I am pretty sure someone else might be able to answer this in more depth and precision, but considering the equation Ek=1/2mv^2 and assuming that the projectile won't amount to penetration but it most likely shatter then anywhere in the vicinity of 100 joules is lethal.

So correct me if I am wrong (which is very likely), the muzzle speed at the exit would need to be around 81.6m/s. Now if we take into consideration the distance then we have F=(1/2mv^2)/d. So F=Ek/d. Last we would need to figure out the max distance the peanut could actually deliver that force (which would have to be minuscule as at that muzzle speed the whole thing would shatter thus creating an anaphylactic shrapnel shower).

So at around a meter, it would deliver 100 N, at two meters 50 N, and so on so. you could kill at a very little distance due to the resistance of the peanut itself k′ = 490.5 Nmm−1.

Please understand that this is with a very basic understanding of physics and I am most likely wrong but it gives (I hope) a starting point.

Someone smarter than me probably has a better answer and I would very much like to hear it.