A number of outfits that sell AR kits, parts and accessories have and do offer a "Bayonet Adaptor" This is imply a two peice ring with a bayonet stud on it which you place on your barrel in the p[roper place to mount your Bayonet on the A1 or A2 style flash hider and screw te two parts together so they grip the barrel. WHen I mount a flashlight with mounts something like that I have had success with the mounts not moving with the light on them by putting a band mad from a cross section of the finger of a rubber or nylon surgical glove under the mount and tighening things down.
Putting such a device on your rifle will change its barrel harmonics, who knows which way? So will mounting a bayonet on a rifle anyway.
I use my bayonets on military style and surplus rifles about every time I shoot them. Many places do not have secure racks for setting rifles in so I carry a page of newspaper or a sheet of wax paper if it is damp, lay that on the ground, "FIX..... BAYONETS!" then stand the rifle up by shoving thebayonet into the ground through the paper which protects my muzzle. I either use chamber type safety flags or remove the bolts of old bolt actions. Unlike leaning a rifle against a table or leaving it in its case on a bench I have yet to have a bayonet "stand" fail.
Check the range safety rules where you shoot before doing this.
Clean and protect your bald after this use.
I would have sworn that the M9 had a very hard plastic grip when they first came out. They were insulated to allow one to cut some live electrical wires. The test/review one the manufacturer sent me worked OK on chain link fencing near my office and two strand barbed wire near there as well. My favorite feature of the original ones was the bottle opener built into the cross guard. I seem to recall we used a photo of a Dr.Pepper being opened with it for the article.....could have used that feature in Cold War (cold Beer) Germany!
Aftrer playing with the M9 abit when it first came out though, I still preffered the M3 style blade on the older M7 bayonet and did not like the round grip of the M9 either. The designer appearently (based on phone conversation) made models with the issue round grip and a more traditional M5-6-&7 grip for troop trials and the troops chose the the round.
-Bob Hollingsworth