I took a chance on a used FRanken-AR at an LGS yesterday. Receiver clearly M&P. However, when I said that Smith didn't make a middy, the clerk said that they had done so in the past. Is there anything on an upper that would indicate it as a Smith?

I have a m&p15 that's a mid length. It's a magpul moe mid length version. Reciver is co branded smith and magpul. Not sure if this helps or if they make others.

I also have a S&W Mid-Moe. Great gun. I quickly browsed Smith's current web page and didn't see it listed any longer. However, the 15T II is still available and is a mid-length.

IMO the mid length is much better than the carbine for the following reasons:
1) The Longer sight radius = better sighting.
2) The longer gas tube = higher velocity in same length barrel.
3) Can correctly mount a GI bayonet.
4) longer hanguard provides for better handling or more area for rail mounted devices.

#3 is really high on my list, too.

Just a point here, the longer gas tube means lower gas pressures inside the BCG during operation, more forgiving of ammunition variation. Has no effect upon bullet velocity.

My S&W VTAC II is a mid gas. Rail is marked Viking Tactical. And it has a Comp/FH combo.

ARCHIVE: M&P(R)15 VTAC(R) II Viking Tactics(R) | Smith & Wesson

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Watch & Pray

Alright, it looks like I did pretty well overall, thankfully. Bummer part first.
Since it had the full issue AR buttstock with storage well, the buffer tube is not inletted for an adjustable replacement. It looks like it was installed with no backplate, nor even a castle nut. The spring for the rear take down pin was protruding out, mashed into a perfect L-shape, and I couldn't see the detent pin on the front side. Okay, meh...not too much of a biggie.

Once I got the optic off, I could see that the front of the top rail was branded S&W just behind the Hexnut. So, upper and lower match. Got the handguards off, and turns out is has a nice barrel....
Faxon 16" Gunner Profile AR15 Barrel, 5.56 Nato, Pinned Gas Block - Faxon Firearms

I could see the bolt carrier was branded with an 'A'. Turns out it's this....
5.56 Bolt Carrier Group, Complete - Black Nitride | Aero Precision

Oh, and I had looked THROUGH the optic in the LGS, but not AT the optic. Found that it's this....
Nikon P-TACTICAL Riflescope .223 3X32 | 4.8 Star Rating Free Shipping over $49!

Thanks for the input, folks.

My mistake on velocity, that was assumed on my part since mid length uppers generally require heavier buffers but IIRC there are other factors such as port size and dwell time etc.
The shorter gas system is more punishing on your bcg than a mid length.

My favorite is the 20" Barrel and full stock.

Standard A1 & A2 Stocks use a different buffer tube/receiver extension than collapsible stocks. And the force of the fixed stock pushed against the AR lower is what holds the detent spring in (no castle nut or plate).

There's a lot of schematics you can find on line that shows how these mate together.

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Watch & Pray

My issued M & P 15 is mid length.

Its.........okay.

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Un-Reconstructed Southerner

That is a pretty decent combo. When my AR barrels burn out, they will be getting replaced by Faxon Gunner profiles, or a Faxon Match series.

UNLESS, I can get a barrel built on a Feddersen blank.