Re: 6.5-284 or 257 Weatherby???
Well, I guess somebody has to speak up for the 257 Wby.
This is the fastest of the factory cartridges in 25. It has held that distinction for over half a century and will most likely continue for a long time. It will roll a 120 gr bullet out the barrel at 3200-3300fps with very little effort. This will carry 1000# of energy to about 600 yds. If you are comfortable with the gun and put a little extra powder in the case and a little extra barrel length you can stretch it further. With smaller weight bullets the velocities get to be close to 4000 fps. Weatherby introduced the general American shooting public to the concept of high velocity and part of this is by providing an ample amount of freebore. Kirby discusses freebore in another post within the last few day. Freebore does a couple of things. It allows you to get high velocity by jumping the bullet down the barrel. This jump also help some of us who get a little sloppy with our reloading because it buffers our over pressure mistakes. This extra velocity comes at a price of accuracy because best accuracy is usually accomplished by having the bullet close to the lands with very little if any jump. Secondly, some designs of bullets with a short bearing surface may not survive the jump to the lands still lined up with the bore. Strangely enough very light flat based hollow points will usually do all right. Very acceptable hunting accuracy ( say 0.5- 1.0 MOA) is not difficult to achieve with the Wby family of cartridges. Every reloading manual list the cartridge and loads. Fed 215 primers and IMR 4350 are pretty basic places to start.
I have only about one year(long ago) of experience reloading the 257 Wby and have dealt with the 25-06 a lot more. The 100 gr Nosler partitions work fine on big mule deer and antelope (I was using a 25-06). Nosler now has a 110 gr accubond out with a good BC and it has a good length of bearing surface to jump down the freebore. I would believe it would work well. If it was me and I was doing a new barrel I would go with a faster twist. (Actually if it was me doing a custom gun in 25 I would just get the 257 AM and be done with it, on the other hand the 25-06 is one fine cartridge and it is easy to live with and is more on the order of the 6.5-284, but it is not what you asked about)
I have two custom guns in Wby cartridges and both of them have a standard factory chamber. This is something you will have to decide on. Reducing free bore means that you are going to swap some amount of speed for some amount of accuracy. Shooting deer and antelope you have a fairly large target area to shoot at and will be allright with the standard chamber. If you are shooting fleas off a prairie dog’s ear at a mile then you will need all the accuracy you can get and might reduce the freebore. While you will not see many 257 Wby at 100 yd bench rest matches, once you get out to the 1000 yd range the 6.5-300Wby, 7-300Wby and 300Wby have been and are used with good results. There is nothing wrong with belted magnums and double radius shoulders. They have been around for a long time and work fine.
A lot of people want their barrels to last a long time and that is fine, because it is their choice. I pretty much prefer lots of powder and blazing speed. That is why I shoot Wby cartridges. It is also why my cars have engines in them that require premium gas. I spent most of today shooting a 7mmWby and a 240 Wby. Like the 257 Wby, the 240Wby still is king of the hill for factory cartridges in its caliber. The 7mm has been superseded by the STW and RUM, Roy jr. needs to neck down the 378 one more notch!
I guess I am not the voice of great experience that you asked for with a 257 Wby but I like the Wby family of cartridges.
The Smokin Fur Rifle Club