Re: When headstamp is incorrect...
You're absolutely correct. Utmost care must be used to ensure your ammo is used only in your rifle. Especially in the situation like yours where the original brass is necked up. That .338 bullet will absolutely not go down a .308 bore without serious damage. It is less dangerous when necking down. Then the bullet goes down the bore...not accurate, but relatively benign. You might try marking the cases with a marking pen (black or green maybe), or some other method so that it is plainly visible that something is not ordinary about the round.