14 Feb 22
At a DTI Urban Rifle Course in SC last weekend, a student brought an M4 chambered for 300Blk. Ammunition was factory reloads from a reputable source.
The rifle was running normally when the bolt abruptly failed to close, and of course, the rifle to fire.
Looking into the chamber with the aid of a flashlight, I could see a deformed primer cup wedged into the space
between two locking recesses!
We had to knock the stuck case back out from the muzzle end, using a cleaning rod. We were then able to persuade the wedged primer to fall out.
I’ve seen blown primer cups (as well as separated anvils) end-up in strange places within receivers. Most of the time the phenomenon has no effect on the rifle’s normal functioning. Once in a while, as happened this time, a blown primer finds its way into just the right place where it causes serious problems.
With the issue thus resolved, the student reloaded and continued with our normal exercise regimen. Twenty rounds later, the identical state of affairs repeated itself!
Yet another blown primer was discovered jammed into a locking recess and preventing the bolt from going into battery.
This time (after fixing the problem again) we examined the rifle in an effort to find out if any additional blown primers were lurking about (we found none), and I instructed the student to switch to new/factory ammunition.
When he did, he was able to complete the Course without further incident.
Blown primers are usually the product of hot ammunition, combined with tight chambers and no, or little, freebore.
For that and other reasons, I don’t like tight, temperamental rifles!
However, blown primers can occur with any rifle, and any brand of ammunition.
I thus encourage students to carefully examine their rifles for blown primers every time they perform user-level maintenance.
As noted above, most blown primers, even when they come to rest somewhere within the receiver, never affect anything, but the upshot of the ones that do may well be a fatality on the part of the Operator who finds himself holding a rifle that suddenly won’t shoot!
“NCOs look after the men, so that officers can find ways to get them killed.
That’s the difference between ‘maintenance’ and ‘command.’”