Simply, why the difference?

Why was the commercial buffers tube developed with different dimensions to the mil-spec.

I am buying a Colt 6920 and know that the Magpul stock with its commercial dimensions will not fit.

I'm not certain, but I'll bet it has something to do with $$$$$.  Are you saying a Magpul commercial stock won't fit, or a Magpul mil-spec stock won't fit?

Commercial buffer tubes are extruded 6061 aluminum which is a cheaper manufacturing process. All about the money.

a mil spec stock will not fit on a commercial buffer tude and same for a mil spec tube wont let a commercial stock work

Notice difference in external diameter, as well as difference in threading.  The threading in commercial tubes is equal to, or smaller than, the overall tube diameter.  Threading on mil-spec tubes is "raised", or larger than, the overall tube idameter.













 

 

double tap

Ease of manufacture.
Just say no to commercial.

In the late 70's and early 80's there were no milsurp CAR stocks to be had and the AR industry was in its infancy.  
Some one(s) stepped up to answer a small demand for the stock and it was born.

I forget who the maker of slanted back tubes was but they do not represent all tubes. The (commercial) tubes I have in my box o'stuff all have flat backs and full threads. There are examples from the 70's, 80's, 90's and 00's

As far as "milspec' vs commercial tubes go, a thin wall tube of 7075 should be as strong as a thick wall tube of 6061.
The only difference that should make is the 6061 tube should bend a little farther before breaking with the same load applied to both tubes.

Impact extrusion is only one method of forging  [youtube]https://www.youtube.com/watch?v=zMhVPId-pRA[/youtube]
Many will beat the drum that milspec tubes are ( drop) forged and machined  like uppers are but the claims have never been substantiated.

A half rack of craft beer to whoever posts a milspec or letter stating the manufacture process of how milspec tubes are to be made.

All bets are off with airsoft stuff.


What I am saying here is no matter which tube you have its not like its going to fall off your rifle and leave you as buzzard bait on the eve of some zombie apocalypse

There have been more reports of broken bolts in the last month on this forum than broken buffer tubes in the last couple years.

It's unlikely that I would ever have to use my buttstock for a weapon.
My issue with commercial is that it introduced an unncessary second standard.

Mil spec ID buttstocks as well as Commercial ID buttstocks.
Confusion and more cost for everyone. See below.



Now here the OP seems to think that all Magpul buttstocks are commercial.

Generally speaking commercial threads are cut and Mil-spec tubes have the threads rolled on.
 

Back when commercial stocks emerged there were no mil dimensioned stocks available on the market.
Why Colt would not sell them aftermarket and why commercial stocks went with a larger O.D. is any ones guess.
Up until just a few years ago commercial was the defacto standard of the civilian market.

I don't think it matters either way except if a person is building a detail correct XM177 or M4 clone.

Do a search in the uppers and lowers forum. Read the FAQ.