It looks like a very used and abused Colt Single Action Army.
The barrel length is 4 3/4 inches as measured from the face of the cylinder to the end of the muzzle.
Caliber is .38 Long Colt.........NOT .38 Special although you can fire mild .38 Special ammo >IF< the gun is in safe firing condition, which is does NOT appear to be.
Serial number 186066 shows as being a late 1899.
1900 began the smokeless powder models but your's has the smokeless powder cylinder pin retention plunger so it's probably a transition gun.
Due to the poor condition value won't be very high.
For much more information post this on the Colt Forum....Some genuine experts are over there and they can tell you a lot more.