I used a standard factory Remington 700 300 RUM, rifle a few yrs ago to feed 4.020" COAL cartridges out of the Wyatts mag. I have a Bridgeport mill so it was easy and looks just like a factory built rifle. But you have to mill the reciever back about 1/4" with a carbide end mill, that has the same radius as the factory reciever. Rotate and indicate reciever to follow and finish out factory original lines
Then mill a little off the bolt stop so the bolt will come back far enough to pick up the cartridges. Then use the new mag well as a template to remove a small amount of metal off the cutout
for the factory mag, until the new one fits. Make some 4" long dummy cartridges and tune mag lips. Mine is total factory Remington, I ordered a 30" Bartlein 8 twist for it but thought I'd do a 308 Win with a long barreled rifle first, then rechamber for the 300 RUM. I liked the long barreled 308 so much I never chsmbered it for 300 RUM.
The 300 RUM factory 26" barreled rifle shoots 250 gr ATIPS at 2985 fps with the 4.00" modifications...but the 10 twist will not stablize the big long bullets at 100 yds. Why I went to the 8 twist, plus it holds BC better for long range according to Hornady. This was for extreme range shooting...but I need to get another Bartlein barrel...plus to get way out there to 2000 yds I gotta drive longer to get out to the desert. Getting old and got bored with LR shooting after 25 yrs of it. I threaded the barrel for a Precision Armament M4-72 Severe Duty 308 brake. Really noisy with a back blast but I, being old, can shoot a light, but altered 4" action in 300 RUM from the prone with 250 gr bullet with over 100 grs of powder in a fairly light rifle in comfort. It will group the 225 eldms nice but I wanted the 250 Atips for better BC and larger bullet splash, to adjust scope after impact. I haven't been back to this project because I'm working on other projects. Being a retired machinist with lots of time and a shooting hobby, I tend to modify and experiment with everything. Right now working on modifing Stainless Steel 6.8X51 mm case head cartridge for and 308 and 6.5 Creedmoor with fantastic velocities, like 3150 fps for 140 gr match in the Creedmoor, 502 gr bullets in the 450 Bushmaster 16 twist, the 8.6 Blackout subsonic with 300 gr Berger & SMK, a 6 twist 5.56 AR. rifle, 2500 plus fps 9mm luger carbine with light copper bullets. Plus making dies reforming cases, 35 Rem from 308, 8.6 blkout from 308 or 30-06...even modified 308 cases to take shotgun primers, so in a primer shortage a 308 can use any available primer with moderate loads..or any large primer in a modified 5.56 case. And a host of other projects...projects and "I wounder if", seem to be my new hobby.