Search  for anything...
Monstrum

Monstrum Guardian Series AO Rifle Scope with Parallax Adjustment

  • Based on 1,371 reviews
Condition: New
Checking for the best price...
$79.95 Why this price?
New Year Deal · 53% off was $169.99

Buy Now, Pay Later

As low as $13.33 / mo
  • – 6-month term
  • – No impact on credit
  • – Instant approval decision
  • – Secure and straightforward checkout

Ready to go? Add this product to your cart and select a plan during checkout.

Payment plans are offered through our trusted finance partners Klarna, Affirm, Afterpay, Apple Pay, and PayTomorrow. No-credit-needed leasing options through Acima may also be available at checkout.

Learn more about financing & leasing here.

Selected Option

Free shipping on this product

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.

To initiate a return, please visit our Returns Center.

View our full returns policy here.

Availability: In Stock.
Fulfilled by Amazon
Arrives Friday, Feb 13
Order within 7 hours and 9 minutes
Available payment plans shown during checkout
Protection Plan Protect Your Purchase
Checking for protection plans...

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: Friday, Feb 13

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.

  • Klarna Financing
  • Affirm Pay in 4
  • Affirm Financing
  • Afterpay Financing
  • PayTomorrow Financing
  • Financing through Apple Pay
Leasing options through Acima may also be available during checkout.

Learn more about financing & leasing here.

Similar Products

Top Amazon Reviews

    • MONSTRUM ALWAYS PROVIDES QUALITY AT AN EASILY AFFORDABLE PRICE!
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. ... show more
Reviewed in the United States on December 5, 2023 by Gil
    • Excellent Scope
Style: Guardian | 3-9x40 AO
Review Image by Melissa Follett Melissa Follett
Review Image by Melissa Follett Melissa Follett
Review Image by Melissa Follett Melissa Follett
Review Image by Melissa Follett Melissa Follett
Review Image by Melissa Follett Melissa Follett
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! ... show more
Reviewed in the United States on June 6, 2023 by Melissa Follett Melissa Follett
    • Optic glass clarity surprised me and under $100 shipped taint bad!
Style: Guardian | 4-12x44 AO
First day: Surprisingly good optic for my Ruger Mini 14 in .223/5.56. shooting 100-200 yards. This scope will obviously shoot farther. Under $100 shipped this Monstrum 4x12x44 replaced a Banner 3x9 and exceeding my expectations. You could put this scope on many short action rifles and even an AR15. It's a good choice for and varmint plinking rifle. A 4x16x44 optic might be bit of overkill for 22LR? Feel and quality of turrets and dials above average. AO adjustment is easy peasy. Lighted reticle "fair in bright sunlight. Only issues: 2032 battery dead for lighted reticle (20 seconds to replace) and the Scope rings included were too tall and cheap. If issues occur, I'll update post. For now... ++Recommend++ ... show more
Reviewed in the United States on February 10, 2024 by GE Utah
    • Good purchase
Style: Guardian | 3-9x40 AO
Works as intended but don't think your American sniper with it but its definitely worth it for a starter scope easy to install and fits perfectly for my 270 Remington
Reviewed in the United States on December 10, 2025 by Jay
    • Good scope.
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
    • Economic choice
Style: Guardian | 6-18x50 AO
Lotta scope for the money!
Reviewed in the United States on December 18, 2025 by Michael
    • Monstrum Guardian= Junk
Style: Guardian | 4-12x50 AO
This scope came highly recommended from piers, I am not sure if I had a faulty scope or not, however I went through 2 boxes of ammo after getting it professionally mounted and bore sighted. So here is the story.. Went to local gunsmith got 30-06 with Guardian mounted and bore sighted and I asked for him to come with me to the gun range so we took off to shoot. Set target up at 100 yards and got it adjusted only after 3 shots then fired 4 more, perfect bullseye 3 out of 4 shots. Move target to 200 yards first shot low right by 4" to the right and 5" low (weird) shoot again and this time 3 out of 4 shots never hit the paper. I let the gunsmith try and same results. We tried a few more shots then back to 100 yards and we were all over the paper at 100 yards. No consistency. Back to the gunsmith shop, bore sighted and back to the range once again pretty much the same thing! 300 yards and not even a fracture of a bullet hitting the paper. This was done in a vise with two different people. ... show more
Reviewed in the United States on December 26, 2023 by Amazon Customer
    • Awesome
Style: Guardian | 4-12x44 AO
Great scope
Reviewed in the United States on November 2, 2025 by chris burnett

Can't find a product?

Find it on Amazon first, then paste the link below.
Checking for best price...