I am looking to SBR a 9mm SMG-style platform or possibly convert my AR-15 to an MCX-Spear 300BO "subgun" impersonator.  I do not want another MP-5, though an MP-5k will show up at some point.    

I was initially looking at the Beretta PMXs but, I also don't get the benefit of a career in Italy using a PM-12 so I question that option even though I can get one for ~$1,000.  Then you have the B&T options, with even the GHM9 being pretty darn expensive.  Sorry but, the CZ Scorpion just doesn't do anything for me.  The Sig MPX might be a reasonable option but, I question the high cost and comparisons against the B&T options in a similar price range.

Then there is the MCX-Spear 300BO!  I could run that one ~15 minutes after it hits my mailbox since I already have a tax stamp for its 'parent' and no NFA wait.  Compared to the 9mm options, I think the smaller bearing surface of the 7.62mm bullet has merit along with the heavier payload.

Am I overthinking things?  If you are not going to get an MP-5/SP-5 option, what SMG-themed pistol/SBR would you get for ~$2K?  Would the additional size of the MCX Spear 300BO option rule it out of the SMG role?  I am looking for something easy to transport discreetly that is effective out to 100y (or m) against someone, or their dogs, who intend to harm me or those I care about.

TIA, Sid

Sp10a3!

I'm sure you've been doing all the comparisons and such between them all.

So I'll say this: When I got to handle a Beretta PMXs, I had a bit of buyer's remorse over my GHM9. The Beretta felt that nice in my hands. I'm sure it has fewer issues with optic mount compatibility, too. However, the market response to the PMXs has been tepid at best. Beretta came in to the US PCC market too late and for too high of an MSRP, and that turned a lot of people off. Beretta has a history of abandoning products that it doesn't deem commercial successes, and the fact that they still haven't bothered releasing a stock or brace (even for the people who want to Form 1 it) should be telling.

I nearly bought the Beretta, but found a new GHM9 on sale for the same price, and the better aftermarket support and flexibility to use other magazines with a lower swap was worth it to me.

Capt speak for the others, but considering a 300 BLK seems like a different ballgame than the 9mm PCC options.

I agree, a 300 BLK will be slightly larger, louder suppressed with subs, MUCH louder with suppressed supersonic, MUCH more expensive ammo, MUCH more powerful with supersonic ammo, more accurate out to 50, and MUCH more accurate out to 100+. Additionally, suppressors will be MUCH larger and heavier on the 300 BLK to even get the gap close on sound. Virtually all 9mm cans suck on 300 BLK, especially supersonic.

Really in a whole different class.

So you already you have a standard AR lower registered as an SBR?

Sounds also like you want to go bufferless?  I would recommend you look at the CMMG Dissent then.
In 9mm, the Dissent is using the (Radially Delayed Blowback) system and finally using a fixed ejector which doesn't fail like the CMMG Banshee RDB in 9mm does.  
IMHO, suppresses way better than an MPX.

You can also get a Dissent in 300BLK.
https://cmmg.com/upper-receivers

All the Dissent models no longer use the buffer tube and the uppers are like 99% proprietary.

I know many of you are buffer tube haters but I love them since it allows me to leverage aftermarket flat springs and hydraulic buffers.
I have the best of both worlds with my hybrid Dissent build discussed on my site here: https://c3junkie.com/?page_id=1733

I wanted to retain the buffer tube for tuning and also to trip the auto sear in full auto.  I also don't like side chargers and wanted an SD build which you can't do with an off the shelf Dissent.
But if you don't care about any of that then the Dissent may be a good fit for you.

I would suggest the Stribig Sp9A3. I bought one for around $1k, which in some circles is apparently overpriced still, but eh, lots of shit is overpriced, like the scorpion micro I bought and sold when I discovered it was nowhere near as awesome as I was lead to believe.

But if you are looking at a $2k budget, that leaves you with a lot of wiggle room.

I own an MPx, B&T P26 (predecessor to the PMx) and three GHm9 variants (two K models and one SD). What do you think I recommend? The MPx is grossly over-complicated mechanically, but is soft shooting. It’s overpriced for what it does. The P26/PMx are fine, but not as refined as the GMH9 series. There’s allot of plastic on them and, for reasons I don’t understand, the PMx did away white 3-lug. The GHM9 is extremely well built, has swap out lowers (B&T, Glock, MP5 and Sig…maybe CZ Scorpion also). Pasta extremely soft shooting and has a superb trigger. I own own probably 25 PCC variants and I still recommend the GHM9 as a great all-around option. I would argue it offers 95% of an APC9 in terms of overall experience.

Recently picked up Dissent 6.5" 9mm upper and mated it to a Mega Arms lower. I own several PCCs and Dissent really impressed me. I recently got a B&T RBS SQD Fullsize suppressor approved and it pairs nicely with the Dissent. The lower back pressure of the can and the delayed action virtually eliminates any gas to the face. Here's a video of me shooting my the B&T RBS SQD Fullsize on my APC9 and Dissent. Also own a PMX and a GHM9, I would choose the PMX over the GHM9. My GHM9 had feeding issues with the B&T mags and B&T USA wasn't able to fix the feeding issues. Switched to Glock mags and now it feeds everything. But I hate running Glock magzines on PCCs. I much prefer to run a staggered feed mag than a single feed magazine.
https://youtube.com/shorts/dyVcqhas074?si=6137PrtgFVFCgtNb

I agree with the B&T advocates. I have an APC9 and MPX. The MPX is its own thing and I wouldn’t even cross-shop it with other PCCs. As mentioned above, it’s overly complicated and just a hassle to own compared to the B&Ts.

I think the GHM9 is the sweet spot for current PCCs as far as price, quality and performance goes. Being able to run Glock mags is a real advantage and 147grain 9mm is subsonic and can be shared across other handguns, keeping the ammo logistics a bit simpler.

If you actually want to shoot and enjoy it, then a suppressed 9mm PCC is just as fun as .300blk for a third the ammo cost.