ORIGINAL: Rammer
I personally HATE internal box magazines, and if I was building a custom 700 I would be going with the HS Precision box magazine. But to each their own, I have personally never seen an extended internal box magazine before, or heard of anyone attempting such a thing....
The agency rifle I was using had one and I loved it. That is the only rifle I have seen that had one. I mean it's done, it's just not common.