Well, I stand corrected. The HK91 is based on the MG42 which is about 60 years old as I remember. The Spanish CETME was the missing link between the MG42 and the G3/HK91. I wouldn't call it blowback because blowback is just held by the mass of the breechblock and the spring. The HK uses a delayed roller lock where the floating bolt head forces the rollers to cam out of engagement with the recesses. So I guess it is recoil now that you mention it but about 60 years old, too.