The Armalites were designed and manufactured back in the mid-90s, when the AR10 was essentially defunct and collectible samples from the 60s were all that was available. Stoner had started making their SR25 reborn AR10, and had utilized the original magazines, which were not easy to come by at all, and no one was making new ones yet. So when Armalite decided they wanted to market a more affordable AR10 for the modern age, they faced either using original and expensive AR10 mags that were in very short supply, or developing new tooling for making new AR10 mags, or adapting an existing magazine for affordability's sake.
Thus, with cheap Korean M-14 mag copies available, Armalite decided to merely contract a version of those, which was the most affordable option at the time.
Not long after that, DPMS decided to compete in this game as well. But by the time DPMS got involved, demand for genuine AR10 pattern mags from Stoner customers who had gone with the SR25 was so high that even DPMS had been approached about manufacturing them. So when DPMS jumped in, they not only did so while designing their variant to take the original mags, they also began to source newly made ones as well. Not long after that Magpul came up with theirs.
As we have seen over the years, just about all the makers of AR10-type rifles have gone with the "DPMS-pattern" lower receiver design. (A few out there went with the Armalite-type of "slant-back" receiver, but even those all went with the original magazine pattern.)
And so nowadays, the Armalite versions are kind of oddballs and not similar enough to the others to be able to utilize any parts commonality. (Not that there is much between all the others to begin with.) Now the modified M-14 Armalite mags are harder to find and cost more than the ones everyone else is using,...and that more than anything else is what impacts the price and values on the Armalites in my humble opinion. They are decently made guns. But if I have to spend more money to get mags for them, that will affect my decision to get one vs. another option.
