300 Blackout Build Advice

[hillbilly grandpa]

I'm putting together a 300 Blackout for home defense. 7.5 inch barrel. PSA had a deal I couldn't pass up. I plan on shooting suppressed, subsonic.

1. What buffer should I get?

2. Is an adjustable gas block recommended?

3. What additional questions should I be asking?

Thanks in advance for your guidance.

 

[yeehall]

First, if you are planning to use subsonics specifically, you need to look at bullets that have good expansion characteristics at subsonic speeds, otherwise you're just shooting the equivalent of subsonic .45 ball. There is a lot of supersonic ammo that expands/frags at supersonic speeds, but it may not perform the same at subsonic. Lehigh Maximum Expansion, Discrete Ballistics, Makers, etc, all seem to put down hogs and deer consistently at subsonic speeds due to the expansion. But premade ammo with these bullets are expensive.

Second, the difference between different loads is really extreme with 300bo--subsonic vs supersonic, and even different kinds of subsonic, all have different characteristics. Here's an old page that looked at port sizing for different kinds of ammo: 300 BLK Port Size Testing, and you can see that even the subs required different port sizes.

If you want to test a lot of different stuff you will find it easier with an AGB but they can introduce other problems. Or you can remove the uncertainty by sticking with known ammo and always shoot with a suppressor installed or not. I have a Rosco 8.5 that I drilled out to .125 and installed a Riflespeed so I could switch between different types of ammo with or without a suppressor, but if you stick with a known good configuration you won't need to do that.

As an example, I bought a couple of boxes of Hornady with the flat polymer tip and it would not cycle reliably in any configuration until I opened the gas port all the way, increased back pressure with a test can, -and- only used PMAGs (the polymer tip did not work well with duramags). One element, Hornady puts non-combustible filler in their powder, which (I assume) is meant to restrict the burn rate and keep the ammo subsonic, but it also leaves particles behind all through the gun (mag and can too) which is a pain to clean out:

I have been testing AAC Sabre Blade Black-Tip subsonic lately and it performs well, shoots reliably without a can, seems to burn relatively clean, but I can't find any solid data on expansion, and there are several reports of popped primers, so I'm not doing anything else with it at the moment. I have also tested some supers and am able to shoot them without the can but I have to dial the Riflespeed all the way down in that configuration (even without the can), and that's not really a desirable configuration for home defense. I am going to buy some of the premade expanding subs next and see how they perform, but it's pricey.

Easy to see why a lot of people load their own, for affordability and reliability.

 

[Alea Jacta Est]

I

I'm putting together a 300 Blackout for home defense. 7.5 inch barrel. PSA had a deal I couldn't pass up. I plan on shooting suppressed, subsonic.

1. What buffer should I get?

2. Is an adjustable gas block recommended?

3. What additional questions should I be asking?

Thanks in advance for your guidance.

I let Daniel build mine. He done good.

Good luck.

 

[John from MD]

Yeehall has some good information. I have been shooting the 300 BO since it came out. Until about two years ago, I was shooting all supersonic in 16 and 8.5 inch barrels. Currently I have a dedicated supersonic in 16 in, an 8.5 inch that will shoot supers and subs and a 6.5 inch that is dedicated subsonic.

Since you have a 7.5 and plan to subs using a can, your work up will be easier. First thing you need to know is that PSA is a hit or miss operation. I understand the need to save money but PSA's reputation is proven spotty.

That said, the first thing you need to do is determine your port size. I have been able to make .100 work fine both for subs and suppers. The easiest way to measure is with a pin gauge. You can get one here for $20. https://www.mcmaster.com/products/pin-gauges/class-z-go-plug-gauges/

You will need an adjustable gas block in order to tune it to work with a lighter spring and buffer and keep the weight down. This is the one I recommend.

UD Gas Block - Ultradyne

The Ultradyne adjustable gas block allows you to control the amount of gas cycling your firearm. Fine tune your rifle to reach the best performance.

ultradyneusa.com

I would start with a standard carbine spring and buffer and adjust to run the ammo you choose. Remember, with subs, you always start with more gas turning down to less gas. This will keep you from getting stuck bullets and having to pound them out. The procedures for adjusting are located all over the web and there is a 300 Blackout Forum located here.

http://www.300blktalk.com/forum/

As yeehall stated, defensive loads that work are as scarce as hen's teeth. There are a number of people who have shot at ballistic gel and have recorded their results on youtube. Non that I have seen yet are 100% effective. In fact, most folks that shoot the 300BO a lot believe the best bullet to use when worrying about shoot throughs is the 125gn at super sonic speeds.

If you need any help, I will be glad to assist if I can.

 

[outrider58]

First, if you are planning to use subsonics specifically, you need to look at bullets that have good expansion characteristics at subsonic speeds, otherwise you're just shooting the equivalent of subsonic .45 ball. There is a lot of supersonic ammo that expands/frags at supersonic speeds, but it may not perform the same at subsonic. Lehigh Maximum Expansion, Discrete Ballistics, Makers, etc, all seem to put down hogs and deer consistently at subsonic speeds due to the expansion. But premade ammo with these bullets are expensive.

Second, the difference between different loads is really extreme with 300bo--subsonic vs supersonic, and even different kinds of subsonic, all have different characteristics. Here's an old page that looked at port sizing for different kinds of ammo: 300 BLK Port Size Testing, and you can see that even the subs required different port sizes.

If you want to test a lot of different stuff you will find it easier with an AGB but they can introduce other problems. Or you can remove the uncertainty by sticking with known ammo and always shoot with a suppressor installed or not. I have a Rosco 8.5 that I drilled out to .125 and installed a Riflespeed so I could switch between different types of ammo with or without a suppressor, but if you stick with a known good configuration you won't need to do that.

As an example, I bought a couple of boxes of Hornady with the flat polymer tip and it would not cycle reliably in any configuration until I opened the gas port all the way, increased back pressure with a test can, -and- only used PMAGs (the polymer tip did not work well with duramags). One element, Hornady puts non-combustible filler in their powder, which (I assume) is meant to restrict the burn rate and keep the ammo subsonic, but it also leaves particles behind all through the gun (mag and can too) which is a pain to clean out:

I have been testing AAC Sabre Blade Black-Tip subsonic lately and it performs well, shoots reliably without a can, seems to burn relatively clean, but I can't find any solid data on expansion, and there are several reports of popped primers, so I'm not doing anything else with it at the moment. I have also tested some supers and am able to shoot them without the can but I have to dial the Riflespeed all the way down in that configuration (even without the can), and that's not really a desirable configuration for home defense. I am going to buy some of the premade expanding subs next and see how they perform, but it's pricey.

Easy to see why a lot of people load their own, for affordability and reliability.

The filler is to assure even and consistent powder burn in subs because there is so much space inside the cartridge due to the small powder quantities. It becomes unnecessary when you use magnum or bench rest primers.

I'm putting together a 300 Blackout for home defense. 7.5 inch barrel. PSA had a deal I couldn't pass up. I plan on shooting suppressed, subsonic.

1. What buffer should I get?

2. Is an adjustable gas block recommended?

3. What additional questions should I be asking?

Thanks in advance for your guidance.

Like John said, PSA is hit or miss. If you can afford better, you'd be much better off. Keep in mind, every new load or cartridge brand you try, you will try without the suppressor first to make sure the round leaves the barrel in a stable manor. The slightest wobble in the bullet will likely result in a baffle strike.

If you reload, you are much better off, ammunition wise. My Blackout has never fire factory ammo.

You will definitely want an adjustable gas block. As yeehall said, 300Blk subs can be very diverse. However, once you settle on a particular round, you will adjust to that round and never need to readjust unless you try other ammo. I'm not super finicky about recoil and I shoot both subs and supers of varying weights and construction. I have my gas tuned where the gun handles everything I feed into it without running on the edge of dysfunction nor beating the gun up.

My gun has a 10.5" barrel so drawing parallels might not work for you, as far as springs and buffers. I will say, generally speaking, the 300 Blk prefers a lighter recoil system. I did have my gas port opened up a little to allow a wider rang of function:

Buffer is an H(or H1-same thing)

Buffer spring is a Tubb's light weight flat wire spring- https://www.davidtubb.com/ar15-buffer-spring-stainless

You should also consider Sprinco buffer springs- especially if you have trouble tuning your gun. They are good springs and very affordable and come in a wide variety of tensions.

My go-to gas blocks are SLR and Superlative Arms

 

[smdub]

That said, the first thing you need to do is determine your port size. I have been able to make .100 work fine both for subs and suppers. The easiest way to measure is with a pin gauge. You can get one here for $20. https://www.mcmaster.com/products/pin-gauges/class-z-go-plug-gauges/

The ends of drills are far cheaper than a single pin gauge. You don't need sub thousandths accuracy here.
FWIW, MSC sells ZZ pin gauges for $4-$5. McMaster is horribly overpriced on them.

Remember, with subs, you always start with more gas turning down to less gas. This will keep you from getting stuck bullets and having to pound them out.

BS. A gas block that is too closed will simple short cycle the gun. A completely closed gas port turns the rifle into a bolt action. A bullet will not get stuck in the barrel either way (a closed gas port actually robs less energy from the bullet making it slightly faster, not slower.)

 

[smokey]

Sure you want to use subs for defense? Supers are by far more effective. You're looking at the difference between about 400 ftlbs vs 1100 between subs and supers to cause trauma. You also go from "it might penetrate iiia" to "it will penetrate iiia".

 

[outrider58]

BS. A gas block that is too closed will simple short cycle the gun. A completely closed gas port turns the rifle into a bolt action. A bullet will not get stuck in the barrel either way (a closed gas port actually robs less energy from the bullet making it slightly faster, not slower.)

This^^^

 

[erwos]

I'm putting together a 300 Blackout for home defense. 7.5 inch barrel. PSA had a deal I couldn't pass up. I plan on shooting suppressed, subsonic.

1. What buffer should I get?

Carbine.

2. Is an adjustable gas block recommended?

No. Run full-open, especially if you're focused on subs.

3. What additional questions should I be asking?

Get a suppressor.

I will echo what others have said: subs are not a good choice for home defense compared to supers. You may as well just use a pistol if you're shooting subs.

 

[alucard0822]

300 tuning is a bit simpler than 5.56, and generally responds better to buffer weight tuning than restricting gas, and usually nowhere near as overgassed. In many cases you might need MORE gas than the port offers. With a decent barrel, you should aim for 100% function with supers both suppressed and unsuppressed, and suppressed subs. If it is a <10" barrel, and it runs subs unsuppressed, you need a heavier buffer. The barrel choice is critical, the huge variety of bullet shapes to feed, bore dimensions, chamber specs and port sizes can make a good barrel just work, and a cheap barrel cause a ton of problems. No real need for adjustable blocks unless you are chasing the best suppression numbers, for a defensive build a simple and free flowing fixed block is better. Most of my builds run fine with a standard 3oz or H1 3.8oz buffer and M4 spring. I just try a couple buffers to find the best weight at the range, then if I need one I don't have, just swap tungsten and steel weights I keep on hand.

As far as ammo choice, subs can be viable for defense. Of course supers have better performance, but being the bullets are very long with a very high SD, they generally outperform a shorter/fatter handgun slug of the same weight. Even a non-expanding sub will yaw and cause more penetration and damage than FMJs tend to. Expanding subs also usually outperform 9-45cal handgun slugs, but of course they need to feed and function reliably.

 

[outrider58]

300 tuning is a bit simpler than 5.56, and generally responds better to buffer weight tuning than restricting gas, and usually nowhere near as overgassed. In many cases you might need MORE gas than the port offers. With a decent barrel, you should aim for 100% function with supers both suppressed and unsuppressed, and suppressed subs. If it is a <10" barrel, and it runs subs unsuppressed, you need a heavier buffer. The barrel choice is critical, the huge variety of bullet shapes to feed, bore dimensions, chamber specs and port sizes can make a good barrel just work, and a cheap barrel cause a ton of problems. No real need for adjustable blocks unless you are chasing the best suppression numbers, for a defensive build a simple and free flowing fixed block is better. Most of my builds run fine with a standard 3oz or H1 3.8oz buffer and M4 spring. I just try a couple buffers to find the best weight at the range, then if I need one I don't have, just swap tungsten and steel weights I keep on hand.

As far as ammo choice, subs can be viable for defense. Of course supers have better performance, but being the bullets are very long with a very high SD, they generally outperform a shorter/fatter handgun slug of the same weight. Even a non-expanding sub will yaw and cause more penetration and damage than FMJs tend to. Expanding subs also usually outperform 9-45cal handgun slugs, but of course they need to feed and function reliably.

As I mentioned above, I had to open up the port on my 10.5" gun because it would not cycle shit. Even supers were hit or miss. This gun was built in SOTAR's first 'build your own' class. The barrel is a Rainier Ultramatch. Not a cheap barrel. I eventually tried a Nemo 300 Blk buffer spring, and while it reliably cycled subs, both suppressed and unsuppressed, supers were a bit much for it. I actually mashed the buffer bumper at one point.

Once we opened the port(I don't recall from what size to what size), the gun ran like a dream, and still does to this day. Putting an adjustable gas block allowed for better tuning where, it runs everything from 110 V-Max supers to 225g Gallant PC subs and just about everything in between, with excellent accuracy.

 

[alucard0822]

As I mentioned above, I had to open up the port on my 10.5" gun because it would not cycle shit. Even supers were hit or miss. This gun was built in SOTAR's first 'build your own' class. The barrel is a Rainier Ultramatch. Not a cheap barrel. I eventually tried a Nemo 300 Blk buffer spring, and while it reliably cycled subs, both suppressed and unsuppressed, supers were a bit much for it. I actually mashed the buffer bumper at one point.

Once we opened the port(I don't recall from what size to what size), the gun ran like a dream, and still does to this day. Putting an adjustable gas block allowed for better tuning where, it runs everything from 110 V-Max supers to 225g Gallant PC subs and just about everything in between, with excellent accuracy.

Ultramatch barrels almost universally have small gas ports, BA hanson do too. Usually around .1" is ideal for 8-10" , although some
just drill them to .125" And use an adjustable block. You can also cut down on port pop, and make it quieter at the ear by going with a light buffer and choking down gas till suppressed subs just barely cycle.

 

[John from MD]

Ultramatch barrels almost universally have small gas ports, BA hanson do too. Usually around .1" is ideal for 8-10" , although some
just drill them to .125" And use an adjustable block. You can also cut down on port pop, and make it quieter at the ear by going with a light buffer and choking down gas till suppressed subs just barely cycle.

That is how I usually do it.

 

[Frankced]

Man.. honestly after building some ARs. You might as well get a factory built AR. More likely than not, something you want already out there. Now if you WANT to build one that’s different, I think building one is something everyone should do to get familiar, but get a good barrel, trigger, suppressor and you can work most things out with an adjustable gas block. You’re not shooting 300 yards+

 

[outrider58]

Ultramatch barrels almost universally have small gas ports, BA hanson do too. Usually around .1" is ideal for 8-10" , although some
just drill them to .125" And use an adjustable block. You can also cut down on port pop, and make it quieter at the ear by going with a light buffer and choking down gas till suppressed subs just barely cycle.

Thanks for the tip. The gun was built originally as a dual purpose gun, which is why I insist on its wide functionality.

I'm in the process of planning the build of a shorter HD gun. Something in the 7.5-8" length. I haven't yet decided on.a 5.56 or 300blk. Whichever I choose will be always shot suppressed.

 

[alucard0822]

Thanks for the tip. The gun was built originally as a dual purpose gun, which is why I insist on its wide functionality.

I'm in the process of planning the build of a shorter HD gun. Something in the 7.5-8" length. I haven't yet decided on.a 5.56 or 300blk. Whichever I choose will be always shot suppressed.

I did a PDW build with 6" faxon, with subs, it is easily the most fun "rifle caliber" AR15 I have. It's a .125" gas port and functions very well with suppressed subs, or supers with or without a can. Not sure if it's the really short dwell or what, but with subs it is noticeably quieter with less gas to the face than most every longer barrel 300 I've built on. A low pro gas block and Griffin linear comp just clear the 7.25" handguard, and tuck about 1.5" of the can. Even so, my 9" 300BO with 9" handguards is my HD choice, more room on the handguard, and better performance with supers.

 

[outrider58]

I did a PDW build with 6" faxon, with subs, it is easily the most fun "rifle caliber" AR15 I have. It's a .125" gas port and functions very well with suppressed subs, or supers with or without a can. Not sure if it's the really short dwell or what, but with subs it is noticeably quieter with less gas to the face than most every longer barrel 300 I've built on. A low pro gas block and Griffin linear comp just clear the 7.25" handguard, and tuck about 1.5" of the can. Even so, my 9" 300BO with 9" handguards is my HD choice, more room on the handguard, and better performance with supers.

This sounds like it's right up my alley. Thanks J!

 

[alucard0822]

This sounds like it's right up my alley. Thanks J!

can't wait to see what you come up with. The little 6" is so easy to shoot, my 12YO loves it, really soft shooting and quiet, less recoil than most any blowback 9mm PCC, but it will also ring plates out to 200 relatively easy. I was a late adopter of 300BO, but soon as I got into it, it quickly became a favorite.