A cheap Nikko Stirling scope. One of these: https://www.amazon.com/Nikko-Stirling-Mountmaster-Riflescope-NMC3940W/dp/B00P54UGAK/ref=cm_cr_arp_d_product_top?ie=UTF8

Got it cheap because it was being discontinued. I figured how could I go wrong for $20?

The biggest problem I had with it is that the screws that hold it to the rail kept loosening up. I had to tighten them again every couple of shots. But I had the same problem with my red dot as well, so I don't know what I was doing wrong.