Tightening the Fitment Between the Upper & Lower on your AR15 Rifle

AR15 Rifle Upper and Lower
AR15 Rifle Upper and Lower
Shield Tactical
Shield Tactical

Shiner, Texas –-(Ammoland.com)- As an AR-15 builder and especially as a builder in the retail market, Shield Tactical has seen just about every mix and match of upper and lower receiver you can imagine.

The fit is never the same even when the fit is good. Because of the nature of how the pieces go together and because of the acceptable variances in manufacturing, you will almost never be able to replicate the fitment of a receiver pair.

For example, a customer of ours was so pleased with the way his Spike’s Tactical lower receiver fit on his Bravo Company Manufacturing upper that he wanted to build another gun with the same pair. When the receivers came in, the pieces fit together as well as could be expected from unmatched pair, but they were not as tightly fitting as his original gun.

It is no slight on Spike’s Tactical or BCM. Both are high quality manufacturers that build excellent parts and well within tolerance. In fact, most manufacturers will bias the fit slightly towards loose to ensure fit regardless of the mix.

As a military armorer, to be able to swap and mix parts from different manufacturers of the same weapon system is an invaluable characteristic that saves on cost, training time, and reduces the logistical footprint of having to manage and maintain these guns.

AR15 Lower
AR15 Lower

A Colt lower fitted to a Sabre upper with an FN barrel and CMT parts should have the same relative fit and performance as an FN lower with a Sabre barrel and a Colt upper. So generally speaking, because these parts must fit each other and must do so every time, the manufacturer will be sure there is enough room for pieces to reliably fit together.

Now that isn’t always the case with manufacturers of complete guns, especially in the consumer (non-military, non-LEO) market. LaRue Tactical goes the extra mile and makes sure their guns are built to such exacting tolerances that a new gun will almost certainly need a tap or two from a small hammer and nylon punch to break the takedown pin free.

It is that kind of attention to detail that will result in the reputable performance that Mark LaRue’s guns always seem to deliver.

Complete manufactured guns will tend to have a more reliably tolerable fit as well as matching finishes and most manufacturers understand that blasting and anodizing of receivers will often need to be done in the same batch in order to have exactly matching finishes. Of course some finishers are so dialed in they can achieve very close finishes in even separate lots.

Does that mean your “Frankengun” will not achieve the same performance as an out-of-the-box rifle?

Your home built rifle has the same potential for accuracy and consistency that any store bought gun does. The real question is how to harness that potential. To continue reading…

About Shield Tactical;
Shield Tactical is a family business. Though we are not all related by blood, the bond of 2nd ammendment supporters is just as strong. My name is John W. Harrington and I am the president and founder of Shield Tactical. I have been involved in student based firearm instruction for over ten years. Over that period of time, my philosophy has been simple – make it about the shooter. Every member of our team has heard me say (more than they care to count) “People don’t come to us to hear about how great we are or what we have done, they come to us for what we can do for them”. Visit: www.shieldtactical.com

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I found this to be a more discreet fix than either the big orange ear plug or the o-ring route. Turn over your upper and see the milled obround slot recessed 1/8″(approx) deep toward the block drilled for the rear pin. In a UK DIY store I found a blister pack of various cistern/watertank ballcock seals. Around 1/8″ thick, black, solid rubber seal of a diameter that would spring/press fit this circular recess toward the rear pin. You could make one easily. When closing the upper, a very light squeeze together is needed to get the rear pin in or… Read more »


Is it possible for you to:
(1) post the name of the manufacturer of the ballcock seals you described?
(2) post a picture of how the are mounted to your rifle?

Thank you in advance!


There is not enough width along the ledge of the slot or “sear-relief” to safely support anything pliable that is under the pressure of closed receivers. Chances are very high it will ooze out. Anything pliable has to be bonded to something solid to prevent ooze. The reason I know this is we manufacture a product called an AR15 Flat Jack. It is formed high heat rubber bonded to a piece of shaped aluminum that friction fits the sear relief.. The drawback is achieving a good reliable relief fit. A fit that will not allow the quick removal of the… Read more »


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