[tobnpr]

I wouldn't use anything other than a high-density, structural foam- even for .223 recoil. While the recoil is not significant in itself, many thousands of rounds could- I suspect- compromise the integrity.

That said, I guess much (how much, is above my pay-grade) of the recoil force is absorbed by the buffer spring. Could always reinforce with a lightweight backing block (structural balsa) if needed.

Did get me wondering though as I hadn't thought much about it before- is the recoil force not absorbed through the buffer just transferred into the lower through the takedown pins?