Help with 300 ACC Blackout build

[detroitcharlie]

I have S&W M&P15 Sport lower I want to mate to a new 16" 1:7 or 1:8 300 ACC BLK barrel and was hoping for some suggestions. I don't have a can yet, but I'm planning on getting the 762-SDN-6. Not looking to break the bank either. Planning on going subsonic with the LEE 245 grn cast boolit.

[xacex]

I have both the 1/7, and 1/8 twist. I prefer the 1/7 twist because it is more accurate with subsonic in my experience. If you have a choice go with a pistol gas carbine length. It allows you to use a wider variety of powders, and you can tune it to your liking with an adjustable gas block, or gas key. There are a ton of manufactures now producing barrels. I recommend finding one with a reputable name. One of mine is a CMMG, and although I do not recommend them it shoots great. 16" 1/7 twist pistol gas. There was an issue many years ago with them not cutting the threads concentric with the bore. I have not had a problem with mine in combination with my suppressor. I think green mountain is cutting barrels in 300 bo now. I have had good luck with their barrels for over 15 years. The one they offer is carbine gas though.

Product Description16" 300 BLACKOUT, 1:7 Twist, carbine length, Chrome bore and chamber, phosphate exterior, made of 4150 steel, 7 flute, 5/8x24 muzzle threads. Not pinned for front site base.

http://www.gmriflebarrel.com/gm-m27-16-300-blackout/

[detroitcharlie]

I have both the 1/7, and 1/8 twist. I prefer the 1/7 twist because it is more accurate with subsonic in my experience. If you have a choice go with a pistol gas carbine length. It allows you to use a wider variety of powders, and you can tune it to your liking with an adjustable gas block, or gas key. There are a ton of manufactures now producing barrels. I recommend finding one with a reputable name. One of mine is a CMMG, and although I do not recommend them it shoots great. 16" 1/7 twist pistol gas. There was an issue many years ago with them not cutting the threads concentric with the bore. I have not had a problem with mine in combination with my suppressor. I think green mountain is cutting barrels in 300 bo now. I have had good luck with their barrels for over 15 years. The one they offer is carbine gas though.

Product Description

16" 300 BLACKOUT, 1:7 Twist, carbine length, Chrome bore and chamber, phosphate exterior, made of 4150 steel, 7 flute, 5/8x24 muzzle threads. Not pinned for front site base.

http://www.gmriflebarrel.com/gm-m27-16-300-blackout/

Thanks for the recommendation I'll check them out. I've heard a lot of warnings on buying 300 BLK barrels and that there are some scams going around. Wanna get it right the first time.

[wordsmith]

I have both the 1/7, and 1/8 twist. I prefer the 1/7 twist because it is more accurate with subsonic in my experience. If you have a choice go with a pistol gas carbine length. It allows you to use a wider variety of powders, and you can tune it to your liking with an adjustable gas block, or gas key. There are a ton of manufactures now producing barrels. I recommend finding one with a reputable name. One of mine is a CMMG, and although I do not recommend them it shoots great. 16" 1/7 twist pistol gas. There was an issue many years ago with them not cutting the threads concentric with the bore. I have not had a problem with mine in combination with my suppressor. I think green mountain is cutting barrels in 300 bo now. I have had good luck with their barrels for over 15 years. The one they offer is carbine gas though.

Product Description

16" 300 BLACKOUT, 1:7 Twist, carbine length, Chrome bore and chamber, phosphate exterior, made of 4150 steel, 7 flute, 5/8x24 muzzle threads. Not pinned for front site base.

http://www.gmriflebarrel.com/gm-m27-16-300-blackout/

Good advice here, although I've had great luck with both barrel twists rates. I have 3-300 BLK's, and I've built 3 more for others. Surprisingly, the AR-Stoner barrels at Midway have given excellent performance, as in sub-MOA with 110 VMax rounds and solid subsonic accuracy. The carbine length gas system works well in the 16in as well, if the gas port is sized appropriately at the factory. I've drilled out a couple that were undersized.

I have 4 cans, including the SDN-6, and the key with non-user serviceable cans is to PC or HiTek coat cast bullets so they don't leave lead deposits in your suppressor. If you shoot naked lead rounds, you're asking for trouble.

[detroitcharlie]

Good advise here, although I've had great luck with both barrel twists rates. I have 3-300 BLK's, and I've built 3 more for others. Surprisingly, the AR-Stoner barrels at Midway have given excellent performance, as in sub-MOA with 110 VMax rounds and solid subsonic accuracy. The carbine length gas system works well in the 16in as well, if the gas port is sized appropriately at the factory. I've drilled out a couple that were undersized.

I have 4 cans, including the SDN-6, and the key with non-user serviceable cans is to PC or HiTek coat cast bullets so they don't leave lead deposits in your suppressor. If you shoot naked lead rounds, you're asking for trouble.

Excellent! Thank you.

[xacex]

Good advice here, although I've had great luck with both barrel twists rates. I have 3-300 BLK's, and I've built 3 more for others. Surprisingly, the AR-Stoner barrels at Midway have given excellent performance, as in sub-MOA with 110 VMax rounds and solid subsonic accuracy. The carbine length gas system works well in the 16in as well, if the gas port is sized appropriately at the factory. I've drilled out a couple that were undersized.

I have 4 cans, including the SDN-6, and the key with non-user serviceable cans is to PC or HiTek coat cast bullets so they don't leave lead deposits in your suppressor. If you shoot naked lead rounds, you're asking for trouble.
I have a stoner barrel in one of my rifles. It has been a performer, but the price is a little high for what it is. Those barrels are made by Saturn. I use that one for sonic shooting with iron sights. I can ring steel all day long with it. A2 dissipator style build. Fun gun. You will like the 300 BLK once you get set up and find a few good loads for it. Out of all the calibers I shoot the 300's get the most action, and praise with other shooters when I bring them to the range.

[detroitcharlie]

I have a stoner barrel in one of my rifles. It has been a performer, but the price is a little high for what it is. Those barrels are made by Saturn. I use that one for sonic shooting with iron sights. I can ring steel all day long with it. A2 dissipator style build. Fun gun. You will like the 300 BLK once you get set up and find a few good loads for it. Out of all the calibers I shoot the 300's get the most action, and praise with other shooters when I bring them to the range.

I have no doubt- thanks!

[Electric88]

Ballistic Advantage makes an excellent 16" barrel for the 300 blackout. Also, Radical Firearms sells complete uppers for a very low price and are a pretty decent quality

[cmdrted]

besides proper gas tube holes etc., one very overlooked area is installing the gas block. most barrels and builds are set up to take the carbine retaining punched metal part. this is supposed to bear tight against the barrel step. if you do not leave this small gap when installing the gas block, sometimes (alot) in non 223 cartridges the gas port in barrel doesn't line up with corresponding hole in block and will give failure to cycle properly. all the above is spot on. kak industries were selling green mountain overrun barrels this summer. snaggged a great one for under 100.00 shipped!

[Hamish]

Here you go, $100 dollar 300 BO KAK (GM) barrel:

http://www.kakindustry.com/ar-15-parts/upper-parts/barrels/contract-300-16-inch-pistol-melonite-barrel

[xacex]

Here you go, $100 dollar 300 BO KAK (GM) barrel:

http://www.kakindustry.com/ar-15-parts/upper-parts/barrels/contract-300-16-inch-pistol-melonite-barrel
That's a great find! Shoot, I definitely do not (need) anymore blackouts, but may jump on one of those for a family member. The nice thing is they have the pistol gas port. Even if you decide you want to shorten the barrel at a later date it will still work down to 8.5" without problem. shooting full power sonic loads will beat the buffer a bit at 16", but there are adjustable gas blocks out there that work like a switch between gas settings so it is not an issue. I want to do a side charge upper one of these days so I can turn off the gas, and have a straight pull bolt action. "The" quietest load I have tried was with red dot, and it did not cycle the action anyway. As quiet as a kids bb gun with my suppressor. It is almost unreal to hear more noise at 100 yards from the bullet hitting steel, than the noise that comes from the gun.

[detroitcharlie]

Ballistic Advantage makes an excellent 16" barrel for the 300 blackout. Also, Radical Firearms sells complete uppers for a very low price and are a pretty decent quality

I know of BA, gonna check out Radical's uppers, awesome thanks!

[detroitcharlie]

besides proper gas tube holes etc., one very overlooked area is installing the gas block. most barrels and builds are set up to take the carbine retaining punched metal part. this is supposed to bear tight against the barrel step. if you do not leave this small gap when installing the gas block, sometimes (alot) in non 223 cartridges the gas port in barrel doesn't line up with corresponding hole in block and will give failure to cycle properly. all the above is spot on. kak industries were selling green mountain overrun barrels this summer. snaggged a great one for under 100.00 shipped!

Awesome advice thanks!

[detroitcharlie]

Here you go, $100 dollar 300 BO KAK (GM) barrel:

http://www.kakindustry.com/ar-15-parts/upper-parts/barrels/contract-300-16-inch-pistol-melonite-barrel

Whoa! Nice deal.

[detroitcharlie]

That's a great find! Shoot, I definitely do not (need) anymore blackouts, but may jump on one of those for a family member. The nice thing is they have the pistol gas port. Even if you decide you want to shorten the barrel at a later date it will still work down to 8.5" without problem. shooting full power sonic loads will beat the buffer a bit at 16", but there are adjustable gas blocks out there that work like a switch between gas settings so it is not an issue. I want to do a side charge upper one of these days so I can turn off the gas, and have a straight pull bolt action. "The" quietest load I have tried was with red dot, and it did not cycle the action anyway. As quiet as a kids bb gun with my suppressor. It is almost unreal to hear more noise at 100 yards from the bullet hitting steel, than the noise that comes from the gun.

8.5 ? I thought they 'stopped' at 9" ?

[xacex]

8.5 ? I thought they 'stopped' at 9" ?

The shortest I have seen is 7.5". I have a 8.5" on my SBR. Runs good, but has a tight chamber. Sucks because I have to size cast bullets to fit that one. If I don't I get stuck bullets in the chamber while the rest eat them up..

[detroitcharlie]

The shortest I have seen is 7.5". I have a 8.5" on my SBR. Runs good, but has a tight chamber. Sucks because I have to size cast bullets to fit that one. If I don't I get stuck bullets in the chamber while the rest eat them up..

I'm starting to think I want to go both sub and supersonic, but not sure I want pistol length. What is the ideal length for accuracy but can still do super? Pistol or Carbine? Upgrade the buffer?

[wordsmith]

I'm starting to think I want to go both sub and supersonic, but not sure I want pistol length. What is the ideal length for accuracy but can still do super? Pistol or Carbine? Upgrade the buffer?

This is a little bit of a tangent, but an increase in barrel length is not equal to an increase in accuracy. In fact, all things equal, shorter barrels exhibit a tendency toward more accuracy due to a lower slenderness ratio, which helps attenuate standing waves and "barrel whip". Point is, a short stubby barrel can be more accurate than a long slender barrel by virtue of it's physical ability to resist bending, torsion and harmonics.

That said, this advantage is often negated by environmental factors such as wind, which tend to act more on the slower rounds coming out of the shorter barrel, which can give the appearance of more accuracy from the longer, faster barrel. Point is, don't obsess over barrel length and accuracy, as they are only very loosely related through external factors.

Pistol versus carbine length choices relate to appropriate dwell time for the barrel length. The dwell time is defined as the bullet travel time (in msec) between the gas port and the muzzle. Generally, 300 BLK 16in barrels have ideal dwell time with the carbine gas system, and less than 16in is best served by pistol. However, you can make pistol work with 16 or less pretty easily, so you see a lot of them.

Rather than worry about a buffer (heavy or light), I would suggest an adjustable gas block that lets you dial in the gas for subs and you will likely be in good shape with supers without adjusting. This is more accurate than just assembling and firing both with the unregulated factory gas port diameter. If you're so inclined, you can turn the gas down when going to supers, but honestly I don't have the memory or motivation to do so, and have not encountered any issues.

[Jupiter7]

The shortest I have seen is 7.5". I have a 8.5" on my SBR. Runs good, but has a tight chamber. Sucks because I have to size cast bullets to fit that one. If I don't I get stuck bullets in the chamber while the rest eat them up..

spikes had a 6". The "Honey Badger", came as a factory suppressed completely built gun. YouTube has a few really nice videos about the gun.

[xacex]

spikes had a 6". The "Honey Badger", came as a factory suppressed completely built gun. YouTube has a few really nice videos about the gun.
That's right! I thought I saw something shorter, but could not remember off hand. I like that stock system they used. I have not seen commercially available 6" barrels out there, so it slipped my mind.

I'm starting to think I want to go both sub and supersonic, but not sure I want pistol length. What is the ideal length for accuracy but can still do super? Pistol or Carbine? Upgrade the buffer?

You can do both super and subs with a pistol gas system, and a carbine gas system. The only advantage is being able to use a wider selection of powders with a pistol gas system, and you are able to cut the barrel down at a later date if you want and it will still function. It may not be a factor for you, but I like keeping my options open. An adjustable gas block is cheap if the recoil is to swift for you. It is noticeable running wide open running supers on my pistol gas set up. During a SHTF event I would run one wide open no matter what if it were adjustable, but I have found a nice sweet spot with mine using the powders, and loads I have for both super, and subs. My SBR 8.5" will do .66" groups with the Barnes tac tx bullets at 100 yards. I do not think my carbines would better that.

[detroitcharlie]

This is a little bit of a tangent, but an increase in barrel length is not equal to an increase in accuracy. In fact, all things equal, shorter barrels exhibit a tendency toward more accuracy due to a lower slenderness ratio, which helps attenuate standing waves and "barrel whip". Point is, a short stubby barrel can be more accurate than a long slender barrel by virtue of it's physical ability to resist bending, torsion and harmonics.

That said, this advantage is often negated by environmental factors such as wind, which tend to act more on the slower rounds coming out of the shorter barrel, which can give the appearance of more accuracy from the longer, faster barrel. Point is, don't obsess over barrel length and accuracy, as they are only very loosely related through external factors.

Pistol versus carbine length choices relate to appropriate dwell time for the barrel length. The dwell time is defined as the bullet travel time (in msec) between the gas port and the muzzle. Generally, 300 BLK 16in barrels have ideal dwell time with the carbine gas system, and less than 16in is best served by pistol. However, you can make pistol work with 16 or less pretty easily, so you see a lot of them.

Rather than worry about a buffer (heavy or light), I would suggest an adjustable gas block that lets you dial in the gas for subs and you will likely be in good shape with supers without adjusting. This is more accurate than just assembling and firing both with the unregulated factory gas port diameter. If you're so inclined, you can turn the gas down when going to supers, but honestly I don't have the memory or motivation to do so, and have not encountered any issues.

Not off in the weeds at all, great explanation.

[detroitcharlie]

That's right! I thought I saw something shorter, but could not remember off hand. I like that stock system they used. I have not seen commercially available 6" barrels out there, so it slipped my mind.

I'm starting to think I want to go both sub and supersonic, but not sure I want pistol length. What is the ideal length for accuracy but can still do super? Pistol or Carbine? Upgrade the buffer?

You can do both super and subs with a pistol gas system, and a carbine gas system. The only advantage is being able to use a wider selection of powders with a pistol gas system, and you are able to cut the barrel down at a later date if you want and it will still function. It may not be a factor for you, but I like keeping my options open. An adjustable gas block is cheap if the recoil is to swift for you. It is noticeable running wide open running supers on my pistol gas set up. During a SHTF event I would run one wide open no matter what if it were adjustable, but I have found a nice sweet spot with mine using the powders, and loads I have for both super, and subs. My SBR 8.5" will do .66" groups with the Barnes tac tx bullets at 100 yards. I do not think my carbines would better that.

That's how it's been explained to me, regarding the powder selection. And it seems, the general key is to find that happy mid-point with the gas that accommodates both sub and super. I want to be able to fire my LEE 245grn HiTek coated cast boolits and then easily switch over to FMJ's at sonic without making mods to the gas system.

[detroitcharlie]

So, let me back this discussion up a bit. Now that I have more info- what would people suggest is the most adaptable way to go? What would make sense that I can put together today, be legal while waiting on tax stamp/can and be able to shoot sub and sonic. Barrel, gas block combo...

[xacex]

So, let me back this discussion up a bit. Now that I have more info- what would people suggest is the most adaptable way to go? What would make sense that I can put together today, be legal while waiting on tax stamp/can and be able to shoot sub and sonic. Barrel, gas block combo...
Depends on what your laws are in your state, and what you want to use it for. Can you have SBR's or AR pistols, do you want to go that short, hunting, target shooting, or home defense mall ninja stuff? I have got deer with my 8.5" running supersonic loads with no problem. That was when it was a pistol before I filed paperwork for SBR'n it. It is more or less a 100 yard firearm for hunting. The rile with a big 16x scope can reach out there further, and I can do 200 yards with it and keep all shots in a group on paper which is the theoretical limit of the 300 blackout for an ethical kill on deer. For target/fun short is good, as is home defense but I do not recommend the 300 BLK for that. A 5.56 with hornady 50 grain critical defense in a 11.5 to 14" barrel seems to be better suited for that role due to the fragmentation of the projectile after 2 layers of sheet-rock. You have to consider the people on the other side of the wall unless you live by yourself in the woods. 300 blk subsonic will go through quite a bit of stuff before stopping based on sectional density and weight. Good for car windows/doors, but inside a house it leaves something to be desired. The subsonic hunting thing I cannot advocate unless you keep the shots close, and use a mushrooming, or fragmenting bullet, and can hit the head, spine, or heart confidently.
As a cast round it is great. More bang for the buck, and fun for targets, but understand it is not a do all. As long as you understand the limitations of the round it will serve you well. I really shouldn't say this, but I advocate getting 2. One in the 8.5-10.5" range, and a 16". For hunting I always take the 8.5", and with a suppressor it is about the same length as the 16". But, for practice and fun at the range shooting further I like the 16" with the 16x scope. I think a 16x scope on a 8.5" pistol is a bit of a stretch, but you probably would find the same accuracy as the 16".

[detroitcharlie]

Depends on what your laws are in your state, and what you want to use it for. Can you have SBR's or AR pistols, do you want to go that short, hunting, target shooting, or home defense mall ninja stuff? I have got deer with my 8.5" running supersonic loads with no problem. That was when it was a pistol before I filed paperwork for SBR'n it. It is more or less a 100 yard firearm for hunting. The rile with a big 16x scope can reach out there further, and I can do 200 yards with it and keep all shots in a group on paper which is the theoretical limit of the 300 blackout for an ethical kill on deer. For target/fun short is good, as is home defense but I do not recommend the 300 BLK for that. A 5.56 with hornady 50 grain critical defense in a 11.5 to 14" barrel seems to be better suited for that role due to the fragmentation of the projectile after 2 layers of sheet-rock. You have to consider the people on the other side of the wall unless you live by yourself in the woods. 300 blk subsonic will go through quite a bit of stuff before stopping based on sectional density and weight. Good for car windows/doors, but inside a house it leaves something to be desired. The subsonic hunting thing I cannot advocate unless you keep the shots close, and use a mushrooming, or fragmenting bullet, and can hit the head, spine, or heart confidently.
As a cast round it is great. More bang for the buck, and fun for targets, but understand it is not a do all. As long as you understand the limitations of the round it will serve you well. I really shouldn't say this, but I advocate getting 2. One in the 8.5-10.5" range, and a 16". For hunting I always take the 8.5", and with a suppressor it is about the same length as the 16". But, for practice and fun at the range shooting further I like the 16" with the 16x scope. I think a 16x scope on a 8.5" pistol is a bit of a stretch, but you probably would find the same accuracy as the 16".

I live in MI- we have very generous laws (SBR friendly) here. I'm not looking to take out ISIS, but home defense/hunting boar is kinda my goal.

[detroitcharlie]

Here you go, $100 dollar 300 BO KAK (GM) barrel:

http://www.kakindustry.com/ar-15-parts/upper-parts/barrels/contract-300-16-inch-pistol-melonite-barrel

I had to jump on this. Best adj gas block for under $100 to run at 16" until I get can and go SBR? Any thoughts?

[Jupiter7]

Me personally, 10" with pistol gas is the sweet spot for velocity with supers and keeping subs subsonic(also more dwell time aids in function). I can get 125's up to 2200, my 245grn subs run about 980. I finally went the SBR route, much better than shooting the pistol I originally built. This caliber suppressed is about as much fun as you can have without a stupid switch. Recently got the Handi rifle in 300blk, loaded with pistol powder and 150grn cast, it is backyard quiet.

[Jupiter7]

So, let me back this discussion up a bit. Now that I have more info- what would people suggest is the most adaptable way to go? What would make sense that I can put together today, be legal while waiting on tax stamp/can and be able to shoot sub and sonic. Barrel, gas block combo...

pinned and welded 14.5" with pistol gas. Otherwise, build the pistol in the barrel length you want and wait for the stamp. A buffer tube can be wrapped with 550cord as not to easily accept a stock. Cutting a barrel would be a last resort.

[wordsmith]

Me personally, 10" with pistol gas is the sweet spot for velocity with supers and keeping subs subsonic(also more dwell time aids in function). I can get 125's up to 2200, my 245grn subs run about 980. I finally went the SBR route, much better than shooting the pistol I originally built. This caliber suppressed is about as much fun as you can have without a stupid switch. Recently got the Handi rifle in 300blk, loaded with pistol powder and 150grn cast, it is backyard quiet.

100% agree. I did a Quickload study to compare barrel velocities for different lengths, and corresponding accuracy nodes that could be obtained with given barrel lengths based on the 110 supers (Barnes) that I wanted to shoot. The answer is the 10-10.5" barrel. With suppressor (SDN-6, for example), it's not much longer than a 16" barrel and retains 80% of the 16" barrel velocity.

BTW, I'm not saying that you can't get reliable function without an adjustable gas block - you can. In fact, my 10.5" does not have an adj. gas block. However, I did have to drill out the port about 0.005" from the factory to get subs to reliably cycle the action. If the gas port is undersized to start with, you'll have to drill it out in that case. However, if it's properly sized (most are), then the adjustable block lets you really dial it in to minimize recoil and suppressor blowback in your face.

If you want the ultimate, add an LMT Enhanced Bolt Carrier along with an adjustable gas block. Once optimized, it will stay as clean as a typical AR...all while shooting suppressed. I replaced my piston system on my long range varmint rig with this setup, and it has greatly reduced blowback and build up in the chamber, receivers and magazines.

http://www.lmtstore.com/parts/bolts-carriers-and-groups-complete/556-enhanced-fa-bolt-carrier-group.html

[wordsmith]

I had to jump on this. Best adj gas block for under $100 to run at 16" until I get can and go SBR? Any thoughts?

http://castboolits.gunloads.com/showthread.php?288040-AR-15-Gas-Block&p=3382204#post3382204

[detroitcharlie]

100% agree. I did a Quickload study to compare barrel velocities for different lengths, and corresponding accuracy nodes that could be obtained with given barrel lengths based on the 110 supers (Barnes) that I wanted to shoot. The answer is the 10-10.5" barrel. With suppressor (SDN-6, for example), it's not much longer than a 16" barrel and retains 80% of the 16" barrel velocity.

BTW, I'm not saying that you can't get reliable function without an adjustable gas block - you can. In fact, my 10.5" does not have an adj. gas block. However, I did have to drill out the port about 0.005" from the factory to get subs to reliably cycle the action. If the gas port is undersized to start with, you'll have to drill it out in that case. However, if it's properly sized (most are), then the adjustable block lets you really dial it in to minimize recoil and suppressor blowback in your face.

If you want the ultimate, add an LMT Enhanced Bolt Carrier along with an adjustable gas block. Once optimized, it will stay as clean a typical AR...all while shooting suppressed. I replaced my piston system on my long range varmint rig with this setup, and it has greatly reduced blowback and build up in the chamber, receivers and magazines.

http://www.lmtstore.com/parts/bolts-carriers-and-groups-complete/556-enhanced-fa-bolt-carrier-group.html

That's awesome, thanks for the reco

[detroitcharlie]

http://castboolits.gunloads.com/showthread.php?288040-AR-15-Gas-Block&p=3382204#post3382204

Good thread, thanks!

[Electric88]

I had to jump on this. Best adj gas block for under $100 to run at 16" until I get can and go SBR? Any thoughts?

The Syrac Ordinance adjustable block is good. I personally like the adjustable gas block by SLR Rifleworks :D