I've had my 7.5" AR pistol for a few years now and have kicking around converting it to .300 for a while. In .223 it is not too bad, but I know that velocity takes a big whollop in a barrel that short, and the thing is just silly loud. Here it is in current config:



Either way, I'd like to hear from .300 pistol owners that run supersonic rounds. How they are to shoot, how loud they are. Living in Illinois, silencers are still not an option but once they are, I'd need to go that road as well.

I've got three 300 BLK SBRs and I enjoy them for what they are. I have one set up with 8.5 barrel for supersonic HD, a 10.5 for supersonic hunting, and a 8.5 clone of the HD gun but set up for subsonic. Mine are all suppressed though.

Where the 300 BLK pistol (supersonic) will shine is in short barrel ballistics. Check out this pdf on the benefits of the round: http://300aacblackout.com/resources/300-blk.pdf

If you can't shoot suppressed, you'll miss out on any of the benefits of the subsonic round. To your ear, there will be no difference from supersonic 5.56 and supersonic 300 BLK.

Go for it...it's only a barrel change and you can always throw the 5.56 one back in.

Never had a 7.5" 5.56, but I have an SBR with a 10.5" 5.56 and another with a 7.5" BLK. Both have Spikes Barking Spiders on them. The BLK I can shoot with subs and not ring my ears, I don not recommend this but was caught in a hunt without my earpro and it didn't even bother me when I took the shot -225gr subs unsuppressed-. The BLK is my go to hunting/plinking just because of the lower impulse and blast.

 

If you hand load, you can get some pretty quite/soft shooting rounds for plinking that will cycle the action. Being in CA we can't have suppressors either, but the 300blk 8/5" edges out my 10.5" 223 wylde pistol for fun. The bullet variety and availability is great for a hand loader.

I'd just build another upper in 300blk and keep the 7.5" 223 as-is.

I have an 8.5 inch 300 Blackout pistol. I was surprised how accurate it was. I thought a short barrel would really hurt accuracy but it didn't. It's fun and easy to shoot. I shoot it more than I do my Colt 6720 lately.

I have two 300blk pistols and one 5.56mm, the 5.56  even with a 11.5" barrel is stupid loud, and the velocity takes a big hit, it's worse with a 7.5" I would imagine. I have 300blk I use as a truck gun 8.5" and with subsonic rounds and a linear comp, it's not bad, probably still want to wear ear plugs if out at the range, but works great for coyotes. I also have a 14.5" for supers, I wanted to get as long a barrel as I could have under state law which states if over 16" it's a rifle, for deer hunting in pistol/shotgun zones. The 14.5" gives me the max velocity from the 300 blk supers I was looking for.

I got 300 blackout as my first ar,  pistol. I liked the bigger bullet and shorter barrel. My American made AK.  

I only shoot supers through it. About to order some subs but didn't go 300 for subs at all.


Still learning a lot and trying to decide if I want to go 556 for a 1-200 yard target gun,  or build another 300 with a longer barrel and full shoulder stock.


I reload em',  so cost per bullet is way down.

I have an 8" Blackout and 10.5"  5.56. Only shoot supers unsuppressed in the blackout, it is very pleasant shoot compared to the 5.56. Still need hearing protection but I don't notice the concussion that makes the 5.56 so unpleasant to shoot. The 5.56 has a STD linear comp the .300 an A2 flash hider.

Another vote for building/buying a complete 300 upper.  I like my 300 but she is expensive to feed.  The cheapest 300blk ammo I've found is $0.42 a round but most of it is double that.   That's 30-50% more than 223, depending on which ammo you,re using.

For comparison I did some testing with my two 300blk pistols both shoooting supersonic Barnes 110gr VOR TX. My 14.5" avg was 2265fps. My 8.5" avg was 2050fps, so roughly lost 200fps while loosing 6" of barrel length.

I know what you are talking about my .223 pistol upper is stupid loud.
I bought this 8.5" .300blk upper to hunt with & found it to be a very accurate & some what quieter. The .300 blk super is about the same sound sound level as a full length rifle in.223. The sub-sonic rounds are much quieter than the supers but my pattern opens up & drops about 10" to 12" at 100 yds. Shooting supers I get a hardball sized pattern at 100 yds. That makes it a great light weight easy to handle short range deer gun. I guess I'm just tough, one shot to drop a deer in the quiet woods doesn't hurt my ears at all, it's s no louder than my shotgun that I used to hunt with.

I have an SBR with a 7.5" 5.56 and a twin. 300BLK upper. I shoot the
300 much more often then the 5.56. I buy the plated 150 grains flat points and load them light. Light recoil and much less concusion/blast from the muzzle. I can load them real cheap with my own concerted brass. I don't hunt or have a suppressor  (yet) so just blasting at the range.

I think there is a huge difference in the noise between .223 and 300BLK.

If I fire a .223 without hearing protection, one shot will make my ears ring. However, I can fire several shots from my 8.5" barrel of super-sonic 300BLK without ringing my ears.

Shooting any AR pistol without hearing protection isn't a good measure of loudness. Caliber, barrel length, and type of muzzle device all affect it.

Addressing the velocity, running a .300BO at 8" is about the same as 5.56 at 10.5". It's still apples and oranges, what we are really measuring is the expansion room of the powder inside the barrel, how much surface area at the base of the bullet is exposed to absorb kinetic energy, and the amount of time it's exposed to that pressure before exiting. As a rough measure, the larger the bore the shorter the barrel to make the same power levels. That is just a rough guideline but why you see 5.56 recommended no shorter than 10.5 but 6.8 and .300 as short as 8 to make the same kind of number.

That is more a dynamic concept of operation, as many posters could show all sorts of numbers arguing the opposite. But it's what the gun designers are coming up with and the packages they sell in volume. The first one was the XM177 in 1965, so this has been going on for over 50 years now.

How much a velocity drop? Much less than some think. A 5.56 from a 20" barrel is typically said to run 3300 fps. We don't see that now, as the M4 runs a 14.5" barrel, and our lookalikes at 16" are comparably close. 2900 fps is normal for a 55 gr round. Move to a 10.5"  tho, it drops to 2700 - not that big a bite, with the 70 grain OTM running over 2500. That draws in the 1000 foot pounds of force to a shorter distance, about 80 meters.

Looking at handgun specs, the designers of the guns for the Army Pistol trials of 1954 specified 400 foot pounds of force were needed for handguns, which typically only have a 50 foot effective range - about 17 meters. The gain in effective range and power of a rifle caliber pistol is simply huge in comparison, which is why we do it. Add having 50% more ammo, too.

For that, we carry something quite a bit bigger and much harder to CCW, if ever. So, no, there isn't that big a velocity loss, and second, you make up for it by making the bullet heavier, bigger in diameter, or lengthening the barrel.

At this point it's all good right? Nope. We haven't discussed ammo costs. And that is where it starts to hurt, as there is NO cheap surplus 6.8 or .300. There is some cheap import stuff, but not a milspec NATO power level combat round for use in self defense or hunting. If you want to practice for cheap then the commercial rounds invented to upgrade the power levels aren't going to be and you will pay extra for the ammo. As much as 2.5 X the price. And that affects training. Those on an unlimited budget who get to blow away ammo all day long 5 days a week don't care, but the other 99% of us have to consider it.

That is why after I had already built a 6.8 AR for deer hunting, I choose to build a 10.5" pistol in 5.56. I don't shoot that rifle much. It's nice, my son likes it deer hunting, but it's not my first choice in the field. That pistol in 5.56 is - because it's a pound lighter, and because I can shoot it at the range for half price, which means twice as much per $20 for ammo. If I can shoot twice as much for the money spent, then that is a big factor.

The deer won't be able to tell the difference, mapping the hunting area I've worked over for 40 years, all the shots were under 80m anyway. 1,000 foot pounds of force is ethical and effective, it's shot placement that counts. and practicing twice as much with a 5.56 pistol means I'm at least as good as with a rifle and red dot. Less effort in the field, too with a lighter shorter gun.

You could do the same with an 8" .300BO supersonic but the ammo costs are the difference. It's going to be more expensive round for round. There is no cheap surplus.

My 8.5" setup shooting 110gr supersonics is quieter and less recoil than my 16" AR15 in .223.
ETA:

Any issues with feeding?

That is the odd part. In my 14.5" gun, they feed just fine when using lancer mags, but not magpul mags. In my 7.5" gun, they fed fine with magpul gen 3 mags, but not lancers. Instead of needing to use certain mags with certain guns, a little bit of fine dremel work smoothing the ramps in the extension and running them down a bit more into the upper and now they both work with either lancer or magpul gen 3 mags. They will all still feed (and always did) longer pointy bullets;)

Added...it took me a while to find a good overall length and load, but they all work now and they will do baseball size Groups at 25 yards. For me, that is all I expect from blasting ammo.