Was there ever serious experimentation with putting something between line infantry and incoming fire? I assume early guns didn't have great penetration.. why not advance with a metal wall in front of you going into pitched battles?
Those early guns were usually very heavy and were definitely two-hand weapons. The idea of the shield wall only works if you have a free hand for the shield and then a free hand for a sword or a spear.
So you'd have to have a big heavy gun, all of your paper, powder, reloading tools, probably a little knife or other hand to hand weapon, and carrying a big heavy shield.
Plus, in Europe, where the first versions of what we would call a gun really took root, they had basically abandoned the type of warfare that made the shield wall effective.
The shield wall is a technique used by foot soldiers, arranged into what we call a phalanx. A phalanx is just an organized system of rows of men often staggered a little within the ranks so that there will be one row of men, and in the next row there will be men that fit in the gaps between the men in first row. basically looking something like this:
and so on and so on.
This way the men in the front hold the shield wall, backed up by the ranks and ranks of men behind them, making the wall of shields into a literal wall, with the men in the back ranks providing the strength to the men in the front. Your goal is to slowly advance your shieldwall forward as the men behind you stab at your foes with sword and spear.
By the early modern period in Europe, as the gun took rise, there were already many people using cavalry, massive groups of archers, and other more mobile or long-distance tactics that destroyed the strength of the shield wall and the phalanx.
A wall of men and shields can stop other men on foot, but its not going to do shit to 300 mounted horsemen with full plate, and its not going to stand up well to continuous barrages from thousands of archers.
So, there may have been some basic experimentation in the early years of large-scale gun-using warfare, it would probably not have included bringing back the shield wall, since it would have been physically very inconvenient and not very effective considering the innovations in military tactics, and the basic organization of that culture's war machine.
Edit: This is a joke, don't believe anything here. Here be lies.
Although never mentioned in most texts, shields weren't popular in the musket era simply due to a series of odd events that made them become unpopular.
It first starts during the Long War in 1591-1606 between the Habsburgs of Austria and the Ottoman Empire. In an effort to push the Ottomans out of Hungry and into the Balkans, the Habsburgs started to fund small bands of raiding dragoons that would push and harass any Ottoman force.
A Dragoon is a very odd mixture of cavalry and infantry. While they are mounted on horses, they are armed with muskets or carbines in order for them to ride around with long range weaponry while not overburdening their horse. While a Dragoon has always been seen as a form of Mounted Infantry, during the Long War, they were seen as a form of light cavalry.
However, the Janissaries of the Ottomans were experts with all weapons, from a scimitar to a musket. So they didn't use a specific fighting style that could easily be defended. Due to some work by noted Austrian historian Wilhelm von Shwarzkopf, in his Der lange Krieg, mentions that these Dragoons were often given large shields that were left over in an effort to arm these Mounted Infantry on the cheap.
Surprisingly, he found that the few records that existed of these soldiers often noted how capable they were at defense, even against Ottoman musketry. Here's a quote from his book:
The Mohammedan is a skilled and ferocious fighter, but seems to be a poor shot. While they are able at hand to hand, they always seem to hit the large shields that the Crown has provided for us. Either their muskets are deficient or the shields are stronger than we realize.
However, the Austrians seemed to find that the shields were rather strong against even their own muskets.
On the morning of the 18th, we had given one of our shields to a captured Moslem Magyar. In an effort to further our skill, we started to shoot at it, one by one. Our shield held up until one of the new soldiers ended up pouring too much powder and shooting through the shield, killing both the Magyar and the poor Christian whose musket exploded in his face.
However, with the rise of the bayonet has been attributed to the fall of the shield, at least according to Shwarzkopf. Due to the size of the shields and their weight, it would be easy for a soldier to pull the shield away and stab the poor soldier in the heart.
However, oddly enough, Napoleon did read this when he was considering a defensive action against the Ottoman Empire. Napoleon was well known for his wide breath of knowledge and large apatite for books, especially histories. After his defeat of the Austrians in the War of the Third Coalition, Napoleon had easy access to the Imperial Library. In a seemingly average text, he finds this concept of a protective shield interesting. However, it was very much against the French method of battle (The French have a phrase, attaque toujour l'attaque or "the attack, always the attack.) So, he would put the idea aside.
However, things did turn South for Napoleon. In 1813, Europe was turning against him due to his destructive economic policies. So Napoleon had to play defensive for the first time since Italy. So he calls for a copy of the history to be procured and from that creates metal versions for what would become the Battle of Liepzig. During the first day, the shields work well, allowing French light infantry from defensive positions with relative safety by placing the shields into the ground and firing from behind them. However, the battle turned rather quick, this is from La Tragédie de Leipzig: La Chute de l'Empire Français by Andre-Charles Villeneuve:
Very bravely, the voltigeurs were fighting off the backstabbing Austrians but the tide quickly turned during the second day. The Saxons whom turned sides against Napoleon quickly found a way to punch through the strong iron shields that Napoleon had employed against the Allies. In this, the Saxons would reload their musket but throw the ball away, but rather discharge the powder, they would add another charge in so that there would be double the gunpowder behind a ball.
It was effective but deadly. The soldiers first did this in a volley, the Saxons had fired in a line formation, but several soldiers died when some of the muskets exploded from the higher than standard pressure. So to fix this, the Saxons sent their soldiers out in open order, so that if there was a musket explosion, it would only hurt the user.
Much must be said about the bravery of these Saxons for willfully handling a weapon that could easily kill them. These brave turncoats stood forward and had a chance of seeing their death in order to fight what they thought was an evil army.
Oddly enough, this is the last time that shields were used. However this may be due to the use of stronger gun powder over time. As time continues, gun powder gets more powerful thus making metal shields too heavy.
[To inform you, this was a joke. I have a real answer somewhere that I'll find for you, as this question has come up before.]