While you wait for an indepth answer you can take a look at a similar answer given by /u/WARitter on armor and how 'bullet proof' they would be.

Given the metallurgy of the time a bulletproof metal shield would be prohibitively heavy or uselessly small. It could be done, and was done but guns got better faster than shields did, so they were ultimately dropped completely.

European armies did not use shields because they ceased being useful well before the British employed musket mass volley fire tactics.

The shield is important in ancient hoplite-style warfare, the soldier holds a sword or a spear in one hand while the other hand holds the shield. This gave professional heavy infantry an advantage against undisciplined or conscripted armies, nevertheless, the cavalry would reign as the dominant combat arm.

This all changed with the introduction of the Swiss Pike in the 15th century. Unlike spears, pikes are so long that they can only be handled with both hands, thus making it impossible to hold a shield effectively. They were devastatingly effective against both infantry and cavalry. Please see this photograph of Swiss Pike reenactors: http://www.badassoftheweek.com/images/31684493594/swisspikemen31.jpg

Since heavy cavalry was still a major threat, musketeers or men armed with arquebusiers would stand behind a protective wall of pikemen which could counter an armored cavalry charge with a bristle of 15-foot long pikes. Muskets were still new special weapons, not everyone had them, and they were used strategically. Please see following dramatized artist's depiction of soldiers shooting guns from behind a defensive wall of Pikemen: https://i.imgur.com/sj8RLmt_d.jpg?maxwidth=640&shape=thumb&fidelity=medium

Eventually, the bayonet replaced the need for having a blended unit of pikes and muskets because the muskets themselves could be used as pikes to defend against cavalry charges. Please see the following photograph of British troops practicing pike-style formations: http://www.oocities.org/athens/rhodes/1266/Naauwpoortsoldiers.jpg

As you can see, this image is from the Boer War circa 1900. This was still a tactic that was practiced until the 20th century because cavalry was still being used at that point. Moreover, they are armed with Magazine Lee-Metford or Lee-Enfield "Long Lee" rifles which are quite long. The Brown Bess and P53 Rifled Musket were even longer. Most pre-WWI rifles were designed to be this long because of the rifleman's dual role as pikeman and rifleman, so there was no room to hold a shield.

So to recap:

• The question is framed entirely wrong. It is being asked from a standpoint that ignores the fact that hoplite style tactics had not been used all that much during the middle ages when the balance of power shifted from aristocratic heavy cavalry to pikemen.

• Heavy cavalry was the dominating force on the battlefield. In the high to late middle ages, armies employed mercenaries and professional infantrymen which utilized a new weapon, the long pike. These pikes could only be used with two hands, no shields.

• When muskets became more commonplace and standardized throughout European armies, the bayonet eliminated the need to employ two separate infantry roles on the field. Instead, each soldier would be a pikeman and musketeer unto himself.