I consider the para return spring assembly to be less reliable than the standards stock. Not that either are unreliable, but stacking 3 springs on top of each other is more complicated than one spring. I think the one long spring is simpler and therefore at least theoretically more reliable. I have built a standard stock model using the para spring, as that allowed me to cut a wood stock short.
But the bolt carrier for the para is different than a standard so that the 3 springs and guide can run inside it. You cn remove the spring guide from a para top cover and use it on a standard, but you cannot use a standard bolt carrier in a para.
T. Mark Graham, Master Gunsmith
Arizona Response Systems, LLC