Having owned a "Bob" for thirty years, (custom M98 Mauser w/E.R.Shaw bbl), and having shot it side-by-side with .243's, I'll take the "Bob" over the .243, any day! (I've used/owned a couple of other "bobs", too. A friends Ruger No.1, and a Ruger M77MkII). I also own a Weatherby .257mag...
I've owned several .243's. The last deer I shot was with a .243, (Dec.12) only because I'd picked it up on a whim last Jan. with gift cards from Academy Sports recieved from my family as "stocking stuffers". It's a Savage Axis and with 65gr Horndy v-max gets 1/2moa @ 3,660fps, and /w 100gr Sierra's, 2/3"moa 5-shots). I "wanted" a .243 because I had several hundred bullets and cases on hand from range pick-ups and bargain shelves.......
The rifle is a "beater" imo, and is what (this year) sits in the kitchen... doe was shot out my kitchen window at ~140yds....
(btw, the PrviPartisan 100gr Spts are good... if you trim the meplats).
But, back to the "Bob"...
The .243 seems better on "paper" but in the field, the larger bore of the .257 and better construction of the bullets, makes it a more reliable performer on big game.
Good grief, at how many deer I looked for in an attempt to retrieve "evidence" of deer shot/poached, and lost, with a .243. (back in the late '70's and early '80's, the Rem. 742 in .243 was the 'night hunters gun of choice'.. (The problem with the .243 is bullet performance... blow ups at under ~50yds, and failure to expand at over 250yds).
On the evening I had a chance to compare them side-by-side, a co-worker and I were "culling" deer on an airport where they had become so populated as to be a hazard to aircraft. Because it was inside the city limits of a municipality with an ordnance agaisnt discharging firearms, the only recourse was to have state game 'n fish personnel do the "culling" due to a provision in the city ordinance.
My co-worker had/used a Rem. M788 (state owned and purchased ammo) and I had my .257.
Longest shot was 378yds, later checked by survey grade stakes where hill-side was being cleared. Shortest was ~40yds.
He was using 80gr Remington factory @~3,300fps and I was using 75gr Hornady handloads at a chrono'd 3,500fps (max load of H414).
The .257 was at NO ballistic disadvantage. No bullets were recovered. (amazing, except that no shots under 150yds were body shots....head shots). Even the 378yd shot (mine) was a spine hit that completely penetrated broadside. And 75gr Hornady HP is supposedly a "varmint" bullet.. (our varmints run ~120lbs....
The .257 exhibited a noticeably louder "ka-whap" on impact than the .243. Even my co-worker was amazed at the difference. Both of us were sighted in for ~250yds, so out to max distance we could see to shoot and/or make a reliable "hit", there was no advantage either way. Point and click...
But the number of deer that went down "right there" was much to the .257's advantage. None were lost with the "Bob"; two "got away" from the .243, though were found the next day by the buzzards.
The "Bob" has always been hampered with medicocre or worse factory ammo. Only the Hornady with 100gr and 117gr bullets, and the Federal +P 120gr Nosler Partiton'ed "Premium" (what a joke... still "mild" loads) came close to tapping the "Bobs" potential.
A good friend had/has a .25/06. My handloads with 120gr Sierra's chrono's exactly the same as his .25/06, only I'm using 5.0gr LESS powder.... And, I've got a 22"bbl to his 24"... But then again, he's got an "Interarms" MkX, and mine is an E.R.Shaw bbl...
BTW; There IS a reason that the 117gr Hornady RN "Interlok" is still loaded in factory .257WbyMag ammo. Out to 300yds, the trajectory is "no issue" and the bullet is superlative on game... I've got several hundred that I picked up covered in dust at an 'old timey' gun shop and here and there. Most were under $8/box..... I'll probably never use up all the 100gr Hornady PtSpt and 117gr RN Spts' in .257" I've got in my lifetime. Especially if I keep "playing" with such as the .243, and .223, .22-250, .375Ruger, .338MX, ect, ect.