40L vs 50L

Be sure to look at the main pack volume. The ULA Ohm, for example, is:

Main body: 2100 cubic inches
Extension collar: 500 cubic inches

So in reality it only has about 2600 cubic inches or 42L of interior space.

Thanks for that observation. I emailed the very responsive CS folks at Boreas and asked how they measured volume. Consistent with the above post, Boreas uses main body only, ignoring the rather substantial hip belt pockets, and very generous stretch side and front pockets. So, since the main compartment of a Buttermilks 40 is actually 42 liters in a size large, the "total" capacity of this pack must be to close to that of the Ohm2/Circuit and a bit more than the GG Gorilla.

Forget the extra volume in the extension collar. It is only for short durations and ergonomically not the best choice.

As far as I know at ULA for the circuit and ohm packs they count the extension, hip belt, mesh and add that all up. I think the Boreas Buttermilk would be quite similar in size. Choosing between the Buttermilk 40 and 50 I would also think about the difference with the pack I already own. If it's very close you might not get enough 'other' use out of it.