The Reptilia RECC E Stock is currently only compatible with AR15 pattern rifles. However, Reptilia is making an AR10/SR25 model that uses an A5/SR25 carbine buffer system which should be released in Q2 of 2023.
This model has has standard ar-15 & lr-308/ar-10 carbine buffer tube length, so any 308 carbine buffers and springs should work OK. No rifle or A5 buffers or springs in this configuration. The A5 length tube and its counterpart receiver extension aren't included. Its basically a UBR-lite with a little bit of play since the locking surfaces are polymer.