Sounds like a hunting situation where you will need the magazine. Even though your chamber will accomodate a longer bullet, your magazine will not. So now you need to determine what the longest oal will fit in your magazine and that length is now the max oal that will function in your rifle. Chances are it will still be over saami reccomended max length and chances are it will still be more accurate as it is still closer to the lands.
I'm kind of surprised though as my remington adl in 270 will accomodate a cartridge that touches the lands.
If you make one that barely fits in the magazine and then make them shorter in increments you can still likely find a sweet spot for your rifle.