Before I changed my front sight/gas block to a railed gas block I have ran 4-16x42 like Crracer with no problems. I even had a 8-32x42 scope using the same tall rings (1") and 1/2" riser. I have found that raising the scope too high really limits your ability to adjust your scope to shoot reasonable distances. There is no limit to the length of a scope to the rifle my scope is 18 1/2" long. Note the following pictures with the original sight and with the railed gas block. Worked great both ways. I just put the railed gas block on to move the scope forward a little ways. These pictures were taken in just a 6 week time period, still trying to find my style with it.
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