Put a weight on the inner JAC barrel and you will get some serious recoil. Some Pare's in the phillipines converted theirs to jackhammers.
Some 'bolt blowback' rifles are alright, done before, but not the greatest performers when compared to a JAC done up right. I think COKSAI was the only one to do it right, YE (Youth Engineering) had some QC issues.
If you saw the internals of a JAC you'd understand I am sure - not worth the effort.