A replica musket was among items stolen from a car parked at Pontefract Castle.
Gill Birtles, 26, was due to take part in a Civil War reenactment at Pontefract Castle last weekend when the musket was stolen from her car.
She said: “I went over on Friday and I left my car in the car park. It had my musket and Sat Nav and a few other things.
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“I last checked on it at 9pm and when I got back at 8am the next morning the back window had been smashed and everything had been taken.
“It was covered up so you wouldn’t have known what it was, I think it was opportunistic."
The gun is worth around £200, but Gill said it would not be easy to sell or pass on.
“It’s registered as a shotgun,” she said. “I’ve got a certificate from the police and it’s all on file. It’s got a serial number.
“They wouldn’t be able to sell it very easily because any gun shop or anything would be able to see straight away that it was dodgy.
“Any information would be really helpful.”
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Gill, who lives in Castleford, hopes that a local resident may have seen something useful. She said: “It’s quite a quiet area so I thought someone would have seen something.
“All the rest is replaceable, I’d just like to get the gun back.
“If people could check gardens or skips or bushes - it could be that they realised that running through Pontefract with a gun bag wasn’t worth it.
“It’s quite obvious what it is.”
Contact West Yorkshire Police on 101, quoting reference number 13190277637.