I bought a 300 RUM a few years ago in a Rem 700 LSS 26" 1 in 10". Its an awesome caliber to say the least. Very powerful and very, very accurate. I shot about 1200 rounds in 2 years through mine and burnt the barrel out, so I feel I have some helpfull information on the cartridge. I was getting 3200fps w/200g SMK's and AB's. I also shot the 210g VLD @ 3000fps because thats where it was most accurate at. I could push it to 3100, but the accuracy was not as good. I also shot the 220g SMK @3000 as well, that was a little hotter load, but accuracy was once again superb. All these loads were using H-1000 powder as thats what my rifle seemed to love from the start, so I never went any further. These bullets with H-1000 gave me great accuracy to 800 yards, usually around .75 MOA, for a factory sporter rifle, not to shabby, and if I was on my game, it would do .5 MOA. In my remington, I could seat all the above bullets to the lands and still use it as a repeater, which was a bonus. You dont need a wyatts extended magazine.
I would not build a 300 Jarret because of what everyone else has already stated. Why go the extra length, especially if your not gaining anything??? I personally dont see any reason to other then the already stated "WOW" or "cool" factor. However, the 300 RUM has its own "WOW" factor after shooting it for a couple years. I actually feel its a little to much gun for most circumstances and feel its better suited for shots no closer then 600 yards, this is just my opinion. I slammed a few cow and spike elk with that 200g AB doing 3200fps and it was quite impressive, from 75-525 yards, and it left fist size holes all the way through the shoulders and the elk simply didn't know what hit them. I also shot my biggest black bear to date at about 250 yards with the 200g SMK right behind the shoulder and he only went about 10 yards and was dead on arrival. Also got my farthest coyote kill at 775 yards, on the 2nd shot, with the 200g SMK.
The 300 RUM is an awesome caliber and I dont think you'd be dissappointed with it. Good luck with whichever way you decide to go.