.300 Whisper OAL for semi-autos?

[Garrett]

I have been loading .300 Whisper for some time for use in a single-shot rifle. Cartridge overall length has only been limited by where the rifling lands started in relation to the chamber.

I am now starting to work up some loads for use in an AR15. I am aware of the issue with the front grooves in the magazine potentially causing problems with long, heavy bullets. I am also aware that the magazines can be modified to fix this, and I will probably mod a couple of mags just for this purpose.

However, does anyone have any good data on how long the bullet can be seated and still function reliably in an AR15 magazine? Do you get to the point where the shortened round will no longer feed reliably? Or do most people just shorter the OAL (and adjust the powder charge accordingly) until it runs well in the magazine / gun?

eta: Oops! Wrong forum. Can this be moved to the Reloading forum?