For a while about 20 years ago, I was shooting a longbow with arrows that were around 5.6 grains per pound and never had any problem with penetration. Of course, those arrows were full length 2419's and weighed 670 grains. I'll let you do the math.

It wasn't long after that I went to 55 pounds for all my traditional bows and used arrows around 560 grains for all my shooting. Since then, I've gone as low as 450 grains, but have never liked the way my bows feel with that arrow weight. If I had those same light arrows now, they'd be rigged with brass inserts and 200 grain points to get the weight up, and have the bonus of a big bump in FOC.