Looks like Rob_s intended for this to be a living thread, so here's my contribution to the knowledge base.
Inspired by recent pricing, I ordered 3 of the Surplus Ammo & Arms stripped lowers from AIM. Soon thereafter, Palmetto State Armory started an irresistable $50 sale on their marked lowers as well. After shipping and transfer, my final cost was $72.28 for each SA&A and $65.33 for each PSA.
Now that I finally have both sets of lowers in hand, a comparison might be helpful or interesting.
Just to disclose where I'm coming from, I know that this review compares a variety of inconsequential cosmetic minutia. To be clear, I'm not building any kind of safe queen on a $50 lower. I only present this information as a resource, for those readers to whom these details are of interest. I also apologize in advance for my lack of knowledge of the correct nomenclature (what is that ring where the receiver extension mounts called anyway??!?)
Corrections and comments welcome.
SA&A (top); PSA (bottom)
Both lowers are reputedly made by Aero Precision. In fact, when I contacted AP on Facebook regarding the SA&A lower, AP responded and accepted the compliment. However, there are several differences that make me question whether both lowers are from the same manufacturer.
MARKINGS
SA&A (left); PSA (right)
The lettering and pictograms on the SA&A are markedly (no pun intended) deeper and more distinct than the PSA. Also, the pictograms are larger on the SA&A.
SA&A (top); PSA (bottom)
Note the keyhole marking on the SA&A immediately in front of the trigger well, absent on the PSA. My AR arcana is weak, but it is my understanding that this indicates that the raw forging came from Cerro Forge, an established and respected maker.
TOP
SA&A (left); PSA (right)
Above you can see a difference in the shape of the void adjacent to the selector holes. The SA&A has a distinct bulge machined in. This does not seem to be a high-shelf/low-shelf issue. Any ideas on the function of these two shapes?
SA&A (left); PSA (right)
Another minor difference is the shape of the reinforcing ring around the receiver extension mount.
ANODIZING
The tone and finish of both the PSA and SA&A lowers is very similar. Side-by-side and in indirect sunlight, the PSA seems very, very slightly more matte and a somewhat colder/bluer tone than the SA&A. However, this isn't to say that the SA&A is glossy. The differences are very slight and are not especially visible in the photos. Both seem a little more matte than my LMT lower or BCM uppers, but those have all seen a lot of oil, dirt, dust, powder, abrasion, etc. I mention this just for the record, since the obsession with color-matching uppers and lowers seems IMHO a huge affectation, up there with Accuwedges, color fill, upper-to-lower slop, "does this company's coyote match Magpul's FDE" and other such non-shooter interests.
I hate to note it, but all 3 SA&A lowers have several areas of thin anodizing where the silver-toned metal is showing through. I know that anodizing is neither spray nor paint, but the subjective effect is a hurried spray paint job. To be fair, all areas of poor anodizing are in places that would not be visible once assembled.
All three SA&A lowers have metal showing through between the front and back trigger guard ears, around the PG near where the selector detent spring inserts, inside the bolt catch channel, inside that mysterious shallow hole where the receiver end plate covers, and inside the well near the takedown pin. The PSA lowers are heavily and evenly coated in all of these areas.
Selected detail shots of the gaps in the SA&A anodizing:
MACHINING
Both are very cleanly machined, at least as good as my LMT and much cleaner than many Colts I have handled. IME a good place to look for this is on the front pivot lugs, the front of the magwell, and inside the trigger well. If anything, the SA&A is slightly smoother in the front pivot lugs and on the "ceiling" of the trigger well. In the curved seam behind the grip (what I call the "beavertail" area for lack of a better term) the PSA is smoother, where the SA&A displays a somewhat prominent flash.
SA&A (left); PSA (right)
MAGWELL
The only potentially significant difference between the two is the flaring of the magwell. Both lowers easily drop PMags, with the PSA being slightly more generous fit. However, the PSA magwell is significantly more flared than the SA&A, even on the front wall which you don't see too often.
Both pics: SA&A (left); PSA (right)
One other functional difference I found was that the channel for the takedown pin detent spring is threaded for a 4-40 set screw on the SA&A. This is a nice touch, but one that I will not utilize since I don't change receiver end plates too often and because I am not completely comfortable with trimming a coil or two off of the spring to make it work. The PSA is not threaded for this set screw.
SUMMARY
Both are very nice receivers. I've only assembled an LPK on one of the SA&A so far - everything installed easily and the lower fits and runs great with either of my BCM uppers. I expect the PSA to accommodate an LPK just fine. However, the much more generously flared magwell on the PSA is a deal-clincher for me so for my use, the PSA lowers will be getting the better furniture and more frequent use.
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