What I mean is the stock currently known as the M4 stock didn't exist until 2002 according to one source I found (that seems later than I remember though). So up until that point all stocks, be they commercial or mil spec had flat backs because all the collapsible stocks up to that point were flat, not angled. Commercial stocks have been made since at least the 80s.
Okay, I finally found some time to answer this.
The current M4 stock is fairly recent, but collapsible stocks on guns like the XM177 existed in the 1960's. They were originally all based on what are now called the Mil-Spec dimensions, as were the original lower receiver extensions (hereafter called tubes).
The Commercial dimension tubes (and stocks) appeared some years later, as a result of manufacturers attempting to save a few $ on guns built specifically for the civilian market. The tubes are thicker, because the extruded 6061 aluminium that they are made from, is weaker than the original spec, which called for forged 7075 aluminium.
There may well have been some flat ended Commercial spec tubes made originally, but most of the ones that have been made, have slanted ends, to the point where almost every stock manufacturer put out a photograph or diagram similar to the one posted above, and the slanted v straight end became the go to difference to look for when calipers are not available. I have never seen it written anywhere, that the reason for slanted ends on Commercial tubes was specifically due to the slanted butt plate of the M4 stock, especially given that it was originally designed for military use, and in Mil-Spec diameter.
Recently, more Commercial tubes have appeared with flat ends, so the old standby of slanted v straight no longer applies. Additionally, some manufacturers are putting out tubes made from extruded rather than forged 7075. These are Mil-Spec diameter, but are not the same as Mil-Spec.
What difference does it make? For civilian users, probably most will never notice, unless they have to mortar the gun to remove a stuck cartridge/case. However, manufacturers who skimp on the tube, have a tendency to be a bit fast and loose with other parts as well.
If you want to be sure, then measure the diameter. If you don't have any means of measuring, then look at the threads.
I don't have time right now to dig through our library of two dozen plus books and manuals on the AR15/M16/M4 FOW for exact dates, but that is the general story as I understand it. If you have access some written material regarding the origins for the slant, I would be interested to read it.
Regards.
Mark
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