FAQ/Contact Us

Modulus Arms was founded by AR enthusiasts with highly technical engineering backgrounds. Our products are designed for ease of use with the end-user in mind, and employ decades of engineering experience. We believe our 80% Lower AR Jigs and related accessories are amongst the highest quality on the market, with unmatched functionality and fitment.


Our customer support team understands the importance of post-sale support, and aims to educate our customers on how to get the most out of their Modulus Arms products. Our mission has always been to take the guess work and hassle out of fabricating 80% lowers.


We have achieved success through expert product design and simplifying the end-user fabrication process, while maintaining the perspective of being enthusiasts ourselves.



Please review a selection of Frequently Asked Questions.

Q: Is [insert product] in stock?
A: All products are in stock unless otherwise noted. The product page will display an estimated ship date for out of stock or backordered items.

Q: What is your return policy?
A: Please review our shipping and returns page: Shipping & Returns. Returns without an RMA will not be accepted and returned to the customer. Please contact us for a RMA number and return shipping address.

Q: Why hasn't my order shipped yet?
A: With the exception of out of stock or back ordered items, Modulus Arms aims to ship orders within 3 business days from the original order date. Transit times will vary based on your geographic location. View our Shipping and Returns page for more information: Shipping & Returns.

Q: How does the Modulus Arms jig compare to [competitor's] jig?
A: It doesn't. Modulus Arms Universal Jig products are the only truly universal router based milling jigs on the market for AR-15 and AR-308/AR-10 80% lower receivers.

Q: How many 80% Lowers can I complete with my Modulus Arm's Universal Jig?
A: Both the AR-15 and AR-308 Jigs should be capable of making well over a dozen lowers with the parts supplied with the Jig. If you need to make more than that we offer replacement pieces to be able to make as many as you want without replacing your entire jig. The Heavy-Duty Universal AR-15 Jig has steel wear surfaces making it the most durable Jig available. NOTE: Replacement parts will only be available for the Router Jig Extreme. Previous Jigs do not have replacement parts. 

We have switched to exclusively producing the Router Jig Extreme, which is significantly faster and easier to use than our previous jig models. We apologize for the inconvenience caused by not having these parts available. 

Q: Can I use a hand drill to complete my 80% Lower Receiver?

A: The Modulus Arms Jig is designed to be able to complete your lower with a hand drill. The drill guide will wear out faster than if you use a drill press but you should have no issues with your build. If you are going to use a hand drill we recommend placing the jig and vise on the floor when drilling the holes as it makes it easier to keep the drill upright. The drill MUST be kept upright to avoid premature wear to the jig. Contact us for my help or hints.

Q: How long will my Solid Carbide End Mill last?
A: The end mill will outlast the jig if used properly. It will break well before it will wear out. Modulus Arms as well as many customers have completed dozens of lowers with a single end mill. The end mill can break if the process is rushed or the instructions are not followed.

Q: Is there anything special about the end mill needed? Will my [insert end mill] work on your jig?
A: The Speedmills last 4-5 lowers on average, but can last many more if properly taken care of. The key to longer endmill life is taking slow shallow passes, and using a lot of lubrication. The faster and deeper you go, the faster your bits will wear out!Q: What router should I use?

A: We recommend the Dewalt DWP611, Craftsman 28212, Ridgid R24012, Makita RT0701C, Porter Cable PCE6435, Dewalt D26670 and Porter Cable 7310. Harbor Freight and Black and Decker have been known to have poor results. There are many that will work and the important thing is that is has a 1/4" chuck and is variable speed. For more information check this link.

Q: What speed setting should I adjust my router to?
A: Each router is different. We recommend a setting  of "5" or towards the middle for most routers.  You should start slow and ramp it up until it feels smooth and relatively easy to move. Take your time. You will know when it is right.

Q: What's the best way to mill with the router?
A: You should always start in an open hole without touching any material. This is why you drill the holes. You are very likely to break the end mill if you start touching a side or turn the router on before you place it into the jig. The router should never be spinning when pulling it out or putting it into the jig. When the router is fully up to speed in the jig you should first try to join all the drill holes taking small semicircular cuts trying to cut with the side rather than the front of the end mill. Continue doing this until all the holes are joined. When they are joined go around the larger hole you created until you are touching the template.

Q: My router isn't cutting very well, what is wrong?
A: Check to make sure aluminum isn't clogged in the end mill. You should check after each pass or if you hear an audible change in the noise or feel of the router. Use more cutting fluid. Attach a shop vac and suck out the chips as you cut them. Check to make sure you did not break the teeth off your end mill. Take shallower depth of cuts. Slow down or speed up the router speed.

Q: Why is my jig different than the pictures on the website?
A: The jigs are being updated and changed from time to time to make them more accurate and easier to use. The pictures on the website are not updated right away. You may have a newer version jig.

Q: Can I have a Military/Fire/Police/EMS discount?
A: Yes, we offer a 10% discount to all Military (active duty and veterans), and active Fire, Police and EMS personnel. Use the form below to contact us for details. Cannot be combined with any other sales, deals, coupons or similar.

Q: I damaged my lower during milling, can I have a new one?
A: We will be glad to offer you a replacement at a discounted price. We offer 20% off a replacement if your lower was damaged beyond function. We do not offer a discount for cosmetic issues that will still allow the operation of the lower.

Q: Do you share or save my customer information?
A: We do not sell or share any customer information with anyone at any time for any reason. We are not able to store or view your credit card information it is securely processed with via We may need to contact you from time to time if there is an issue with an order. This is done via email or phone.

Q: I live local to your location, can I pick up in person?
A: We prefer to do online sales only.