It seems like an advantageous tactic for a military between the 1400s-1800s to employ a Roman style shield wall in unison with line infantry firing from the cover provided. However I do not think any military ever employed this tactic effectively. Are there any anecdotes of experiments involving this? I imagine there must be unforeseen complications that make this tactic unfeasible. Too much weight? Too awkward? Limited effectiveness?
As I imagine it would be pretty hard to find any documentation of this specific topic, a broader question would be could shields even be made to effectively stop early gunpowder weapons?
Any insight on the topic of use of shields in a gunpowder dominated era is appreciated. I am aware that riot control forces do use this tactic regularly. I am interested why it can’t and presumably never has been applied to broader military applications.
It's been done. Since early infantry guns were usually two-handed, the shield was usually rested on the ground, rather than carried by the gunner (i.e., the shield is a pavise). This was done in both Europe and Asia:
Germany, 1504: https://commons.wikimedia.org/wiki/File:Schlacht_Schoenberg.jpg
Japan: https://commons.wikimedia.org/wiki/File:Ashigaru_using_shields_%28tate%29.jpg
There are two problem with this kind of tactic.
The first is the weight of the shield - even if the infantry don't carry it while shooting, somebody needs to carry it and put it in place. An iron shield designed to stop late 16th century European guns would need to be about 3mm thick, and if of reasonable coverage, say 140cm by 60cm, and of uniform thickness, would weigh about 20kg. If made of wood, it would need to be thicker, and could easily weigh much more. While 20kg might not be unreasonable, as guns grew more powerful in the 17th century, such shields would have needed to be thicker, and thus heavier. Portable shields would no longer be very portable, and one might as well build non-portable field fortifications (or permanent fortifications).
The second problem is that handguns were not the only gunpowder weapons on the battlefield. In particular, there were cannons. A cannon outranges muskets, and artillery would be an ideal weapon to use against relatively immobile shield walls. If the shields are thick enough to stop artillery, then all is well, but they are no longer portable - we are back to field fortifications or permanent fortifications.
One way to make field fortification that are very effective against muskets and artillery is simply to dig trenches. This has the advantage that the soldiers only need to carry picks and shovels, rather than heavy and bulky pavises which would not even provide sufficient protection against artillery. Trench warfare is an implementation of the idea of infantry firing from behind cover - compared to your shield-wall suggestion, it lacks portability, but is effective against even modern rifles and artillery.
There were experiments with carried shields and gunpowder weapons. One method that avoids the infantryman needing a third hand to carry the shield is to make a combination shield/gun:
Europe: https://commons.wikimedia.org/wiki/File:English_-Gun_Shield-_Walters_511414.jpg
Europe: https://commons.wikimedia.org/wiki/File:Gun-shield,_perhaps_Italy,circa_1540-Higgins_Armory_Museum-_DSC05680.JPG
China: https://commons.wikimedia.org/wiki/File:%E6%94%BE%E8%BF%85%E9%9B%B7%E9%8A%83%E5%9C%96.png
Gun-shields like these weren't found to be effective enough to ever become more than a novelty.
Cavalry often carried shields late into the gunpower era, and some of these shields were quite bullet-resistant. Bullet resistant rawhide shields were discussed in https://www.reddit.com/r/AskHistorians/comments/8eqzq1/is_there_any_truth_to_zulu_shields_being/ by u/GeetchNixon and me. These shields are much smaller than those that would be required to form a shield wall, and are therefore much lighter.
