I do not know the specifics.
But I do know a few things;
The skillpoint boost is a percentage boost. I.e. if you have an attack score of 20k and have 100 skillpoints in attack, your final attack score will be 40k.
The reason why you are doing the same amount of damage to the same person is because the system reworks the difference between your attack score and his defense score and downsizes it to fit his max health. (For example, if he has 100 health max, he can never lose less than 8 health or more than 35 health, in one hit (I don't know the exact numbers, but something like that)). So the difference between having 400 Norinco and 400 Minimi's is 20k, but after downsizing it, it may land on the same number of health points. But this is only for this person. If you try it on other people with more or less max health, you will notice different results.
Just know that having Minimi's instead of Norinco's is a lot better :)
//Formidable - DeathWish MGS