SafeTacMag,LLC has developed an training magazine for M16s and M4s, specifically designed to fire blank ammo.
"During training exercises, blank ammunition often "jams," causing the service rifle to malfunction. ... The standard-issue magazine was never designed to fire blank ammunition," according to a recent press release. "As everyone in uniform knows, the much shorter blank ammunition cartridges simply do not feed properly from the standard magazine into the service rifle. Our blank-fire magazines feed blanks into the service rifle seamlessly without causing jams."
SafeTacMags require no modifications to the weapon and "makes training safer because it is simply impossible to load live rounds into our magazine," according to the release.
The mags are blue as an additional visual safety feature. They cost $20 each.
I doubt individual gun owners will want to buy these, but it seems like a good idea for military units to consider if the price were to come down for a bulk order.