I bought a complete AR built from Aero-Precision parts, with a 10.5" upper.

I wanted a rifle so I bought a collapsable stock and 18" upper.

I've changed the uppers - that was easy - but I want to make sure there is nothing else I need to/should change. The 10.5" has carbine-length gas tube while the 18" has rifle-length. That's as expected. But there's nothing else I need to change - in terms of the gas system or anything else - right?

I'm an AR newbie and just want to make sure this thing is put together right before the chamber sees 55,000 PSI.

Maybe get want a gunsmith to check headspace if the 18 was not a completed upper.

It was a complete upper, brand new. Fit seemed perfect. (And to be clear the lower, old upper, and new upper are all Aero Precision. Both uppers are M4E1.)

All I had to do was move the charger and BCG from the 10.5 to the 18. I don't think there's a "wrong" way to install the BCG. It fit! I watched a couple vids before I did it.

You might possibly have to change your buffer weight, maybe not

Thank you - can you please elaborate?

Shoot it.  There’s nothing that could need changed that would cause serious issues or damage.  If the gun functioned with the 10.5” upper and the new upper has been assembled correctly, there’s a 99.9% chance it’ll reliably run as is.

Gas system length doesn’t matter as far as your situation is concerned.  A lower is a lower, so no problems there (so long as it was functioning prior to the upper swap).  You can check headspace, but also a 99.9% chance you’re fine.  I’ve never checked headspace on 5.56/.223 ARs and have never had problems.

The important thing is having the correct spring and buffer for the stock you have installed, the length of the gas tube is not important.
Rifle stock requires a rifle spring and buffer, collapsible stock carbine spring and buffer, you can play around with different weights if you want but as long as the rifle functions you are GTG.

“Have to” is a bit of a misnomer, as the chance you absolutely need to change buffer weight is extremely low.

Shorter barreled guns do tend to use heavier buffer weights to bandaid the decreased operating envelope with shorter gas systems with larger gas ports.

An 18” RLGS barrel will obviously have a longer gas system and will likely have a port sized more appropriately for that barrel length.  This will usually mean a smoother cycling action with the gas port pulling gas from significantly further away from the chamber, meaning way less bore pressure and less “violent” function.  The only situation/issue you may run into is if the original shorty upper had a hogged out gas port requiring an H3 (or similar) buffer and the new upper ends up being gassed lightly.  In that scenario, the buffer may be too heavy, resulting in short stroking type malfunctions.  This could only occur in lower temps or with lightly loaded ammo, or it could be a constant issue.  You won’t know until you test it out, though.

You could weigh the buffer originally in the gun to find out the answer as to buffer weight.  If you don’t have a scale that reads down into the oz range, drive the roll pin out of the back of the buffer and pull the rubber buffer stop.  Inside will be 3 weights and you can tell which one you have based upon the ratio of tungsten (heavy) weights to steel (light) weights.

Thanks for all this great info. And, I've been reading about the AR buffer system for the past half hour.

It does seem there is very little chance I'd need (or even want) a change because even the "pistol" A-P lowers have carbine buffer systems from what I see on their site. For example:

https://www.aeroprecisionusa.com/m4e1-pistol-complete-lower-receiver-magpul-moe-grip-black

"AR15 Carbine Buffer and Spring"

There is another twist, though. The box the lower came in says this:

M4E1 Pistol
Complete Lower
Receiver w/MOE
Grip & BSL Brace
489015

BUT the receiver extension is *not* the pistol type, but the carbine type (with the detented ridge on the bottom for adjustable stocks) as seen here:

https://www.aeroprecisionusa.com/m4e1-carbine-complete-lower-a2-grip-anodized

I don't know now if the private seller changed the extension or gave me the wrong box for the already-assembled weapon.

Anyway given the fact that ALL of their lowers have "carbine" weight & spring there should be no issue.

Ok, this is a really important point. By "stock" you must mean receiver extension. I didn't change that.

The Magpul BSL mounts to a standard carbine receiver extension, so if it shipped with the BSL, that’s what type of RE it came with.

Older non adjustable braces typically used pistol REs.  Initially some companies came out with pistol REs with built in flanges/stops somewhere on the tube to increase LOP.

Modern day, most manufacturers have went the adjustable route.  A lot utilize standard carbine REs to offer that adjustability because it made things more simple and less expensive, although some developed their own proprietary REs.

Yeah, it came with a Magpul BSL!

I threw that away when God (the ATF) changed some things.

As mentioned in this thread, you are probably fine when it comes to head spacing but your scenario is exactly what Urimaginaryfrnd is warning you about, using a bolt that didn't come with the barrel. Personally I wouldn't worry about it, but just because your old bolt and carrier fit into the new upper does not mean the head spacing is good. You need gages to check the critical dimensions between the bolt face and chamber.