diameter as the RCM.
Here is how I did it.
Procedure for necking down 375 Ruger
1) Hornady 338 RCM FL die
2) 300 RUM FL die to taper mouth of case
3) Whidden 338 RCM bushing die with .329 bushing
4) Whidden 338 RCM bushing die with .312 bushing
5) Open up with K and N mandrel, neck turn to .013" neck wall thickness
6) Run brass into Whidden 338 RCM bushing die using .311 bushing
Ordered a 7mm/375 reamer with .317" neck
Two weeks ago I rechambered a 23 1/4" rem takeoff that was a 7 rem mag. Years ago I had that takeoff put on a Mark X and the barrel shank had to be cut off to rethread for the larger mauser shank, that is why it is so short.
Here are the details on the load development:
Seating 168 berger classic hybrid to max OAL to fit magazine of MARK X has a bullet jump of .054"
moly coated 168 Berger classic hybrid and H-1000:
78=3075 shoots very accurately
79=3090
80=3115
81=3135 was also quite accurate
82 3205 vel jump/over the top/some bolt lift resistance
moly coated Berger 180 target VLDs, RL-33.
81= 2988
82= 3010
83= 3040
84=3075 most accurate
85=3124
86=3140 slight bolt lift resistance
Load for Mark X: 84 gr of RL-33 with 180 VLD 3075 fps
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As I said didn't want to buy any custom dies till I decided it would shoot. I bought a 375 Ruger die set and will use the FL die for body bump. Bought a 7 RUM die set and machined a 7 RUM seating die shorter. The 7 RUM shoulder dia is .005" larger per side but both share a 30 degree shoulder angle. Setup seating die so brass would kiss shoulder area and got low runout and decent accuracy.
To reload once fired brass I'll use the 375 die for body bump, the 338 RCM Whidden FL/ bushing die for sizing neck. Pretty straight forward.
A buddy is selling me a #7 fluted SS lilja that is 30" currently a 7 STW with 200 rounds through it for $250. I'll cut off some shank, rechamber and put it on one of my LR rifles for some testing at longer distances. Have some 180 hybrids on the way. Figure I'll get 3100-3150 pretty easily from a 29" barrel with RL-33 and the 180s.
This has been lots of fun. I think this size case is a great size for the 7mm. I hope to wack a coues wt with this rig next season.