[Robert A Cook PE]

The WWI museum at the Somme shows several varieties of german and Allied “portable” armored shields ... and several have been shot through. The high-speed bullets were just too powerful.

Even “tank” armor on the early WWII light tanks could be pierced by large rifle and machine gun bullets.

[Travis McGee]

Simo "Simuna" Häyhä December 17, 1905 – April 1, 2002), nicknamed "White Death" (Russian: Belaya Smert; Finnish: valkoinen kuolema; Swedish: den vita döden) by the Red Army, was a Finnish marksman. Using a Finnish M/28-30 rifle (a Finnish variant of the Mosin–Nagant rifle) and the Suomi KP/-31 Submachine Gun, in the Winter War, he is reported as having killed 505 men, the highest recorded number of confirmed sniper kills in any major war.[2][3]

During the Winter War (1939–1940) between Finland and the Soviet Union, Häyhä served as a sniper for the Finnish Army against the Red Army in the 6th Company of JR 34 during the Battle of Kollaa in temperatures between −40 °C (−40 °F) and −20 °C (−4 °F), dressed completely in white camouflage. Stalin’s purges of military experts caused chaos, and Soviet troops were not issued with white camouflage suits for most of the war, making them easily visible to snipers.[5] Häyhä has been credited with 505 sniper kills.[2][6] A daily account of the kills at Kollaa was made for the Finnish snipers. All of Häyhä's kills were accomplished in fewer than 100 days – an average of just over five kills per day – at a time of year with very few daylight hours.[7][8][9] Häyhä used an M/28-30 with serial number 60974, because it suited his small frame (1.6 m (5 ft 3 in)). The rifle is a shorter, Finnish White Guard militia variant of the Mosin–Nagant rifle, known as "Pystykorva" (literally "Spitz", due to the front sight's resemblance to the head of a spitz-type dog) chambered in the Finnish Mosin–Nagant cartridge 7.62×53R. He preferred iron sights over telescopic sights as to present a smaller target for the enemy (a sniper must raise his head higher when using a telescopic sight), to increase accuracy (a telescopic sight's glass can fog up easily in cold weather), and to aid in concealment (sunlight glare in telescopic sight lenses can reveal a sniper's position). As well as these tactics, he frequently packed dense mounds of snow in front of his position to conceal himself, provide padding for his rifle and reduce the characteristic puff of snow stirred up by the muzzle blast. He was also known to keep snow in his mouth whilst sniping, to prevent steamy breaths giving away his position in the cold air.[10]

[Obadiah]

Thanks. Your post provided more information on why the sleds failed than the entire article did.

[buwaya]

Most of the shields werent portable as such - they were used as part of the trench fortifications. A prefab firing slit. It was dangerous to observe from the trenches, or to fire from them because of course the other side were trying to do the same thing. So armored shields were one expedient.
Personal armor was another. The Germans even had special arnor plates for helmets and even machineguns.
Another was periscope rifles, I have seen several.

Mobile shields on wheels were tried a bit, but were not suitable to the ground.