Like stated above, you can use heavier bullets in the .243, but you have to have a faster twist to stabilize them.
Back in the days when I first started reloading, I was looking at Mauser actions, Remingtons, Winchester Model 70s, Savage 99s, and the occasional Ruger. The reloading manuals then were using these same type guns to test their loads.
Then we started getting new guns and new cartridges. The ammo and component makers didn't or couldn't keep up with all the different guns that could shoot their ammo. They started making the rail mount or 'shooting fixture' guns that were basically an action, trigger, and barrel hooked down to a bench. No more recoil spoiling their shots or making their shoulders sore. Many more tests could be done in a day.
Then we started getting into less expensive guns. No longer hand made parts, they used machines to stamp out the parts. These newer and less expensive guns did not appear to be able to handle the pressures like some of the older guns. They were mass produces and sold thru stores like Montgomery Wards, Coast to Coast, etc. The ammo dealers began backing off a little on the loads and making sure they were safe for ALL of the guns that could shoot that caliber.
Now days we have sooooo many guns and soooo many powders it is getting tougher to find specialized data like we used to have. Other than the opportunity to buy some of the older guns and components that are sometimes put on the market, this is likely gone forever. Yes, Federal, and Hornady, and Remington, etc are all making their new fangled ammo and cartridges and 4000fps wonders. Who can keep up with it all? Not the ammo makers or the makers of reloading components. Now you can have a comupter give you load information in 2 seconds that used to take weeks at the loading and shooting bench to find.
For me, its a continuous journey. I can stop and pull out my old Winchester or old Savage and shoot some loads made per manuals from the 60s and 70s. Newer powder and primers so loads are worked up, but the guns still shoot velocities etc like they did years ago. My newer guns may have the same name like they used to, Winchester Model 70, H&R etc, but my loads come from reloading manuals that have 1000s of loads, not 100s. I have to be careful with my loads because I have seen these newer guns wear out and break faster than some of the older ones.
You jump on the ride and see where it takes you. Hopefully every place is safe and sound.
Steve