It would be a huge error, struck on a cent planchet and thus very thin and not fully out to the rim. 1943 War nickels are a silver/manganese alloy
Silver-alloy nickels were minted from 1942-1945. However, in 1942, they minted two types: one was a copper alloy, one was a silver alloy. So how can you tell the difference? Silver-alloy nickels have either a large "P" or "S" above the Monticello dome as seen below (there was no "D" minted in silver for 1942). Interestingly, there are a few 1944 nickels that are missing the mintmarks above the Monticello; these are counterfeit. But they could also be quite valuable. These could likely be "Henning nickels," named after the man who had counterfeited them. (See photo below and also click the link for my page on coin talk about it) https://www.cointalk.com/threads/opinions-on-my-self-designed-slides-please.309775/#post-2973821
Don't forget about the weight also. It would weigh the same as a Cent Planchet. 3.0 Campared to a 5.0 Nickel.
I have a Bu 1944 p Copper war nickel 5.06 g. And another that 4.87 g. [Edited] KA Bowman collection per Elenore Roosevelt as William P Bowman was a US Army Quartermaster that worked the docks at Delco Remy during WW2.