Hi guys. I have a wthby MK V action currently chambered for 6.5-06 improved. I am thinking of selling the barrel and redding dies and going back to a magnum chambering.

First,...it was a 257 wthby mag, can the bolt face take the 30-378?

Second,.....I looked at the report on the 30-378 and it showed moderate erosion after only 125rnds. If I use a hart barrel, in say, contour 8, how many rounds will I get before groups start to grow?

Third,...with almost the same balistics, does the 300 ultra mag erode as fast, or would this be a better choice? If so, what would have to be done to get the original bolt face to grab the 300 UM?

thanks guys.

378 cases require a larger bolt face than the 257 Wby. Erosion will be pretty much identical between the two. If the bullet weight and speed are the same, you can figure the erosion factor will be pretty close. the ultra mag has a slightly recessed rim, so it will fit in the 257 Wby bolt face. - Dan

THANKS DAN. Now, what if any gunsmithing will be necessary to the bolt face or will it provide a tight fit? Will even several thousandths have to be taken or added anywhere? or will the UM just fit perfectly?

Or should I chamber for 300 wthby mag? will the bolt face accomodate that caliber?

About that bolt face change. Didn't it have to be modified to convert from a 257 WM to an 6.5-06 to begin with? 06 case has a smaller rim diameter than the standard Weatherby mags - and - apparently the 378 wm case has a bigger rim diameter than the other Weatherby cases.

If the bolt works with a 257 mag rim it will work fine with the Ultra. The problem will be getting the fatter case to fit in the magazine and to feed.

The 300 Weatherby Mag would be a whole lot easier to make work.

Just my two cents worth...Joe.

JustC,

A big advantage of the Wby Mark V (or disadvantage, depending on your point of view)is that the action rails are not involved in feeding.

So if you have the bolt face opened and buy the magazine box and follower for a 378 based Wby caliber you will be almost there. The other thing that needs to be done is that the groove under the bolt has to be lenghtened as this forms the bolt stop for the Mark V. 378 based calibers also have a notch on the right hand side of the front receiver ring, which allows the longer 378 based calibers to eject loaded rounds.

I don't know if the 300 Ultra would feed from the 257 magazine system. Biut to do Ultra you will need to lengthen the boly groove and remove spacer from magazine.

As to barrel life, I think with full powered loads the 30/378 would have a much shorter barrel life than the 300 Ultra. Barrel life dramatically reduces when you increase case capacity for bore size when you are at this level.

On the other hand, a 30/378 loaded back to 300 Ultra ballistics might have longer barrel life than the 300 Ultra and of course a longer case life.

Think of it this way. Everyone on the forums gets all excited about loading a 416 Rigby back to 416 Remington and low pressure etc. etc. Loading a 30/378 back to full pressure 300 Wby ballistis is the same sort of deal.

Mike

JustC, if the bolt face is the original 257 Wby bolt face, it will work just fine with the RUM. It will not be big enough for the 30-378, and would have to be opened up. As mentioned, the groove in the bolt will have to be mdified to accomodate the longer cartridge (either one). The magazine box can just be changed out to the 378 size for the 30-378, but for the RUM you will probably have to experiment a little with the 300 WBY size box to make it work. The ejection port is relieved, as Mike says, at the front to allow the longer cartridges to eject freely. This may not be required with the RUM. Hope this helps. - Dan

Thank you gentlemen.

As a side note, yes the bolt face was modified for the 6.5-06 by way of a ring which was inserted into the bolt face, (like a snap ring) which grasped the 06 cartridge.

I just like the 30-378's potential, especially, as has been stated, since I can download it to 300mag, or push it up for heavy bullets and long range work.

Well, then, remove the snap ring, open up the bolt face, modify the bolt groove, relieve the ejection port, change the magazine box, put on a new barrel, and there you go. Have fun - Dan