Why does M-113 have only a 50-cal machine gun? Why isn't there a cannon and a rocket launcher besides?

The M113 actually originated from a requirement for 2 different vehicles - one for the heavy armored forces and another for airborne units. As it turned out the requirements were merged and we ended up with the M113.

The need to make it air transportable kept the size down to a minimum which means light weight and thin armor. Also, the requirement called for a vehicle that could transport troops under armored protection near their objective where they would dismount and fight on foot. Thus the 1-1-3 is a battle taxi, not a fighting vehicle. Thus the light armament of just one heavy machine gun and again the thin armor.

Combat experience in Vietnam as early as 1965 showed the 1-1-3 had severe limitations in combat. The thin armor made the vehicle vulnerable to mines and even small arms fire at short range, let alone anti-tank weapons. Many soldiers preferred to ride on top instead of inside for that reason. The guy operating the machine gun was also supposed to command the vehicle AND the dismount squad - something which is quite impossible. Manning the gun also made him quite vulnerable since the weapon could only be fired with the gunner dangerously exposed. The troops inside also could not fight or even see from within the vehicle unless they exposed themselves through the roof hatch. The Israeli's had much the same experience with the vehicle in Lebanon 1982 and soon stopped using it in forward areas, the troops preferring to walk. Thus it didn't take long for the army to realize the 1-1-3 was not going to cut it and studies for a replacement began quickly, culminating in the M2 Bradley.

But the M113 was cheap which made it a popular export item and it was versatile enough to be used in a wide variety of roles for which its limitations were not a big deal.

The 1-1-3 is no longer used as a infantry vehicle in the U.S. Army but is still found in specialized roles such as ambulance, mortar carrier, engineer vehicle, cargo carrier, etc... The US Army recently announced its intention to phase the vehicle out of service over the next few years.

The M2 Bradley has a 25mm gun due to the requirement to defeat the armor of the Soviet BMP infantry vehicle. It has TOW missiles because of the requirement for the Cavalry Scout Vehicle which merged with the Infantry Vehicle during development. The infantry did not ask for TOW.

The M113 is an ageing battle-taxi which, as far as I know, is not used on active service by the US Army. Retrofitting these old APCs with contemporary systems would be a waste of money, as a 25mm Chain gun and Trigat or similar may set up recoil problems that will require strengthening of the track/roller mechanisms.

That being said, I have a photograph of an Egyptian M113 with a Russian B11 anti-tank gun bolted to the top - I would not like to be a soldier who has to serve this weapon - clinging to the top of a bucking and rolling APC trying to stuff a round of APFSDS into a B11 with bullets whistling around your ears would seem to be a suicide ticket. Personally, as an ex RN cook, I'd rate my survival chances higher by cooking bacon sandwiches for the Golani Brigade!

The M113 isn't used in Active Duty anymore in the US. The only guys that use it are the National Guard and they don't need anything bigger the a .50 Cal. But in Vietnam I know that some were out fitted with Recoil-less Anti-Tank Cannons.

When I was in the Armored Cav in Germany back in the mid 80's, we had Scout M113s with TOW ''Hammer heads'' and Light Scout M113 with Dragon launchers. Regular M113s had a 50 CAL with an optional M60 mounted on the back for air defense. I believe the M113 can be outfitted with many types of weapons. Most of our M113s had battleshields.