http://stevespages.com/ipb-remington-51.html
Which model, what condition, condition, and condition?????
Impossible to answer without knowing if it is a rifle, handgun, shotgun and the caliber/gauge.
Anywhere from under $100 to over $1000, depending on which handgun you had in mind.
It depends on which gun.
100 USD
If you mean the magazine, there is a magazine release button on the left side of the gun. You press the button and the magazine comes out.
Model 51 manual located here - http://stevespages.com/pdf/remington_51.pdf
S&W never made a 25 ACP handgun for retail sale.
$273,880
26
300$