I am ready to build my first pistol, but I'm not sure which caliber and barrel lengths I should be looking at. I currently have a good supply of 300BLK, 5.56, and 7.62x39 ammo. I do not plan to be using a suppressor any time in the near future. I would really love to hear barrel length suggestions for the 3 calibers previously mentioned and pros and cons for or against pistol length barrels for those calibers. Thank you for any advice provided

Why?   What is the intended use?

Truck gun, home defense weapon, possibly hunting under 200 yards. Just picked up a new lower and wanting to build something different without going to a hard to find or expensive caliber

10.5 in 5.56. Although 300 blk might perform better 556 will work and hurt your wallet less when shopping for ammo. When shtf what are you more likely to find ammo wise in the wild?

Thank you for your response and suggestion. I have rifles
in 5.56, 300BLK, and 7.62x39. I understand your reasoning and
agree with your point. And I am considering the possibility of
having both 5.56 and 300BLK pistols. My original question
probably should have been what barrel lengths are best for each caliber and why

.300 blackout out of a ~10.3" or less barrel will have better or at least the same terminal performance as 5.56 out of similar barrel lengths with much much less flash and concussion. Same for x39. Unlike the x39 though, you'll be able to use all your 5.56/.223 magazines and parts (minus .300blk barrel) instead of one specific manufacturer of x39 magazines, must get an extended firing pin, new bolt, and have to make sure you have a barrel that spec'd properly for .311" diameter bullets and not .308 claiming to be x39, if my memory serves me.

Since you're not going to suppress, there's no real reason to go as short as possible, unless you want to for carrying/storing.
Therefore I'd suggest .300blk with a ~10.3" barrel. Short, but only gives up ~150fps to a 16" barrel, and allows you to use most any handguard and give you more hand space. Ballistic Advantage 10.3" hanson profiled .750" gas block journal diameter barrel is what I have and am pleased with.

This is actually a 12" rail over a 10.3" barrel with a 2.25" linear comp that sits 1/16" proud of the end of the handguard. The extra handspace is greatly appreciated by me.

That is a very nice setup. Where did you get those 300Blackout  bands?

https://sighthoundballistics.com/product-category/mag-bands-2/

Does anyone offer a pistol brace with the buffer tube, spring, and buffer for a reasonable price or do I have to get the brace separately? Most braces I have looked at don’t seem to come with a spring and buffer. Which buffer should I get? Does the caliber make a difference on which weight buffer I would need?

If you are going 300blk, most use a standard carbine buffer and spring.   I don't think too many of them come with buffers and springs unless you get them in a kit from a vendor.  I have buffers and springs just laying around from different builds.  You could use this as an opportunity to try out a flat spring.

Flat spring? Please enlighten me

Instead of the metal of the coil being round, it is flat.

Some folks swear by them. Some say they are meh, but at least get rid of the "sprong" sound you get with AR's.

KAK Shockwave blade version 1.0 comes as a complete kit.  Usually $60 or less.  That's what I use on my 11.5 inch 5.56 version.

I have all three calibers as pistols.

- 556 - I  have gone with a 10.5" barrel for ballistic reasons. It's the most boring because it shoots every time and never had any issues with it. Lot's of options for ammo and bullets for reloading.  

- 300blk - Started with a 7.5" no name barrel, then 8.5" KAK econoline, now a 8" Wilson Combat. Accuracy wasn't what I wanted with the first 2 barrels so I moved up to a WC. I reload and this caliber can use a lot of bullets I already have or can easily obtain.

- X39 - 7.5" KAK economy version. Doesn't like steel case ammo and most of the powder burns outside of the barrel. The cost of steel case ammo is less, but if it doesn't feed properly it doesn't really matter. I've got a stash of X39 from years ago and this pistol doesn't like it. Fewer bullet options available for reloading, but you can also reload with 308 bullets if your not as picky as I am or get a Faxon barrel that's bored in 308.    

In order of preference for a pistol: 300blk, 223/556, X39

My advice is to get a good barrel to begin with so your not changing it out later because accuracy is crap.

I have been looking around and the shockwave blade seems like the route I am going to go for a pistol brace. And I just picked up 2 more stripped lowers. I think I'll go with 5.56 and 300 BLK pistols. Not yet sure what the 3rd lower will be for

Great advice. Thank you. I don't think I will go with X39 for a pistol. I'm leaning towards 10.5" for 556. I'm curious about 300BLK though, I have had a couple suggestions for 10.3" or 10.5"
and you get suitable accuracy with 8". I suppose more research is in order before choosing what to go with for the 300BLK

Barrel length has nothing to do with accuracy, that is dictated by barrel quality.
Velocity is dictated by barrel length, which translates to bullet drop and maximum terminal performance range.
Again, .300 blackout will offer better or at least the same terminal performance than 5.56 out of shorter barrel lengths at the cost of more expensive ammo.

So a question you may want to consider is how often do you plan to shoot this particular build?
If it's mostly going to sit in your truck, by your bedside and occassionally used to hunt with, with some fun shooting peppered throughout, I'd go .300 blackout (as you posted above).
If you intend to run this gun pretty hard and still expect it to fill truck, HD and Hunting duties, then I'd go 5.56.

With 300blk you'll gain some velocity with the 2" longer barrel, but length doesn't effect accuracy in most cases.... barrel quality and ammunition does.

With a 10.x" it will give you more room to get a more comfortable grip. A 8" barrel with a 7" hand guard gets you in a tight position with the forward hand. C clamp seems to work best for me since you can't put a vertical grip on a pistol without NFA paperwork.  

The good part about 223 and 300blk is it only takes a barrel change to switch a between the two, everything else is the same....but of course, you'll end up with both anyway.

That was another concern of mine. I'm a big guy with long arms and the extra 2" of length might be a little more comfortable grip position for me. I think that I will be going with 10" to 10.5"  for both 5.56 and 300BLK.  I do appreciate every ones input, thank you all. I suppose now I need to start shopping for my builds and I still have to figure out what to do with my extra lower I  have. It will probably be a rifle, but I don't need another 5.56, 300BLK, or 7.62x39