The "riser" is actually called the scope base. Most bases for a given firearm are the same height. The scope mounting height variation is obtained by the different scope ring heights. This varies predicated upon the make and model of the firearm and are usually offered in low, medium and high variations. Most scope ring/base manufacturers publish charts that list the suggested base and ring heights. If you cannot find a published chart you should either email or call the company that you have selected as the manufacturer of the base(s) and rings. When you ask for the ring height be sure to tell them that you do not have a "smooth" barrel and that it has iron sights mounted that you do not desire to remove (if this is the case). Good luck.
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