The improper use of cheaper, but much dirtier, ball powder ammunition in a rifle specifically designed for stick powder, and the government allowed the omittion of the design specified chrome lined chamber and bore, as well as the lack of issuing cleaning kits; all to cut costs.
Now you know.
It was a combination of several things. Remington, contracted to develop M193 ammunition, changed the bullet from the longer, higher BC bullet designed jointly by Stoner Sierra bullet to a shorter, lower BC bullet that better stabilized in the early 1:14” twist barrels. (Which the US Army changed to 1:12” twist anyway after Arctic testing.)
However, the lower BC bullet lost velocity faster and created problems with a 500 yard 10 gauge steel penetration requirement. Consequently, it needed to be launched about 150 fps faster, which created problems in terms of suitable powders that could achieve the velocity within the required pressure limit.
Both Olin and the US Ordinance officials wanted to use a cheap and easy to produce colloidal all powder, which could be made much faster and with greater safety as it was a wet process. WC846 was chosen, and eventually one of end of the rather wide burn rate specification for WC846 that worked well in M193 was Re designated WC844. But it still required waivers for a higher maximum average pressure and an increase in the acceptable cyclic rate in the M16 - as well as a reduction in the penetration range requirement.
The colloidal ball powders at the time were often made from recycled surplus naval cannon powders, and a calcium carbonate was used as a stabilizer. However, the amount used was about 8 times more than was required and it’s this excess calcium carbonate that caused fouling in the M16 gas tube.
Colt was also at fault as it’s marketing department was making optimistic claims to appeal to McNamara’s whiz kids in the defense department to secure contracts, claiming among other things that the new M16 didn’t require cleaning. As such, it was initially not issued with a cleaning kit.
A chrome lined chamber and bore wasn’t specifically required, but in South East Asia it did help reduce the potential for corrosion in the bore. The chrome lined chamber also meant that any powder residue stuck to the brass case, rather than the chamber and did help keep the chamber cleaner.
Eventually after some contentious congressional hearings, the problems with the powder got sorted out and the M16A1 was issued with a cleaning kit and the famous comic book user manual.
It was eventually a very reliable rifle when kept clean and well lubricated.