I was just talking with Pete Lincoln the other day about this exact same thing. If you dont know who he is I will post a link to his site. I will also include his responce to my question. Bottom line is he builds 338LM's on the HOWA LA with detachable mag systems. If there is an awnser to your question he will have it!
Roedale Precision - Pete Lincoln
I did some trading with you for parts and you tuned my trigger hope you remember? Any way I have some questions? I have my rifle all built now on the Howa LA but the bullets Im using will not fit in the magizine. Do you have any recomindations on how to modify my magizine to fit longer bullets?
you need to extend the feed well. / mag port, there is a nose that sticks out infront of where the trigger is mounted.
machine that away and then extend the factory box magazine,.
its basic metalwork to do and about 1 hours work,