So i just ordered parts for my first pistol build. Aero lower, DPMS LPK, and a Shockwave brace. I will replace the grip but until I shoot it I won't know what I want. I have a PSA Enhanced trigger group (from when they first were released) that I will probably drop in it.

Upper is a whole 'nother beast. Not sure on length yet or handguard. It's going to be a PDW and HDW. Suggestions on how to top the lower off?

What caliber?  

For 5.56 people are generally happy with 10.5" to 11.5" to maintain some performance and reliability.  For 300 blackout 8" to 10.5" is common.  For pistols and SBR's I really like what 300 blackout brings to the table.  The cartrige was designed around short barrels and to work well with supers and subs.  I have a 10.3" Ballistic Advantage Hanson 300 blackout build that I really like.

I would not criticize anyone for what they build or use, this is not that.
I would personally stick with 5.56, especially for your only pistol.
300 Blackout costs more and adds confusion / danger and another caliber to the mix. I'm sure it is a fine round and (may) have it's advantages but at what I see as great cost. Anyway, just my opinion.
For your first and only pistol I think 11.5'' is a good choice. It's still has velocity and is usable for many roles. Going really short like 7.5'' would be a good second pistol but it lacks velocity and ends up being nearly as long as an 11.5'' most of the time because you end up needing a long muzzle device to tame the flash (they can be made to look really cool though). With picking only one I think an 11.5'' in 5.56 is an all around good choice.
Opinions are just opinions, this is just mine.

I went with .300 AAC for my car gun (pistol). I chose it since at 10.5" it still hit with authority and would pas thru car doors and windshields with the same bullet/cartridge from Hornady. Personally I wouldn't go below 16" for 5.56/.223 but that is me.
Add a good muzzle brake and enjoy. My pistol was over 26" and had a VFG which made shooting it accurately at close range rather easy.
You can add tape to the .300 AAC mags to help avoid confusion. Hell, you can put rubber bands on .300 AAC mags and that helps visually and tactile wise.
Full disclosure: I am switching to 6.5 Grendel these days.

Thanks for the info!

I've been going back and forth on whether to stick with 5.56 or go with a 300BO. I have plenty of 5.56 on hand to feed my rifles. However, 300BO has been interesting me as of late. The ability to differentiate between the two wouldn't be an issue as all of my current AR mags are USGI and I would buy PMAGs or Lancers if I opted for for 300BO.

I know that I have to declare the lower a pistol lower when I sign for it from my FFL. But do I have to declare a caliber when doing so?

Oh the choices.....

It won't be declared as a pistol unless you're buying a complete firearm. If it's a stripped lower, it will just be listed as "Other" on the 4473. And you can build it into any caliber you wish.

Personally, I would stick with 5.56 for a first build, but that's just me. To me, 300BO is best suited to a 2nd or 3rd build, and for a primarily suppressed gun. I like 5.56 due to ammo costs and it does everything I need it to do with a 10.3" or 10.5" barrel. Not anything against 300BO, it is a perfectly good round. I just shoot a lot and don't really have a need for anything other than 5.56.

Where in Iowa OP?

Also, you dont have to declare anything when picking up your lower. A stripped lower can be made into a pistol or a rifle, it doesnt matter.
And no, to your question on declaring a caliber.

I forgot to mention, I built my .300 AAC pistol for supersonic loads only.

Thanks for the additional info. For whatever reason, I though that I had been asked by the FFL when I took ownership of my last two stripped lowers if they were for rifle or pistol builds. I knew that for rifles, having them marked "Multi" for caliber covered everything, wasn't sure on pistols.

This will be my 4th stripped lower build but first for a pistol (yes, the only difference is the brace and tube for the lower). I'm heavily leaning towards sticking with 5.56 on this one. If I like what an AR pistol does, then I'll build out another in 300. Probably do another 16 incher at the same time.

I'm still digging into what I want for an upper. I know what I like in my rifles. But for a pistol, I'm thinking a red dot and BUIS. Probably a light and maybe a laser down the road. But do I want a railed hand guard or go traditional plastic? MBUS front sight or F marked USGI style? Different pistols grips I'm sure. More than likely I won't be ordering any of those parts until middle of next month so I have plenty of time to do some research and figure out what fits my needs the best.  

As to where I'm at in Iowa, I'm out east in the QCA.

I prefer free float handguards.  I like having the room and ease of installation provided by a good FF hand guard.  There are so many excellent quality low cost versions now that for me it doesn't make sense or save any money going with a fixed USGI front sight, unless you like the looks.

If pre 8/2008, that was necessary, as there was no "other firearm" option till then.

- OS

If you're planning to mount a light, then a rail would be nice, but not absolutely required. If you're planning to mount a laser as well, you'll definitely want a quality FF rail to have a solid mount that will hold zero. If I were building something new, I'd go with a Geissele 9.5" Mk8/Mk14 rail and a 10.3" .070 GP barrel from DD; for something a little more budget friendly, a BA 10.3" from Andro Corp would be another great option. You can get a BA 10.3" from Andro Corp for $120 right now - that is probably the best bang for the buck on a quality 10.3" barrel. You could go 10.5", but a 10.3" barrel and 9.5" rail just go perfectly together IMO. The thread shoulder starts as close to the end of the rail as possible while still being able to mount a suppressor, whereas a 10.5" barrel has more of a gap - just something to be aware of.

Andro also has a pretty good deal on an assembled 10.3" URG w/ FSB. Keep in mind, however, that an FSB upper can feel a little cramped for room, especially once you start putting on accessories. A full float rail w/ LPG is going to both give you more room and allow for a grip that is much further forward. Unless you really like the looks of an FSB (and don't mind the limitations on rail space and grip position), I'd go with an FF rail w/ LPG and a rail mounted front BUIS (MBUS/KAC/Troy/DD/etc.).

krdt - I get what you're saying and appreciate the info. Really making me think about how I want to spend my money.

I'm wary about dropping too much into this pistol as I've neither handled nor shot one before. I have no clue as to if I will even like this setup. So I'm thinking I will go basic with this first pistol build and figure out what does and does not work for me. If I like the platform then I will build another in line with what works for me. If I don't like it, then I can replace some parts and do a RECCE build that I've been thinking of. Or I can just SBR it and add a new stock. Choices......

Thanks again for the insight and food for thought......

I'll add pics as the build begins......