3.5, frequently a case fired in one gun will be tight going in another gun, even if full length sized. This isn't anything new or anything to even worry about so long as the cartridge is sized and loaded to specs. You've got a generous chamber in one rifle and a tight one in the other rifle. No big deal. Some rifle chambers are cut so small you have to use "small base" resizing dies to make any reload fit in them.

No to worry. But you may want to keep your brass seperate from his. The only concern here is if his chamber is so loose that he may have excessive headspace. But to a point, even this is not as issue as his brass will fire form out to fit it and then it would be best to only neck size his cases.