What is the difference between a Mark V chambered in 300 WBY to one chambered in 30-378. I tried to fit a 378 in the magazine and it is too short? Not being familiar with Mark V actions, I am hoping someone can tell me an easy fix to make this fit. My buddy wants to rechamber his and needs the additional length.

Nobody knows how these two differ??

I'm not sure .... so don't take this as absolute, but may give you a starting point for more research.

I believe there may be some differences between the standard MkV nine-lug action and the 378 (.416 Rigby) class of actions. The head diameter IS larger than the H&H class of magnums .582" vs. .532". I don't know if it is a simple case of opening the bolt face or not.

IF the two actions are otherwise identical I believe all that is needed is to order the new mag box and follower from Weatherby.

Good Luck!

I don't think the actions are different, just not sure if there is a special magazine box or what for the longer rounds.

They are completely different.

The bolt stop on the Mark V bolt is a recessed groove under the bolt and it needs to be machined out to accommodate the longer bolt throw. Teh bolt gace also needs to be opened. Both simpl jobs for a competent 'smith.

Also, the .300 magazien box is designed to stagger 3 rounds. If you fit the larger cartridges in there you will find it holds 2 but only the fist one will feed and the second one will jump the rails every time, based on the follower and speing which is also different to the larger case in design.

The best option is to order a drop magazine from the Weatherby custome shop. These things are wonderful. They simply screw into place as a normal floor plate exchange and are only slightly protruding below the magazine well, so they do not look ugly or upset balance.

The good news gets better because they hold 3 rounds and as the Mark V is a dreaded push feed action, you can slip a 4th round in the chamber and double up on the traditional capacity.

Want more good news?

When you shoot the barrel out, as you will, you are already set up to rebarrel to .338/378, .378, .416 or 460 with no other changes as they are all interchangeable bolt and magazine.

Wonderful stuff.

JW

JW This is what I was looking for. It will be re-chambered to 7mm-300 WBY but we want to be about to seat the 180s out to a decent length, hence the 378 length was desired. I will take both apart and compare them and decide how to proceed with the mill...

Sounds like you want 7mm STW performance in a Mark V?

they are definitely different, the .300wby has the standard .532 magnum head the .30-378 uses a .579 head.your case lengths are different as well 2.825 vs 2.913. so i think you would need a different bolt or a lengthened one and a bigger follower and magazine too. sounds like a lot of work.

I want STW performance without the drawback of the short neck.

So here is a question rediculous as it may be.. Would a vanguard be able to be converted to .30-378?

Just curious.. ..

FMP

Not likely

OK, I took both stocks off today and compared everything between a Mark V Accumark chambered in 33-378 and a standard mark V in 300 WBY. The actions are basically identical. One bolt would function in the other- dimensionally they were within .002". The differences were in the bolts- there is a flute milled into the bottom of the bolt body that slides along the end of a screw which serves as the bolt stop. The flute was milled closer to the bolt face on the 378 action which allowed the bolt to be retracted further rearward- giving me what looked to be about .040-.050 there. The sheet metal magazine box on the 300 was much heavier gauge and somewhat shorter in length- between the two factors, it made the interior of the box about .060 shorter. There is a portion of the receiver that the mag box butts into- this need to have about .040 so milled off it to accomodate the bigger magazine box. The configuration of the box was different, but I think I can bend it and make it work with the 300 which is not a center feed like the 378. Once I get a new box, I will make the cuts!

Dennis,

Could you measure the box and mag follower in the .300 Wby? I'm converting a .257 Mark V to a match grade 7mm Rem Mag and want to have the full magnum length magazine. My smith is removing the mag block for the shorter .257 ctg. and I'm looking for a full length mag follower and spring. I think the mag box is 3.7" without the block and 3.4" with it in place.

DF

I'm curious how you rechamber a 300 WBY to7mm ???