Ok folks, blame @Idiotprofessor4 for this one. So why WERE the artillery units in the AotP so effective? Well it's fairly complicated and somewhat arcane and based on secondary sources and conjecture based on modern experience but... hey you're following me, you knew that already

So, union artillery, but especially in the Army of the Potomac has a well-deserved and fearsome reputation. There were three roots to this. Training, supply, and leadership. In all cases General Hunt had a decisive input both pre war and during.

I'm going to start with supply and equipment. Regardless of his position and responsibilities, which varied in scope throughout the war, Hunt remained keenly focused on keeping guns, carriages, horses, and men in top shape all the time.

In this effort he was GREATLY aided by General Montgomery Meigs who really could not possibly get enough credit for his tireless efforts to keep the armies of the United States supplied to the greatest extent possible.

Hunt and Meigs both saw artillery as a system to be handled as a whole not as small parts, and Hunt had very strong opinions with regards to what they should bring to the field. The 10lb, 3" and 20lb rifles might have represented the future but Hunt considered all 3 flawed.

He preferred the 12lb Napoleon for its far greater battlefield utility, reliability, and handling. These guns formed the backbone of both armies artillery corps and while Hunt found employment for the rifles, he had less use for them and especially the 20lb which he found too

Dangerous for his crews.

He didn't just focus on the guns though. Carriages, caissons, forges, ammunition trains, and every piece of equipment received his attention, including several changes to individual equipment for those serving the guns. Most famously he maintained a "secret" ammunition reserve

Of 20 rounds per gun which he never specifically told Meade or Grant about until well into the fighting at Petersburg. This extra ammunition allowed him to employ his batteries with confidence long after rebel guns had been forced to restrict their fire.

Hunt accomplished this through the massive application of paperwork. Indeed his very limited staff as a brigadier general in the war put a lot of the reporting on him and he wrote like he was running out of time.

Now let's talk organization. At all times after the peninsula campaign, Hunt was in charge of artillery. Sometimes he had more power, sometimes less, but officially he owned the guns. He wanted a full corps organization, with appropriate staff but Congress didn't like him and

Really didn't want to deal with the post war implications of an artillery corps so that never came to fruition, but he still maintained several artillery brigades, 1 for each corps and additional reserve brigades usually under his personal command.

This organization created an institutional knowledge and tradition of how to mass fires as a doctrine and not constantly improvised as rebel commanders had to do throughout the war.

Take Gettysburg where Hunt was able to repeatedly bring to bear massed artillery fires in the last two days while pemberton did nothing useful and Alexander struggled to get useful fires even with plenty of preparation.