Years ago I was an M61A1 mechanic. Based on the information you've
provided I am fairly certain that you've got a LIVE ROUND!!! The "INERT"
stamp applies to the projectile only meaning that it is a target practice (non-
explosive-blue) projectile. The round itself is live!
All the items marked "INERT" in my collection, especially military-marked dummies, are in fact totally inert. This includes fuzes, projos, submunitions, and ammo. If the round is loaded with an inert projo, this is specified by the marking ("Loaded With Inert Projectile") or the inert component is marked as such.
On the other hand, I've never seen a live Target-Practice round with the case marked as "Inert". Normal, live, TP ammo is never marked "Inert" anyhwere.
If you can shake it and hear the powder charge then it is most certainly live. They do not "fill the case with sand" to create ballast.
In the 30mm M848 dummy round "Primer and propellant are replaced with threaded steel bolt attached to the base of the projectile body to maintain same weight and balance as the TP cartridge".
In the 25mm M794 dummy round "The cartridge case cavity is filled through an enlarged primer hole with epoxy resin filler to approximate the weight of the M792 high explosive incendiary-tracer and M793 TP-T service cartridges."
The above from the Army ammunition data sheets, TM-43-0001-27, April 1994.
I even have a Army drawing, cutaway pic of the M51 Vulcan dummy. According to drawing RA PD 212989A, the M51 dummy has a solid case head with no primer pocket (this is actually the "Cartridge Case, M103 Dummy", same M-number as the "live" case). Headstamp is an ink marking "INERT", and the case is filled with "INERT MATERIAL".
From "Airmunitions, General" (Air Force T.O. 11A-1-20, Feb 1966):
"CARTRIDGE, 20-MM, DUMMY, M51. This cartridge (figure 10-1) is a completely inert assembly that is used for drill purposes, for testing the feeder assembly of the weapon, and for ballast [!!!]. The service cartridge is simulated by assembling the projectile of ball cartridge M55 or M55A1 with steel cartridge case M103 (dummy). The cartridge case contains approximately .086 pound of inert material to produce an average overall weight equal to that of the service cartridge."
In larger arty it's common practice to inert-fill shells for various tests. 50/50 water-antifreeze is commonly used as a weight substitute for nerve agents.
"Some target-practice projectiles are cast iron while others are service projectiles loaded with sand or other inert material." ... "Dummy propelling charges are filled with wood grains simulating 'live' propellant grains, and the color of the cloth in dummy propelling charge bags simulates that used in 'live' propelling charge bags." (from "Ammunition, General, TM 9-1900, June 1956).
In "History of Modern U.S. Military Small Arms Ammunition, Volume II", Hackley and Woodin wrote that "Numerous other types of [.60 Cal] inert rounds were assembled for demonstration and other purposes and may be found with a variety of components. Some have been examined loaded with inert charges." They also mention that .50 BMG dummies have been made with inert powder charges: "One round headstamped F A 45 has a blackened primer and an inert powder charge and is marked INERT in black ink on the case." And: "Specially weighted dummy cartridges were used at Frankford Arsenal to verify the accuracy of gage and weigh machines and inspection equipment."
I can't recall where, but I have read of the use of pencil lead as a inert filler substitute for propellant. Problem with that is, it must *look* just like real powder (good idea if you're making a cutaway display, though!).
A "ballast" round looks completely different. Ballast rounds are all orange in colour and quite obviously fake (Ballast=Orange, Dummy=Silver).
I have a M254 Vulcan dummy which is all white plastic. Never seen an orange one, although other countries have made dummies in all colors of plastic. Seems like a good idea! The plastic was introduced because it reduced the wear and tear on the gun and feeder, not to simulate the weight of the service rounds (which they don't).
I have two M51 Vulcan dummies of the "all-silver" type, although I read they are supposed to start off with a chromate finish (it must wear off). One of them has some kind of loose inert filler, by the sound of it more like lead or steel shot!
Check the primer. If there is a blue inner ring around the primer then it is electrically fired.
That's really no indication. I've got a dummy .60 Cal round with what appears to be a perfectly intact unfired electric primer, but it's a dummy (ink-marked INERT and no powder inside).
(AFAIK the Navy doesn't like to use electric primer ammunition if there is an alternative).
Funny that they keep using it... yes, it has its idiosyncrasies but it works.
Although they did come out with the MK 7 link, with the protective tab to cover the primer. I think more to block radio transmitters and radar from setting off the primer, though!
A red inner ring around the primer means that it is percussion fired.
Um, no. It means they used a different color plastic that day. I have live examples of both. I've dissassembled hundreds of electric primers in the course of reloading ammo...
If there's *any* inner ring with an insulator, it's electric-primed.
In any case I would suggest that you get rid of it somehow. Drop it in an amnesty box somewhere or hand it over to the military for disposal.
Or just mail it to me. I'll pay the shipping and throw in $10 for your trouble.
If it is an electrically primed 20mm round, it can be set off just buy a static discharge from your own body.
IF it's got a live primer. I've heard this anecdotal evidence before. And as a M61 tech, you'd certainly be in a position to know! But have you ever directly *seen* evidence that this *has* ever happened? I personally count it as a remote possibility. It takes about 400 volts (trivial static charge) at a good amount of current to light that primer off. Common sense dictates that gluing a piece of 3/4" square of aluminum foil over the primer will prevent any problems.
It's not worth keeping it around. It's a dangerous souvenir ...
Do you really think they'd hand out LIVE HEI rounds to project team members as mementos? Anyhow, modern fuzes are great. The live HEI fuzes require both spin- and setback- arming.
There's really no way of telling with some stuff you come across. I've heard stories of things found on museum director's desks that were filled with 40-year-old live HE! Then you've got some of us active shooters running around making dummy rounds out of mix and match components.
Personally, if I make up a dummy round for function testing or as a gift, I make a point of drilling out the primer pocket so neither will a primer ever be able to be seated in it (prevents "squib loads") nor will it be able to hold powder without it running out the gaping primer hole.