I have a number of different hunting rifle scopes and and I bought all of them online. In my experience, guys in gun shops are just trying to sell you guns. They will tell you just about anything to make a sale and its hard to get good, honest information.
If you go online you can find tons of rifle scope reviews and all the specs you need to make sure it will fit on your scope mount. You can cut through all the sales pitches and get right to the information you need.