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  • Post last modified:August 18, 2023

If you have purchased an aftermarket rifle stock that doesn't fit the buffer tube on your AR, Here is why.

There is a difference between Mil Spec VS Commercial Buffer tube sizes

  • Mil spec stock ONLY fit on mil spec buffer tube
  • Commercial stock can fit on commercial AND mil spec buffer tube

Tighter the fit, less stock wobble!

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Major Spec Differences


Referenced Max Blagg

The difference between mil spec and commercial buffer tube are:


buffer tube diameters

A mil-spec buffer tube

  • Cross Section Diameter: 1.14"
  • Length: 7.25"

It's made from 7075-T6 alloy, which provides 1-3 and 16-16 UN thread rolls and it's shorter than the commercial with a flat end.

General Accountability Office (GAO) defines requirements and standardized measurements of equipment used by the military to guarantee interoperability, commonality and reliability for each part purchased for military use.  

A commercial-spec buffer tube

  • Cross Section Diameter: 1.17" (thread height accounted for)
  • Length: 7.25"

Much cheaper as the extension is manufactured from an extruded tube. The tube features a welded-on cap, normally made from a lesser metal.

The 5 degree slanted back is another way to identify a commercial tube just by looking.

Castle Nut

ar15 castle nut
fix it sticks castle nut

All reciever tubes are threaded the same  1 3/16" x 16 tpi

All steel castle nut will fit even if some companies labeled them differently for marketing purposes.

When torquing the steel castle nut on aluminum buffer tube thread to 40 ft pound, always apply some anti seize compound on the threads to prevent material galling.

Always stake the castle nut to the end plate at least two locations, to displace enough materials into the notch to secure the buffer tube to the lower receiver.

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Is One Better Than The Other?

buffer tube assembly

No. There is no functional differences between the two.

The buffer spring works exactly the same when cylcing.  Even tiny differences in dimensions aren't enough to cause any issues with functionality.

magpul CTR stock black

Some mil-spec buffer tubes are stronger based on the tensile strength of the aluminum used. However, there is also no reliable case studies showing that commercial spec tubes are structurally inferior.

Mil-spec buffer tubes do have more options in the marketplace for stocks. While some of the larger stock manufacturers, such as Magpul, offer many of their products in both mil-spec and commercial.

Before buying stock upgrades, please confirm the specs they offer.

Newer Designs

sig mcx rattler with sig romeo8t

Not all AR15 features fixed buffer tube system. 

brn 180 upper receiver

Piston driven AR style rifle like Sig Sauer MCX Spear LT, BRN180, CMMG DISSENT feature foldable stock.

law tactical adapter

Foldable hinge buffer tube system like the LAW Tactical still requires a buffer tube, but the rifle is NOT functional until the adapter is closed before firing.