Possibly the most accurate- go over o auction arms and gunbroker. Set up an account there. Then you can do a search for a completed auctions. The value of ANY gun is based on make, model, and condition.
Search for acuctions with your make and model, and compare the condition of yours to the conditions of the ones that sell. Ignore auctions with no bids- that was a WISHING price, not a selling price.
If you have a fairly common rifle, post good pics, give us info, we can give you a ballpark number.