I've recently built my lower for my first AR-15  pistol, utilizing a 5.56, 10.5 inch barrel with Shockwave Blade stabilizer, for HD.
I'm thinking to purchase a Complete 16" upper in 300 AAC Blackout so that I can convert my 5.56 10.5" pistol to a 16" AR-15 rifle utilizing an adjustable rifle stock if the need arises.
Is this a 'make sense' route to take regarding conversion for a situation which warrants longer range and more power, and would I be within the law as long as I don't put the rifle stock on my pistol?
I don't have the $ for 2 individual AR's, which is why I thought this solution would cover all bases.

1. Legally, you can't. Once a pistol lower is converted to a rifle, the lower can no longer be converted back to a pistol. I'd suggest just buy a 300AAC upper with 10.5 barrel so that you can have a 300 AAC pistol.
2. The 300 AAC is an expensive round to shoot.
3. Depends on the 300 AAC type, subsonic or supersonic, your buffer, weight or bolt carrier may need adjustments.

I appreciate the heads up. To clarify, I purchased a Stripped Lower which was listed as such on Form 4473. I've read that if I purchase a Stripped  Lower which I make into a Pistol, I can change the upper or barrel, as long as the Barrel is at least 16" in length and convert the Pistol to a Rifle, and back to a Pistol as often as I want to, as long as I don't put a Rifle stock on the Pistol.
I cannot convert an SBR back to a Pistol and I cannot convert a Rifle listed on Form 4473 to a Pistol. But I can convert a Lower which is on Form 4473 from a Pistol to a Rifle, and back & forth.
Is the above accurate?

If your receiver started life as a rifle, it can never be a pistol.

If your receiver was first built into a pistol, you can change it into a rifle and back into a pistol. Just make sure you do not have the stock on it with a barrel less than 16 inches.

If I were you, I'd just be looking for another lower on black friday. PSA should have them dirt cheap, your going to have to jump on the deals though.

Thanks for clearing up the misinformation.

I posted above what I have researched. Thanks for confirming that info.
I am picking up my 2nd Stripped Lower which my FFL dealer just received. But instead of building that lower and installing a complete upper, I figured I'd save a few bucks and purchase just the .300 AAC Blackout upper and swap out the Uppers whenever I want to.
Considering that I've already purchased my 2nd Stripped Lower, the additional costs to build out the 2nd Stripped Lower would be approx. $140, consisting of $75 for the AGL ACT Trigger assembly & $45 for a CMMG LPK. I can see your point on just having a 2nd AR, vs. just getting a 2nd complete upper to convert.
If I'm going to build a 2nd AR-15 for longer range and power, is the .300 AAC Blackout still the way to go?

I like my .300 it is a great deer gun out to about 150 yards, now you can shoot farther with it, I have seen videos of guys shooting out to 5-600 yards with them, but I keep mine to around 150 max and it works out fine.

Dave do you have any chrono data for the .300blk in the 16"?

1. This is completely false.  Built as a rifle it cannot be converted to a pistol, however built as a pistol, it can be made into a rifle and then back in to a pistol and no law was violated. Look at the Glock and M1911 carbine uppers as an example.

2. 300blk can be expensive, but for factory the price is coming down and availability is increasing.  Hand loading is makes it as cheap as anything else.

3.  If you stick with a standard buffer and get a 300blk upper with proper gas hole placement and size, then you will have no issues.

We were talking about that the other day in the reloading section, Using H110 Nosler ballistic tip near max load, I am getting right around 2250 to 2300 on the chrono, all of my notes are out in the shop, I will have to double check my numbers but those are right around what I got last time I ran the chrono.

In fact I just noticed you were asking in that thread as well, I just went out and got my notes, I am loading 18.2 gr of H110 behind a Nosler 125 gr ballistic tip and get a small amount of variance depending on conditions.

This has worked out as a good deer hunting round and cycles with no problem in my rifle I built for that purpose, running a standard carbine buffer, 16" ballistic advantage modern profile barrel with pistol gas, shooting iron sights.

I thought you were saying you had one complete rifle and wanted another upper. Now I understand that you will have 2 rifles plus another upper? You can definitely save money that way, I'd probably eventually buy another cheap lower but thats just me. Nothing wrong with multiple uppers for one lower.

5.56 has better performance at longer ranges than .300 does. The main advantage of .300 blackout is its ability to reliably expand out of a shorter barrel than 5.56. Personally, I (probably) wouldn't build a 16 inch .300, but thats just personal preference.

Ammo availability is increasing, and if the hearing protection act passes we might see the popularity of the caliber expanding too, and its availability with it.

Sorry for the late reply. Subscription alerts weren't working.

Here's an update after researching more:
I decided to put together another AR even though I don't have the disposable money. I'll be speaking to my shrink about this
- I've assembled a 2nd lower and am awaiting my 8.5" 300 Blackout upper from PSA. Once the upper is received, I'll be good to go to the range with my new pistol, unsupressed.
I'll be using a FailZero Exo Coated BCG.
- in the meanwhile, I've decided on the ammo for HD I'll be using. Lehigh Defense 194gr maximum expansion. I'll also be calling Lehigh Defense with a couple of questions.
As for range ammo, the 194gr at almost $2/rd is a little much. I'll shoot several rounds just to make sure I don't have any issues. I purchased 80rds of HSM 300 AAC 220gr subsonic @85¢/rd for the range. I'll also purchase some supersonic for the range.
I have a carbine, H and H2 buffer I'll be trying. The carbine buffer should be fine but I want to have other buffers available just in case.

If I had to assemble all over again, I'd have first put together a 300 AAC Blackout for HD and a 5.56 w/16" barrel for longer range if ever needed. My first AR pistol was a 10.5" 5.56. I went to the range but had a bunch of failure to fire, feed and extract issues. PSA determined it was a bad BCG, and I am awaiting the return of my upper.
My next purchase will be a 5.56 16" upper to convert my 10.5" 5.56 pistol to a rifle, and back and forth as needed.
But first I'd like to shoot my 2 AR pistols, without issue