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.
Yes you can. The bore is the same size.
50-400 usd
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.