The authors explicitly state that updating from an older version (like 2.5) to 2.6.9 requires a fresh character to avoid game-breaking bugs.
(Note: exact item lists and numerical stats vary between community builds; check the mod’s page or changelog for the official patch notes.) fallout 4 modern firearms 269 link
or at least using a clean save when updating to 2.6.9 from a previous version to avoid major script conflicts. Load Order: For the best results, place Modern Firearms at the bottom of your load order The authors explicitly state that updating from an
New weapons are injected into NPC and vendor inventories via scripts. If you don't see them immediately, try sleeping for three game days to allow the leveled lists to reset. How to install Modern Firearms in Fallout 4? Aug 8, 2561 BE — If you don't see them immediately, try sleeping
A separate Vanilla Weapons Replacer is available if you want to replace standard in-game guns with Modern Firearms assets .