AR-15 300blk- best way to build one

[Bowgod02]

After following along with EMs post on the 9mm build, I'm interested in building a 300blk subsonic that will get suppressed. I have the lower and parts kit so far. The more I read the more I get confused on what I should purchase.
Hoping there are some of you here that has had some experience and are willing to share your thoughts.

Thank you

[ftownkev]

From what I hear don't go under a 10" barrel. I have a 10.5" barrel SBR and it runs like a dream. Also if you plan on it being a suppressor host I would highly recommend the JP Silent Buffer Spring they bring quiet to a whole new level.

https://www.jprifles.com/1.4.7.2_os.php

[NathanB]

I've got an 8.5 inch sbr and it's never malfunctioned. Since its suppressed it gets passed around a lot too.

It's been awhile since I researched but the 300 blk ballistics are designed around an 8 inch barrel iirc.

Here's mine

[esh21167]

I believe 9" is the base barrel length for 300 Blackout ballistic measurement.

I'm a bit ahead of you OP, and in the middle of a small struggle with my first 300 using an 8" barrel. Nothing cycles. The manufacturer has told me to enlarge the gas port from .098 to .106. Barrels seem to be designed to shoot hot/light/supersonic and enlarging gas ports doesn't seem that uncommon. The only thing I can deduce is that's for 'safety'. If barrels shipped with larger gas ports to cycle subsonics, they would likely cycle very hard for supers.

NathanB, curious as to what 8.5 barrel you used, and its gas port size if you know it.

[Nate7667]

Ive been running a 8.5" Noveske 300Blk barrel since 2010. First with just an SDN-6 on the end and later upgraded it to an integral Leonidas from Liberty Suppressors.
Never had a hang up other than figuring out which buffer to run once i got some hand loads tweaked.
Pressure changes from shooting with and with out a can. I shoot subs 95% of the time. Thats what this cartridge was made for, esp in the AR system.
I can run either super or subs with an off the shelf carbine buffer and spring.

Friends through the years have ran 12.5 down to 8" barrels. Both carbine and pistol length gas systems. Most issues were gas port size or powder selection when handloading. Ports are easily fixed with a drill.
But make sure that is your issue, after its drilled, your only fix is an adjustable gas block....or a new barrel.

Bowgod, pick a barrel length and gas length (pistol in my opinion) and go from there.
Grab a barrel from a known manufacture.
Are you planning on handloading?
Barrel length and gas length choices give you a broader range of powders to play with.
A1680 will always work. But to me its loud and hard to come by sometimes. But it will cycle a properly built upper with the correct gas port size.

[aubie515]

You must be hearing the wrong info. For 300BO most go between 8.5" to 10.5" because you don't need a long barrel to burn the powder in 300BO.

If you buy the proper barrel, you won't need to open up your gas port. Stay away from CMMG barrels and the cheap barrels and you should be GTG.

If you plan on hunting with your pistol/SBR, make sure you choose a bullet that will expand with subsonic loads. Hornady 208gr Amax will not be a good choice for hunting.

From what I hear don't go under a 10" barrel. I have a 10.5" barrel SBR and it runs like a dream. Also if you plan on it being a suppressor host I would highly recommend the JP Silent Buffer Spring they bring quiet to a whole new level.

https://www.jprifles.com/1.4.7.2_os.php

[ExFlyinguy]

Just to add my limited experience here. I have a 10.5 stainless that I think I got from MAS Defense: http://www.shop.masdefense.com/105-3...037HBBL-SS.htm

I chose the length based on Silencer warranty concerns and not wanting to go less than 10.5"

My experience is that both supers & subs work with can on. Without can my subs wouldn't cycle the ammo using IMR4227. Haven't pursued the matter any further as I don't want to shoot sub's without a can- don't see the point.

[Bowgod02]

Ive been running a 8.5" Noveske 300Blk barrel since 2010. First with just an SDN-6 on the end and later upgraded it to an integral Leonidas from Liberty Suppressors.
Never had a hang up other than figuring out which buffer to run once i got some hand loads tweaked.
Pressure changes from shooting with and with out a can. I shoot subs 95% of the time. Thats what this cartridge was made for, esp in the AR system.
I can run either super or subs with an off the shelf carbine buffer and spring.

Friends through the years have ran 12.5 down to 8" barrels. Both carbine and pistol length gas systems. Most issues were gas port size or powder selection when handloading. Ports are easily fixed with a drill.
But make sure that is your issue, after its drilled, your only fix is an adjustable gas block....or a new barrel.

Bowgod, pick a barrel length and gas length (pistol in my opinion) and go from there.
Grab a barrel from a known manufacture.
Are you planning on handloading?
Barrel length and gas length choices give you a broader range of powders to play with.
A1680 will always work. But to me its loud and hard to come by sometimes. But it will cycle a properly built upper with the correct gas port size.

I will be reloading for subs and have been collecting stuff for it already. I have read about the leonidas uppers. Read good things about them.

[Bowgod02]

I've got an 8.5 inch sbr and it's never malfunctioned. Since its suppressed it gets passed around a lot too.

It's been awhile since I researched but the 300 blk ballistics are designed around an 8 inch barrel iirc.

Here's mine

Nice looking ar there. I like it

[Bowgod02]

I believe 9" is the base barrel length for 300 Blackout ballistic measurement.

I'm a bit ahead of you OP, and in the middle of a small struggle with my first 300 using an 8" barrel. Nothing cycles. The manufacturer has told me to enlarge the gas port from .098 to .106. Barrels seem to be designed to shoot hot/light/supersonic and enlarging gas ports doesn't seem that uncommon. The only thing I can deduce is that's for 'safety'. If barrels shipped with larger gas ports to cycle subsonics, they would likely cycle very hard for supers.

NathanB, curious as to what 8.5 barrel you used, and its gas port size if you know it.

I've been following along with your post as well. Hope you get it figured out soon.