"21 gun salute" should be 21 ROUND salute
Actually, the classic volleys fired over our U.S. military graves are fired by a 7 man firing party in three volleys. So the number "21" is actually correct as far as the number of rounds U.S. military (and veterans organization) firing parties expend in tribute to deceased service members. I think that may be the point of confusion with the classic salute fired for royalty. In cases where reenactors or veterans organizations provide the firing party and they are "short", they make do with smaller firing parties; but still only fire three volleys. I have never seen any number other than seven shooters in any official U.S. military firing party.
I'm 100% certain they are safe, think about this
would the Army send out rifles with a NON-VISIBLE tack weld on the barrel/receiver point but NO OTHER MODS but still fully capable of FIRING regular ammo? Not bloody likely.
At a minimum, the Army would have stamped something like "USE BLANK CARTRIDGES ONLY-UNSAFE TO SHOOT" on the things somewhere to avoid liability concerns (and the Federal Tort Claims Act WOULD certainly allow a suit were there any injuries).
I suspect the real reason they did this is to discourage "LOST" rifles. In other words, the tack weld makes the rifle fairly un-desirable for three reasons. One, people THINK it is somehow defective/dangerous for regular ammo and two, it renders the rifle worth zipola (i.e. parts value) for re-sale.
Most folks aren't capable of changing out barrels, so the discourage take down argument makes not much sense. Three, the tack weld pretty much implies it's a VFW Gun "US Property," so, again, less likely to wander off and less likely to be purchased by gunshops, etc.