There seems to be a bitter fight going on between supporters of the M4, made by Colt, and the HK416 made by Germany's Heckler & Koch.
Field tests seem to show the HK416 superior on the issue of reliability and parts lifetime, and the US Delta Forces have already switched over.
I mention this only because of a huge scandal that happened during Vietnam where Pentagon officers favored the M-14 over the then-new M-16, because the M-14 used the same 7.62 mm ammunition as the M-30 machine gun, making logistics simpler.
There were also, as later uncovered in the congressional investigation, some covert financial ties between the manufacturer of the M-14 and those officers promoting its adoption as the standard US infantry battle rifle.
The scandal happened when the M-16 was sent into field tests in Vietnam, but with ammunition intentionally manufactured to not conform to the original Armalite specifications. The wrong propellant was used, not only in the shape of the granules which altered the burn time, but in the high residue composition which increased fouling of the barrel. This led to a rash of breakages and jammings and quite a few young Americans were found dead in the Vietnam jungles with partly disassembled M-16s in their hands.
Tests showed that with the correct ammunition, the Armalite M-16 not only was reliable, but the .223 bullet tumbled on hitting the target, causing a more serious wound while staying within treaty limits on "legal" ammunition in war zones.
Congress blocked the Army's decision to go with the M-14 and investigated, found out that the field tests had been set up intentionally to fail and discredit the Armalite M-16 and hand the contract to the M-14 manufacturer, Springfield Armory. Congress over-rode the US Army and ordered them to deploy the much less expensive but more effective M-16.
The US Army "retaliated" by requiring the M-16 by changed by adding a "Forward Assist Plunger", that oddball and rather useless device sticking out of the upper receiver at an angle at the right-rear. It's inclusion increased the difficulty of manufacture of the M-16, the price, and is generally viewed by expert shooters as totally useless. The US Army rationale was that soldiers had a deep psychological need to feel a traditional bolt handle on their rifle, but most soldiers found it simply added weight for no good reason. On those very rare occasions when one needed to manually force the bolt closed, this is easily done with the thumb through the ejection port.
While the US Army, never able to admit error, still requires the forward assist plunger on all M-16 derived weapons, upper receivers without this dunsil are available from after-market suppliers for sport-owners of these rifles.
So, as we see another vested financial battle over weapons procurement shaping up, and we start seeing American kids dying with malfunctioning weapons, I have to wonder if history isn't repeating itself here.