Took a couple of guns to the range: a Smith and Wesson 10mm M&P pistol shooting Underwood 180 grain XTP bullets, and the 300 Blackout pistol that I jut built, shooting 200 grain subsonic ammunition through a Dead Air Sandman 2 suppressor.
The 300 Blackout was basically a bolt action rifle. It failed to feed nearly every single round. I am thinking that, since the rounds were subsonic and there was a suppressor involved, I either need a lighter buffer or a gas block with a larger port. I will work on that. The gun was very quiet. Accuracy was great, with every single round punching the bullseye at 50 yards. I put a chronograph on 10 of the rounds in two-five round strings:
String 1: 200 grains, 896 feet per second average speed, with a spread of 40 feet per second and a standard deviation of 14 feet per second. Minimum was 876 fps, maximum 916 fps.
String 2: 200 grains, 904 feet per second average speed, with a spread of 54.6 feet per second and a standard deviation of 20 feet per second. Minimum was 879 fps, maximum 934 fps.
The 10mm did well. Recoil was snappy, as usual. Accuracy was good, with 17 of my 20 rounds impacting within the ‘down zero’ ring at 20 yards. Of the other three shots, 2 were in the ‘down one’ ring, and the last was one I pulled, only striking the bottom of the ‘down two’ zone. I think I anticipated that one a bit. I measured a 5 shot string on the chronograph and got:
180 grain bullets, 1264 feet per second average speed, with a spread of 36 feet per second and a standard deviation of 13 feet per second. Minimum was 1243 fps, maximum 1278 fps. That gives us an average kinetic energy of 638 foot pounds. Underwood lists that load at 1300 fps and 676 ft-lb, so I am right there with expected performance from a 4 inch barrel.
and yes, I was practicing with full power defense loads. I try to shoot a box of full power loads at least once a quarter, even if it is more expensive than the range ammo I usually use.
It was too cold for the outdoor range, so there was no possibility of shooting any gelatin. Still, that load for the 10mm is giving an IPSC power factor of 227.5. There aren’t many autoloaders giving that kind of power. For common defensive pistols, on the .357 and .44 Magnums do better, and they are limited to six rounds with slow reloads. I have a 4 inch .357 Magnum, but I don’t yet have a .44. Maybe one day.
11 Comments
Dirty Dingus McGee · February 2, 2026 at 5:31 pm
MOST .357’s are 6 shot. I have an S&W 686P, holds 7. I bought it a few years back and also bought 4 speed loaders for it, Yeah, the speed loaders were not cheap but it’s better than fumbling around when/if things get ugly. With some practice I’m able to reload the gun in only slightly more time, maybe 1 1/2-2 seconds more, than slamming a new magazine into my EDC Ruger P-95.
Why a 7 round “six” shooter? Tactical advantage. Everyone see’s a revolver and assumes 6 shots. That 7th shot might be enough to save my ass.
Bobsuruncle · February 2, 2026 at 6:04 pm
DM, me and a gunsmith friend did extensive testing on .300 BO piston driven, DI, supressed, unsupressed sonic and subsonic rounds etc, he even developed a DI system for 7SFG that worked with sonic snd subsonic rds. I opted for a PWS self adjusting piston system it worked well, no adjusting on my part. I had all kinds of special loads made from a now defunct local ammo company, hed make em and Id test him and give him data back. For sonic rds, I liked the 125g TNT loaded pretty hot, load them up to max and accuracy goes. For subs I have to recommend the Hornady 190g Sub-X rds. Only bullet designed to expand at those velocities, very reliable round. I moved away from .300 BO when i figured out how to load those 190g sub-X rounds in a .308 subsonic round, for me that made the .300 BO superfluous. I can run .308 sonic, or .308 subsonic. I use other stuff for home defense. you either need an adjustable DI gas block and/or a lighter buffer spring, or buffer same as FRTs and dwell time issues, lighten the buffer or the spring. But…likely once you get it to run subs, it most likely wont run sonics, maybe but not likely. Keep in mind those 190g subs penetrate very well in my tests, anything like a 200g or 220g are worse, i tested 220g also, they sucked at everything, key-holed, no expansion, etc…Honestly, Id run the 125g TNT sonics with a can, those things are like fuggin hand grenades, they are quite gnarly rds. The Hornady 115g ballistic tip are decent as well.
Just some 2 cents from a lot of playing around with .300 BO. Good luck. It can get frustrating. I made a 350 legend, 7.5″ pistol and couldnt get it to run, I tried everything then that same gunsmith recommended putting a pigtail gas tube on it to slow the gas down, works like a champ now. I added a blast device, basically an open ended supressor (I forget its name) on it for flash and bang with only a front sight and it makes a heck of a truck gun, it hits hard.
Divemedic · February 2, 2026 at 6:11 pm
I want a house gun to supplement the 12ga and the Skorpion that are in the safe. I want it to be short and easy to handle indoors, quiet (meaning suppressed) to save my ears if I have to shoot it in the house, hit hard like a truck, defeat obstacles and body armor, and have lots of capacity so I can lay down some serious firepower if required.
That’s why I am looking at the 300. I am thinking of perhaps sonic ammo like the 125gr for body armor, but the tradeoff is louder to shoot.
Honk Honk · February 2, 2026 at 6:59 pm
Getting ready to rid the commie menace once and for all.
All commies are subhuman vermin.
jack · February 2, 2026 at 8:17 pm
I’m running a 300 blk, all suppressed. Had the same issues. Brownells has a 300 blk buffer spring that fixed the problem. Sorry I don’t remember the part number. Give their tech support a call if you can’t find it online.
Winterborn · February 2, 2026 at 8:52 pm
Probably under-gassed. I was doing reloading testing with a noveske barreled 300 blackout upper. I ended up opening the gas port by two or three drill bit sizes. It’s way overgassed to run supers but it runs all the subsonics with a suppressor on it. You can always run an adjustable gas block to reduce gas for supersonic rounds.
Xzebek · February 2, 2026 at 11:02 pm
I’ve mentioned my FN PS90 SBR here before. Very high speed rounds designed to penetrate body armor. 50 rounds so there are lots of opportunities for follow-up shots if a threat is not neutralized on the first couple. Low recoil and quiet while suppressed. It’s completely ambidextrous so easy to move around corners no matter which direction you are coming from. I know it shoots what a lot of people think is Oddball round but I love it.
I had a similar problem with my Steyr AUG that you had firing your 300. It was firing only one round at a time like a bolt action. I finally discovered that the gas Port had been turned to grenade launcher mode. D’oh. I moved it and have not had a problem since.
Gregb · February 3, 2026 at 5:41 am
So you had a GREAT day?
Danny · February 3, 2026 at 5:56 pm
Desert Eagle dude 😀 chambered for .44 magnum
Divemedic · February 3, 2026 at 8:11 pm
Not enough room left over in my pants for a second monster.
Firehand · February 5, 2026 at 11:08 am
The stuff on lighter spring/buffer was covered above, I’ll mention if you want to be able to tune the gas block might consider the Superlative Arms adjustable block. I’ve messed with two of them, and they seem to work well. And with their click adjustments there’s not a locking screw to have to mess with.
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