There is no one answer, because there is no one 38 caliber handgun.
First, assuming you are asking about a .38 Special revolver, and not a .38 S&W, or a .38 Super automatic- the weight will vary based on the size, and the material it is made from.
The current production Smith & Wesson 5 shot Airweight revolver with a 2 inch barrel weighs about 15 ounces empty. A 6 shot target revolver, with a 6 inch heavy barrel, made of stainless steel may weigh a couple of pounds.
No one gun, no one answer.
it is a little less powerful but not by much
Find a gunsmith and ask to be shown how.
100-400 USD depending on condition.
50-400 usd
Yes you can. The bore is the same size.
Not in .38 Special. Several manufacturers make handguns in .38 Super, though.
No. Only use what is stamped on the barrel.
No, i asume you dont have see .380 pistol and a revolver 38 spl, the .380 and 38 spl ammo share de same diameter boot no the longer, the 38 is for a 38 special revolver tipe and the .380 is for an automatic pistol tipe some people call 9mm short.
Impossible to say without a more detailed description of the handgun in question.
You haven't given enough information. It depends on what brand and model. Some 38 revolvers are worth $30 or $40. Some are worth several thousand.
Library, Blue Book of Gun Values, Standard Firearms Dictionary.
I'm not real familiar with the 38-40, but based on what I looked up, I'm going to say no. The 38-40 cartridge uses a .40 caliber bullet, which means the chamber in a 38-40 handgun would be too large in diameter to properly/safely hold the .38 special.