The original AR15 design incorporated the ability to fire rifle grenades as that was one of the things the Army desired in any/all of it's infantry rifles at the time (1950's), the grenade ring is actually more a "grenade stabilizer". The FH itself is actually the mount for the standard (of the time) Energa 75mm grenade, the grenade will mount on the FH w/o the "grenade ring", the ring is more of a spring that stabilizes the rear of the grenade behind the actual "mount" which is the FH, I've seen no reference to the ring/spring in any of the manuals of the time, including the 1960 Colt AR15 manual, the early Air Force manual on the 601 or the 1966 XM16E1 manual. They are military contract parts though but they may have been for use once the rifle was fired a few times w/the grenade as the fit between a new FH & the grenade (talking of the Energa here) is very tight when new & doesn't seem to need the ring, after a few grenade launches the wear from the grenade base on the FH may have been enough to leave a bit of "wobble", the ring would stabilize the grenade & overcome this, I think this may have been the original purpose for these, by the time the AR15 was actually in service the "need" for the rifle to fire rifle grenades was pretty much unnecessary due to the early grenade launchers (think the early one, VN era, was the Law) & the developement of the M79, XM148 & M203, the later rifle grenades, after the Energa were also a bit smaller & lighter in the "head" area of the grenade & most likely wouldn't have created a "wear" problem on the surface of the FH. This "theory" is based on my 601 & 603 replicas w/the Energa grenade fitted, on the 601 the barrel assembly is a NOS 601 unit & the grenade goes on the Duckbill & is very tight & stable, when I put it on the XM16E1 replica which has a "used" 3-prong it's a bit looser, putting the ring on the XM16E1 tightens it up completely. Also w/the ring in place once you fit a grenade to the rifle the "tongs or fingers" on the grenade ring lose some of their "spring" so even using a ring it seems they would most likely be effective for a few launchings & then they'd have to be replaced! It appears (to me) that the grenade ring, which would probably be more correctly identified as a spring was designed/purchased to solve a problem that never came up in service due to the fact that the M16 once finally adopted & issued in lg numbers was never used to launch rifle grenades as that requirement was met by the other pieces of equipment mentioned above.
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