I know it is about a modern 120mm smooth bore round not the 105mm howitzer and tank guns you are discussing but the answer in in the text I pasted:
"The M908 round was developed to destroy obstacles and barriers (concrete, rock, dragon's teeth, etc.) that are set up to stop tanks. The round is a modification of the M830A1 (commonly known as the MPAT) ......."
specifically:
"........where the front fuze is replaced with a steel nose. This provides penetration into the obstacle before detonation. "
The secret in achieving the desired effect is in the design of the ammunition. Do you need it to penetrate or explode on impact, what does it need to penetrate, steel, concrete, brick, mud brick? Do you need to apply an external force to push something over, punch a large hole, or rather penetrate and then shatter a wall or even punch into a building or room and explode it through over pressure.
Even more interesting is the new AMP (Advanced Multi Purpose) round that aims to replace just about all of the above through the use of advanced fusing covering point-detonate, point-detonate delay, or airburst effect to deliver the affect of HEAT, MPAT, Canister or demolition.