From the website:
The third model, the M1A1, or M1 with Block 1 Product Improvement, started in August 1985. In addition to the improvements fitted to the IPM1 tank, the M1A1's major asset was to be the German Rheinmetall 120mm smoothbore cannon. US studies on the gun concluded it was overly complex and expensive by American engineering standards, so a version using fewer parts was developed, and designated the 120mm M256 gun. Along with the new gun came a number of associated changes to the fire control system.
Your comment
however the design had to be altered in order for the US industry being capable of producing it.
That is not what the article said and you are drawing the wrong conclusion.
For starters, Watervliet Arsenal in New York was more than capable of producing the exact weapon to the exact same standards as what Rheinmetal could do. The probelm would be the cost of retooling the production line from the M68A1 to the exact specs of the Rheinmetal design. So a cheaper and more efficeint way was found to use existing equipment as much as possible when manufacturing of the M256 started at Watervliet.
In order to same money the design was modified. It had nothing to do with ability. Additioanlly, most of the changes had to do with how the weapon fit into the gun mantle sicne the gun mantle of the M1A1 was significantly different than the Leo 2. Oh, and some of the parts were changed from Metric to to English since that is what US weapons use.
Today Watervliet is using only 18% of capacity and most of its issues have to do with the labor force...which has migrated out of upstate New York as the jobs have moved elsewhere.
Rethinking Governance of the Army's ... - Google Book Search
Watervliet also had the adeed problem of dealing with old facilities. US Cannons have been made there since 1813. Rheinmetal got to start over again thanks to the plant redesign opportunities afforded them thanks to the RAF & USAAF.
As for the M16/M4 "debate"
You have based your judgements on hearsay and old data. You have never fored the weapon in combat as I understand it.
OFOGS and several others have.
I believe I will go with their assesment.