Considering building a new pistol for a pdw. My plan is to have the weapon suppressed. Wondering if anyone has any experience with either. What are the advantages and disadvantages of both

I've had both, still have the 300. I like the 220gr subs nice and quiet through my Specwar. Also I had lots of issues with the 9mm AR. The CZ Scorpion was much more reliable but still louder than the 300 so that's why I still have the 300.

What size barrel did you get for your 300? I have a 10.5 on my 5.56 but I don't think suppressing it will make it quiet lol

Have both. One of the advantages of the 9 is the low cost of factory ammo.

Prefer the 300 for HD

5.56 is somewhere around 18000 PSI pressure uncorking at that length.
Have you fired it indoors without protection?
Did your ears bleed a little?

I fired it indoors but not without ear pro lol. I have a flash can on it now so blast goes away from me about to put ferfrans crd on it next

That won't help indoors.  The concussion just ricochets off the wall back @ you.  With a KAK flash can on my 16" AR, @ an indoor range someone else w/ an AR asked me if I had a .308".

I'm headed for subsonic suppressed 9x19mm for HD out of Glock mags.  Will be a 2 stamp gun, not anytime soon.  I acknowledge the better wounding effects of .223" over subsonic 9x19mm HP, but not willing to be a deaf family b/c I shoot a burglar.  Other advantage is that it will use the same mags as my CCW/secondary.

.300" Whisper subsonic is expensive, and doesn't appear to have much better wounding effects than good 9x19mm HP.  I could step up to .45" ACP for home, but I'd lose mag compatibility w/ my secondary.

That's exactly the info I'm looking for. My exact reason why I'm considering building either a 300 blk or 9mm. I like how the 9mm Barrel can be shorter

The 9mm can be much shorter than the 300. 9mm is also much cheaper to shoot.

I don't have a 300, but I'm +1-ing the part in bold.

9mm can be smaller, cheaper to shoot, quieter, Glock mags comparable with pistols.

300BO has more down range energy.

.300" Whisper also has the advantage of going a LOT hotter w/ a mag change.  That's not a particular advantage for me inside the house.

Correct......either choice has pros and cons.

After holding off from the 300BO bandwagon for a long time, I finally borrowed a friend's 300BO AR and shot several different subsonic rounds in different brands with my Silencerco Hybrid suppressor.  OK...kind of see what the hub-bub is all about.  I bought a 9.5" 300BO barrel and built up a pistol with a KAK Shockwave brace.  I was using 208g AMAX Hornady subsonic factory ammo, and it performed amazingly.  It's quiet and accurate...at least within 125 yards or so.

I started reloading 208g Hornady ELD-Match in subsonic, and it yields identical suppressed performance.  This is a really quiet gun, and it throws a huge bullet downrange very accurately within its envelope.  I think the advantage of the 300BO over the 9mm subsonic is the much heavier bullet you get and the energy it delivers at 100 yards or so.

What is the performance in gel?  Under 2100 fps, bullet weight and energy generally don't matter - the question is how deep will it penetrate, & how reliably and to what diameter will it expand?

In an AR platform, I'd choose .300blk for better ballistics over 9mm, and the ability to shoot supersonics.

I've got a 16" and a 4" 9mm AR but if I was suppressing 9mm I'd do it thru my Evo scorpion before any AR. I like my 9mm ARs for range plinkers but there's better platforms for suppressing.

One big advantage to a 9mm suppressor is being able to share it with a  lot of pistols & pistol caliber carbines.

I understand your point about optimal bullet expansion and the often lack thereof as it pertains to subsonic bullets, but I think that statement about bullet weight under 2100 fsp not being a very important factor may be a bit broad.  That said, bullet expansion in a 300BO can be accomplished with bullets like Lehigh Defense and such if expansion is a primary goal.

Fat McNasty left the market.  I think you're limited to one supplier.  Correct me if I'm wrong.

Thank you guys for the info. I'm still leaning towards the 9mm ar for the fact that it'll be more compact.

Now you've given me an idea.

Instead of an SBR Glock under an AR9, it would be nice to have a flamethrower.  Some downsides, though.  

I'm waiting on my tax stamps for a daniel defense 300 bo SBR with the 10.5 inch barrel and a silencerco omega suppressor.  I went the 300 route for the supersonic options, coupled with being able to shoot subs up to 220grains.  I reload, so ammo cost isn't as big a consideration.  I'm also going to run the omega on a 300 win mag bolt gun, shooting supersonics, and a 10.5 in .223 upper that i'll use with the dd sbr lower.  
Ultimately, i may get a 9mm mag block and upper as well.

Clearly there's a lot of preference around different calibers in the AR platform.  It provides a ton of choices.  There is another element that has kept me from building a 9mm AR, even though I've been tempted a little.  The main issue is weight.  I have a pair of 9mm carbines in other platforms...a Marlin Camp 9 in a folding stock chassis and a Kel-Tec SUB2000.  Both are relatively light.  Because of that I haven't gotten that revved up about a 9mm AR.  People are different, preferences are different.

I played around with 7x45 Ingram, 7mm TCU and there .300/.221 back in the 1980s when I shot a lot of metallic handgun silhouette with both 10" and 14" barrels.

As a concept it does allow a much heavier bullet than would be the case in the .221/.223 parent case, which was useful for momentum in knocking down 200m rams, and allowed for accuracy that was far superior to that of a traditional hand gun round.

For a suppressed rifle, those same benefits apply, with the added bonus that the higher weight and shorter case with a short OAL, allows for sub sonic velocity as well as feeding in an AR.

There's no free lunch however.   .300 AAC is more expensive due to the higher cost of the rifle bullet and case used. Way back in the day its ancestors were a hand load only proposition, and while that is not an issue today with the .300 AAC, it's still a much more expensive proposition than 9mm.

Factory reman ammo in .300 AAC starts at about $0.30 per round, and increases to about $0.50 per round for new ammo, you'll be paying around $0.70 per round for Federal, Remington, Hornady, etc, and around $0.85 per round for the heavy bullet sub sonic loads from the bigger name companies.

In comparison, 9mm factory reman ammo starts at about $0.10 per round and you can find 147 gr sub sonic ammo at around $0.15 to $0.20 per round.  New sub sonic ammo can be had for around $0.25 per round.

Hand load cost differences are similar, as the costs are driven by the higher cost of the brass, powder and bullet for the .300 AAC.

In contrast, 9mm cases are dirt cheap, powder charges are small, and you can get decent 147 gr bullets pretty cheap.  Just staying within a single company for comparison purposes, 147 gr Hornady XTPs will cost me about $25 per 100 (and will deliver 1.5 MOA accuracy in my AR-15 pistol), while even a 150 gr FMJ in .308 will cost me about $31 per 100.  The heavy .308 bullets for a .300 AAC will cost me around $45-$55 per 100.  

I can also get inexpensive heavy plated 147 gr 9mm bullets that will still give me 2-2.5 MOA accuracy for around $50 per 500 and my total cost for a box of 50 is about $7.50.

In short:

The pros for .300 AAC are potentially greater accuracy and range potential, and some cool factor, but the cons are higher cost, somewhat reduced availability, and higher operating pressure and smaller bore diameter, creating greater muzzle blast (if you are not shooting suppressed).  

The pros for 9mm are substantially lower cost, availability pretty much everywhere, and reduced muzzle blast, particularly, in a 10.5" barrel, if you are shooting un-suppressed. The major con is the shorter effective range - about 150 yards in a 9mm AR-15 pistol or SBR, due to reduced energy and accuracy along with a steeper trajectory (with a 100 yard zero you'll have about 10" of drop at 150 yards).  

So it really comes down to how far you want to shoot it.

If I wanted to plink cheap and quiet, 9mm.
If I wanted to kill something, 300blk with 110gr Barnes supers.
If I wanted to kill something quietly, 300blk with $$$ expanding copper subs.

I already have a 9mm but will also be building a .300 BLK




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?

Most of these can even be found in factory loads:

Maker
Lehigh
Outlaw State

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I went the Glock mag route for magazine compatibility with my CCW and backup too. Mine is a 4.5" Faxon barrel using a direct thread mount on my F1 can. With a 10oz buffer the port noise is nothing, and 147gr and 158gr are scary quiet. It feeds every bullet profile, and I can still use 124gr/115gr/95gr if I want supers.
Nothing the lightest thing in the world but it works 100% and has a lot more options for configuration than other PCCs do. I liked it so much I decided to get started on an identical one in 10mm for commonality with my G20 woods gun
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We're glad you're back.  Your website was an invaluable resource when I first got here - all the info I needed in one place.

Not happy about the reasons, though.  My grandda wore out his inner ears, but was able to work as a sacker @ a grocery store for 20 years after retirement b/c he could hang onto the cart.

Great to see ya about QB

What lower is that on your 9mm build? I'm looking to build a 9 and want last round bolt hold open.

Thanks in advance

This one is from Runner Runner Guns, it's made by New Frontier so no lrbho on this lower

Definitely pros and cons on the caliber issue.  I would like to add one comment about cost.  There are some excellent .30-cal bullets out there at some very good prices.  Those Hornady 208g ELD bullets I mentioned cost me about $35 per 100 on sale at Midway last month...which they do frequently.  Pretty good for a premium bullet.  For me at the moment, I'm only interested in the 300BO suppressed.  This caliber suppressed in a 9.5" barrel length really does it for me.  I may look at 300BO supers in the future.

I've just got my 300 running and am planning on using Speer 180gr spire points for cheap subsonics. I can get them for 19 cents a piece and the guy has many thousand available.
Now to get the can out of jail about July

Thanks for the info on the lower, I'm holding our for last round bolt hold open for the 9mm

Hopefully you won't be holding out too long, there are a few of us working on different ideas for cleanly adding one to existing lowers

Will be watching. Have to finish my 5.56 pistol before starting on the 9mm

NFA/Runner Runner puts the LRBHO activation in their upper.

the 300 is a better round, from sub sonic to full loads you can do alot with the round.

with that being said. i would go 9mm every time.

i have 556 for carbine plinking.

i have 308 for more energy and longer range

i will have a cheep 9mm pcc that gives me 90 percent of subsonic 300 performancr for 1/8 the cost to shoot

i dont have to spend a ton of time reloading, i can practically fire 3 factory 9mm rounds for what it cost for a single 208 amax projectile.

for me 300 doesnt reallt give me anything that i dont have.

for some its awesome.

RRA's LAR-9 has last round bolt hold open with it's RRA magazines.  The oddity is that they supply a modified 25 round Uzi magazine with their new pistols and the modified Uzi magazines do not have the little BHO tab on the follower.

But if you buy a RRA 32 round mag it'll come with a BHO follower in it.

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There have been mixed reviews about BHO in the 9mm.  

Some folks claim that the BHO feature tends to break bolt releases.  Some of these people prefer to run their 9mm AR-15s with magazine followers that do not hold the bolt open after the last round. The rest of these people will use spacers in the buffer tube to reduce the over run of the bolt and just reduce its speed when it hits the uplifted bolt release.  But that can cause other issues, so it's a mixed bag.

Other folks don't have any problems with broken bolt releases, and don't see this as a problem.

Either way, with either a RRA or Colt pattern 9mm AR's and magazines you can get the last round bolt hold open feature.

Bolt stop breakage in the 9mm has to do with buffer length.
There are cheap fixes for this.

Was really tempted to do an 8" 300BO at one point. Did a 7" 5.56 instead.

A pistol caliber carbine made more sense to me as well. Just waiting on a 10" 9mm upper.

300BO is most efficient in supersonic loads through a short barrel. Other than that it appears rather pointless. If I were to go that route an integrally suppressed 16" rifle probably makes the most sense.