As I read on this forum, the A5 receiver extension is basically an AR-10 carbine RE. You take an AR-15 rifle spring and the custom buffer from VLTOR and there’s your A5 system.
So, if that is indeed the case, it seems like the AR-10 version of the JP Silent Captured Recoil Buffer Spring might work.
Has anybody tried it?
I have, although I did have to modify (shorten) the delrin plug that goes into the RE. The one that come from JP is for a rifle RE.
I used it with both a 243 Rem upper and a 308 upper. The setup felt like a rifle length RE setup but with a collapsible stock. 
Nice.
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I wonder, would this combination also work on an AR-15?