Nubo, that weight of grain per inch means nothing when comparing it to spine. You want an arrow with adequate spine or "stiffness". The grains per inch is just telling you what that arrow weighs.

For example, my most recommended aluminum arrow in terms of spine would be a 2314, but I always preferred the lesser stiff spined 2216 and...............the 2216 is heavier due to thicker shaft walls. Perhaps a bad example.

Although the Easton Axis 340 is stiffer(amount of measured deflection), the A/C Slim 400 is slightly heavier due to the construction of that particular arrow. .300, .340, .400, and .500 are a measure of spine(stiffness) only. The weights of different TYPES of arrows have only to do with the construction of those different TYPES of arrows. Although you can compare the spine of different arrows, those different arrows may and many times do weigh different. Hope this helps.