Yes I have heard of it. It also was used by the ATF in the Olofson Case. (Given, ignore the other aspects of the case considering Olofson did indeed have Auto parts in his gun,) this below excerpt from the cross examination of the ATF's testing seems to support your armorers discussion. I have never personally witnessed this myself, although I suppose it is possible in the right circumstances.
Source:
https://www.gunowners.com/Olofson-Pe...Certiorari.pdf
(When asked why the AR-15 fired differently in the
two tests, Kingery explained that it was “due to the
ammunition [he] was using”:
Even though it was commercially available
ammunition [in the first test], it was a military
grade ammunition which has much harder
primer than standard civilian ammunition. [Tr.
107, ll. 16-19.
According to Kingery, the “harder primer” ammunition
was designed to ensure that a firearm’s “firing pin”
would not “strik[e] the primer,” and “accidentally”
cause the firearm to “go off.”
Tr. 108, ll. 1-9. However,
when pressed on cross-examination whether the
utilization of a “softer primer” ammunition grade
would increase the likelihood of “multiple firings,”
Kingery denied that the “softer” civilian grade of
ammunition would have had any such effect. Tr. 126,
l. 12 – 127, l. 19.
When pressed further on cross, Kingery was asked
why during the second test he had failed to test the
AR-15 in the semiautomatic position. Tr. 127, ll. 20-
25. Kingery replied, “because I already knew that it
would function.” Tr. 128, ll. 1-2. Yet, Kingery
admitted that,
by utilizing a different ammunition, it
was “possible” that, even in the semi-automatic
position, Olofson’s AR-15 could have malfunctioned,
either by “hammer follow” or by misfiring more than
one shot at the single pull of a trigger. Tr. 128, l. 3 –
129, l. 3. )
....To add to the internet lore, there has been discussion in the past that the ATF used very soft primer handloads to make the AR15 function in Full Auto without issues.