I have owned a lot of Wby Mark Vs, since about 1970. I have owned all the Wby calibres except for the 224 and 7mm but have shot both. I have also had seven 378s,that's right, seven of 'em Four were via the custom shop.
As to accuracy in my opinion the best are the Made in USA versions since about 1995.
The enemy to accuracy in a Wby chambering is if the parallel free bore section is too big in diameter. The spec is .0005 over bullet diameter. Of the standard issue Wbys the most consistently accurate is the 460. Of course this is after bedding etc. The reason for this is the 460 is a custom shop barrelled action and the freebore diameter is right on spec and sometime even smaller.
It is important to bear in mind that Wby freebore is a parallel section, like a cylinder whereas conventional chamberings it is tapered.
The freebore brings some pluses and minuses.
In a conventional chamber as most reloaders know seating the bullet a little deeper or a little further out can really tidy up the groups. However, this is not a bullet travel issues as such. What happens when you make a small change to the seating depth you alter the ignition characteristics.
A Wby does not allow that option. This results in general that a Wby chambering will not usually offer top accuracy with a wider range of components and simply because you can't really alter the ignition characteristics.
On the plus side and with an accurate load a Wby chambering is more likely to put the first shot from a clean cold barrel into the group or very close. Same when a shot is fired from a barrel with cold hard fouling.
Another plus is the Wby will generally give better velocities from powders that would normally be considered to fast. Try something like 4064 or Varget burn rate in a 300 Wby and you can see some big velocities and top accuracy. This does add to loading flexibility.
One last point concerning the Mark V action. Action feed rails are not involved. In fact the magazine is heavy metal and is similar in operation to the old 303 SMLE. Tabs on the magazine control the feeding. Thus a 257 Wby and a 460 Wby are identical actions except for the magazine/follower and the slot on the bottom of the bolt that acts a the bolt guide and bolt stop is shorter. When calibres like 30/06 were chambered in the 9 lug action the same applies. What this means is conversion of a Mark V from one calibre to another is very simple and trouble free.
In fact if my memory is working AussieGunWriter (when he was living in Australia) had a 270 Wby changed to 416 Wby and it simple. Just a new magazine box/follower and of course opened bolt face.
I hqv ehad most gun makes come through my hands and the Mark V wins for feeding reliability and double so for any of the 378 based calibres. All the 378 based calibres use a centre line feed and a centre line feed (assuming all else is equal) will always beat a stagger feed for reliability.
Personally, after having been a gun nut since about age 15 I reckon there are three factory actions. Rem 700, Model 70 and Wby Mark V.
As a side note Wby has always been regarded as the prize import agency in Australia. They stick with the appointed distributor and won't sell to others. Their Custom Shop is a pure pleasure to deal with. However, if you use the Custom Shop as opposed to buying their stand custom shop offerings on the website, then it does get expensive, very expensive.
I guess if I had to pick one rifle tomorrow it would be a custom shop 270 Wby and with the all steel bottom metal, the Krieger cut rifling barrel and Krieger action work.
Four were via the custom shop.
