A castle nut is a steel locking nut used to secure the buffer tube (receiver extension) to the lower receiver on AR-15, AR-10, and similar rifles. It threads onto the buffer tube, holding the end plate in place and ensuring a stable, secure connection for the stock assembly. The castle nut works with lower receivers, carbine-length buffer tubes, end plates, buffer assemblies, and collapsible stocks. It’s an essential component for building or maintaining reliable, adjustable stock setups on AR-platform rifles.
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