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Extended holiday returns
FREE refund/replacement until Jan 31, 2026 Learn more
FREE refund/replacement until Jan 31, 2026 Learn more
This item's return window has been extended for the holiday season: FREE refund/replacement until Jan 31, 2026
To qualify for a full refund, items must be returned in their original, unused condition. If an item is returned in a used, damaged, or materially different state, you may be granted a partial refund.
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Availability:
In Stock.
Arrives
Monday, Dec 15
Recently Viewed
Style: Guardian | 3-9x40 AO
Features
- Adjustable 3-9 times magnification, for targeting out to 600 yards and beyond
- An adjustable objective lens or AO - that allows for sharper focus of the target image, elimination of parallax, and range estimation
- Aircraft grade 6061 aluminum scope body with a sealed, nitrogen-charged tube for resistance to water and fog
- Precision 1/4 MOA/click windage and elevation adjustments with locking turrets
- Package includes High Profile Picatinny/Weaver Scope Rings and set of flip-up lens covers
Brand: Monstrum
Color: Black | Picatinny/Weaver Scope Rings
Style: Guardian | 3-9x40 AO
Sport: Hunting
Item Weight: 15 Ounces
Material: Aluminum
Magnification Maximum: 9 x
Objective Lens Diameter: 50 Millimeters
Magnification Minimum: 3 x
Compatible Devices: Rifle
Brand: Monstrum
Color: Black | Picatinny/Weaver Scope Rings
Style: Guardian | 3-9x40 AO
Sport: Hunting
Item Weight: 15 Ounces
Material: Aluminum
Magnification Maximum: 9 x
Objective Lens Diameter: 50 Millimeters
Magnification Minimum: 3 x
Compatible Devices: Rifle
Mounting Type: Weaver Mount
Night vision: No
Model Name: SG3940AO
Manufacturer: Monstrum
Reticle Type: Crosshair
UPC: 810088810800
Item Package Dimensions L x W x H: 15.79 x 3.5 x 2.95 inches
Package Weight: 0.83 Kilograms
Brand Name: Monstrum
Part Number: SG3940AO-B
Included Components: Guardian Scope, One Pair High Profile Scope Rings, One Pair Spring-Loaded Flip-up Lens Covers
Date First Available: June 22, 2022
Frequently asked questions
If you place your order now, the estimated arrival date for this product is:
Monday, Dec 15
Yes, absolutely! You may return this product for a full refund within 30 days of receiving it.
To initiate a return, please visit our Returns Center.
View our full returns policy here.
To initiate a return, please visit our Returns Center.
View our full returns policy here.
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Top Amazon Reviews
Style: Guardian | 6-18x50 AO
My Monstrum Guardian Series AO Rifle Scope with Parallax Adjustment, is a Second Focal Plane scope (my preferred Focal Plane for long-range shooting [750yds+]). The glass is perfect (clear, sharp presentation of illuminated crosshairs, and no halo or fog), and the 1" tube is solid and well made. The 1/4 MOA/click windage and elevation adjustable turrets have screw-on covers to keep your adjustments secure once zeroed, and the clicks are smooth and positive. I have a couple of scopes costing 10 times more than the Monstrum Guardian Series AO Rifle Scope, and I bought it "just to check it out" because of the low-end price. I have used several Monstrum cantilever mounts, and other Monstrum products which impressed me enough to try their optics, and the Monstrum Guardian Series AO Rifle Scope with Parallax Adjustment surpassed my expectations. Monstrum provides an outstanding scope at a bargain price in the 6-18 power range. You are reading this review, and I have NO association with Monstrum other than as a customer, so if you're looking for a 6-18 variable optic, I STRONGLY recommend the Monstrum Guardian Series AO Rifle Scope! If you don't like it and bought it from Amazon, you can return it for credit or replacement.
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Reviewed in the United States on December 5, 2023 by Gil
Style: Guardian | 3-9x40 AO
Scope is clear and easy to move all the adjustments. I'd buy again.
Reviewed in the United States on November 7, 2025 by Susan
I built a AR with parts I had laying around. I usually put vortex on all of my rifles. But being short on cash and not wanting to invest a lot of money in my build. I went with the monstrum Guardian series, I am impressed. I ran somewhere around 200 rounds through it and it held zero the whole time. The clicks in the adjustments was there, but not the verbal way. The illumination knob does have a few spots where it wont light up. Not a big deal for me. If i can get something in low light when it matters, i dont think it will be a big deal. It feels tough, about the same weight of the vortex i usually run on my ars. I will carry this AR with me on the property and beat it around. The life time warranty makes me feel better about if something goes wrong. I gave 4 stars because of the illumination knob not working in all settings and I haven't been able to beat it around enough to see what it will withstand. If your looking for a decent scope for the price. I wouldn't look no further.
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Reviewed in the United States on February 27, 2023 by Amazon Customer
Style: Guardian | 3-9x40 AO
I don’t think it is possible to get a better scope for the money. When my scope was delivered, I was amazed at the quality and features it had. Unfortunately, there was a defect in the scope where something was obstructing the view inside the opticals. I set up a return with Amazon and had a new scope delivered in 3 business days. I could have returned this scope and then bought a different one but I liked the look, the feel, the optics, the lighted reticles and the price. I also really like the lens covers as well. Mounting the scope was easy and the mounting hardware is excellent quality. This scope is perfect for my little .22 rifle and is easy to set up and sight in. The objective control and magnification controls are not the smoothest I have ever used, but the optics on this scope are quite simply amazing for the price. You really cannot go wrong with this scope!
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Reviewed in the United States on June 6, 2023 by Melissa Follett Melissa Follett
Style: Guardian | 4-12x50 AO
This is an amazing scope has great eye relief and it’s a great price!
Reviewed in the United States on September 25, 2025 by Dylan Hawkins
Style: Guardian | 4-12x50 AO
I bought this to replace the Viridian junk that came mounted on my 10/22. It's simply awesome. Great eye box, wonderful eye relief, and that big 50mm objective lets in a ton of light; so much so I haven't even needed the illumination at the range. I will say that there is a lot of light bleed at the higher settings of illumination, but as none is typically necessary, it's not a big deal at all. The throw lever is a bit stiff, but is starting to smooth out under use. The picture is clear at all magnifications, and the adjustable objective is a game changer. I've never used an AO scope before, MD this is just fantastic. Unlike the POS it replaced, I see a clear target instead of fuzzy mush. While I don't know if it'll hold up to larger calibers, for .22 it's fantastic. The price point is almost ridiculous for a scope like this. I would 200% buy it again.
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Reviewed in the United States on January 8, 2023 by Amazonian Customerers
Style: Guardian | 4-12x44 AO
It lasted 1 year with little use and mostly sitting in the gun case. The cross hairs flop around and the illumination only works between settings. It only works when the dial is between the numbers and stops, so when you pull the trigger it turns to the closest setting and turns off
Reviewed in the United States on November 17, 2025 by Thejosher
The glass is nice and clear, great functions and solid price point, also looks great on my 10/22…. However I took it out to zero it and to no avail it will not hold zero. Tried multiple types of ammo and different rifles. I’ve messaged the manufacturer so we will see if they make it right. If so I will update with my findings. UPDATE: Manufacture was great to deal with. They sent me a brand new optic and it holds great. Awesome product for the price.
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Reviewed in the United States on June 20, 2024 by Austin Lee