Down the rabbit hole I go!

Discussion about rifles in 300 AAC BLACKOUT (7.62x35mm), hosted by the creator of the cartridge.

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B y r o n
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Down the rabbit hole I go!

Post by B y r o n »

So, I've started my journey down the rabbit hole and started the build of a 300 AAC Blackout pistol. For a little background info, this is not my first AR build. I've built several in various calibers; .223/5.56x45, 6.5 Grendel, .308/7.62x51, direct impingement and gas piston. All of my previous builds have involved either mid-length or rifle-length gas systems. This is however my first attempt at an AR pistol.

During my research before starting this build I have run across several issues involved with building a pistol and the 300 Blackout in particular. Some deal with reliability running sub-sonic as well as full power loads. I've also run into the issue of adding a piston kit with regard to receiver and handguard issues, more on that later. I also intend to run the pistol suppressed in the future.

I liked a couple of features offered by Aero Precision. I liked the Atlas S-One handguard. I liked the looks, top rail removed in the middle of the handguard and I liked the way it locked to the receiver. However it won't allow me to tuck any part of the suppressor inside the handguard. This led me to their enhanced upper and handguard combo. I settled on a 12” handguard as it would allow me to tuck the back of the suppressor inside the handguard with a 10” barrel and yet still allow for a muzzle device to reach the end of the handguard until the suppressor was acquired. Things were looking good and I laid the money down on a receiver/handguard set and then ran across this line; “These uppers are not piston-kit compatible ”. I ran through my options like modifying things or switching plans and building it as a DI but decided against both. Instead I decided to keep the parts once they arrived and use them on a future build and buy another receiver and handguard combo. That brought me back to the regular receiver and the Atlas S-One handguard in a 9” length.

I mentioned a 10” barrel because my research indicated that the Blackout would burn all its' powder in a 9” barrel. No sense blowing unburnt powder out the muzzle. There were many barrel makers with good reputations but one of the things that ruled them out for me was I wanted a 1:7 twist for running heavy projectile sub-sonic loads and a lot only offered 1:8. Ballastic Advantage kept popping up during my searches. I settled on their 10.3” Premium Hanson Series.

Now is where it starts to get a little confusing for me. From what I'd read a suppressor tends to increase gas pressures while sub-sonic's run lower pressures. I figured I'd start with a standard carbine buffer and spring and I ordered some tungsten weights to tune the buffer as needed. The other thing that was throwing me was Adams Arms, my choice for a gas piston kit, uses a low mass carrier in their 300BLK kit instead of a standard weight carrier. My assumption is that it is to counter the low gas pressure with sub loads when a suppressor is not used but I haven't contacted them to confirm this.

This leads me to muzzle devices. Looking into the future for a suppressor I ran across two manufactures who make low back pressure designs. OSS's claim is that bolt carrier speed is only increased by 5% with their can. Another benefit is less gas in the face when firing suppressed. Tuning should also be easier as they claim no changes are need for the gun. This looks promising but is in the future. At minimum it looks like a standard A2 birdcage will be on the list unless I find a better option/fit that doesn't mess with function. I don't want to spend a lot of money on a muzzle devise that will eventually end up on the shelf. Besides, it is not like I need a brake, at least I don't think I will.

Some may wonder why my choice of gas piston. My main reason is less gas into the receiver so less dirt to get on more expensive drop-in triggers and such. Not to mention, less gas in the face.

If you see anything that won't work in this parts combination feel free to let me know what it is and why. If you don't think that a piston pistol is the way to go simply because then don't bother, same as some like Ford some like Chevrolet and some like Pontiac.

Not much to mention about the lower, Anderson lower, SB Tactical SB3 brace, Magpul MAID grip and JM Pro MMR Drop In Trigger and a few other parts.

Except for a muzzle device the parts have been ordered and some are already in. Problem is, I'm in one state and the parts are at home in another right now. Once I get back home I can start the assembly process.

This brings me to another subject, ordering parts that are listed in stock only to be told latter that they weren't. Now I understand that sometime timing issues with system inventories can be off but I'm talking more about companies that list the parts and never actually have them in stock, without any indication listed on their sites. Optics Planet is one that comes to mind. Or, how about a company that has your $20 part and then wants to charge $20 to ship it when it will fit inside a small padded envelope.

Looks like I need to pick up some additional loading supplies as well.
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GunFunZS
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Re: Down the rabbit hole I go!

Post by GunFunZS »

B y r o n wrote:So, I've started my journey down the rabbit hole and started the build of a 300 AAC Blackout pistol. For a little background info, this is not my first AR build. I've built several in various calibers; .223/5.56x45, 6.5 Grendel, .308/7.62x51, direct impingement and gas piston. All of my previous builds have involved either mid-length or rifle-length gas systems. This is however my first attempt at an AR pistol.

During my research before starting this build I have run across several issues involved with building a pistol and the 300 Blackout in particular. Some deal with reliability running sub-sonic as well as full power loads. I've also run into the issue of adding a piston kit with regard to receiver and handguard issues, more on that later. I also intend to run the pistol suppressed in the future.

I liked a couple of features offered by Aero Precision. I liked the Atlas S-One handguard. I liked the looks, top rail removed in the middle of the handguard and I liked the way it locked to the receiver. However it won't allow me to tuck any part of the suppressor inside the handguard. This led me to their enhanced upper and handguard combo. I settled on a 12” handguard as it would allow me to tuck the back of the suppressor inside the handguard with a 10” barrel and yet still allow for a muzzle device to reach the end of the handguard until the suppressor was acquired. Things were looking good and I laid the money down on a receiver/handguard set and then ran across this line; “These uppers are not piston-kit compatible ”. I ran through my options like modifying things or switching plans and building it as a DI but decided against both. Instead I decided to keep the parts once they arrived and use them on a future build and buy another receiver and handguard combo. That brought me back to the regular receiver and the Atlas S-One handguard in a 9” length.

I mentioned a 10” barrel because my research indicated that the Blackout would burn all its' powder in a 9” barrel. No sense blowing unburnt powder out the muzzle. There were many barrel makers with good reputations but one of the things that ruled them out for me was I wanted a 1:7 twist for running heavy projectile sub-sonic loads and a lot only offered 1:8. Ballastic Advantage kept popping up during my searches. I settled on their 10.3” Premium Hanson Series.

Now is where it starts to get a little confusing for me. From what I'd read a suppressor tends to increase gas pressures while sub-sonic's run lower pressures. I figured I'd start with a standard carbine buffer and spring and I ordered some tungsten weights to tune the buffer as needed. The other thing that was throwing me was Adams Arms, my choice for a gas piston kit, uses a low mass carrier in their 300BLK kit instead of a standard weight carrier. My assumption is that it is to counter the low gas pressure with sub loads when a suppressor is not used but I haven't contacted them to confirm this.

This leads me to muzzle devices. Looking into the future for a suppressor I ran across two manufactures who make low back pressure designs. OSS's claim is that bolt carrier speed is only increased by 5% with their can. Another benefit is less gas in the face when firing suppressed. Tuning should also be easier as they claim no changes are need for the gun. This looks promising but is in the future. At minimum it looks like a standard A2 birdcage will be on the list unless I find a better option/fit that doesn't mess with function. I don't want to spend a lot of money on a muzzle devise that will eventually end up on the shelf. Besides, it is not like I need a brake, at least I don't think I will.

Some may wonder why my choice of gas piston. My main reason is less gas into the receiver so less dirt to get on more expensive drop-in triggers and such. Not to mention, less gas in the face.

If you see anything that won't work in this parts combination feel free to let me know what it is and why. If you don't think that a piston pistol is the way to go simply because then don't bother, same as some like Ford some like Chevrolet and some like Pontiac.

Not much to mention about the lower, Anderson lower, SB Tactical SB3 brace, Magpul MAID grip and JM Pro MMR Drop In Trigger and a few other parts.

Except for a muzzle device the parts have been ordered and some are already in. Problem is, I'm in one state and the parts are at home in another right now. Once I get back home I can start the assembly process.

This brings me to another subject, ordering parts that are listed in stock only to be told latter that they weren't. Now I understand that sometime timing issues with system inventories can be off but I'm talking more about companies that list the parts and never actually have them in stock, without any indication listed on their sites. Optics Planet is one that comes to mind. Or, how about a company that has your $20 part and then wants to charge $20 to ship it when it will fit inside a small padded envelope.

Looks like I need to pick up some additional loading supplies as well.
Welcome. I over thought a lot of that stuff too. Sounds like your parts list is good. I haven't tried your piston system, but I don't think it is really necessary.

A common thought is that while it looks cool to have your handguard surround your can, that's a great way to transmit heat to your handguard.

I have found with this caliber, that the main things you will find is that it is picky about bullet profile shapes, and most of the barrels have feed ramps that are designed for quarter inch bullets. Almost every "high end" barrel out there, would be improved with bubba level dremel work in that are. It's a shame. But once you get over squeamishness, and do the obvious smoothing and widening of the feed ways you will never regret it.

Another note. As you start reloading, make a dummy cartridge for any potential load. If you can get it to nearly chamber in a 556, then either use a different bullet profile or seat it out longer. I know you can make a lot of stuff work, but IMO, it needs to be .125 from fitting into a 223 case gauge, for it to be acceptable. Some of the M80 ball type bullets will mimic the shoulder of 223 at reasonable seating depths (for 300 BO), or be close enough that if someone beats on the forward assist, they will chamber. If you are a social shooter, it's a good idea to keep your ammo stuff separate, and make it so even if that safeguard fails, the dumbest one of your friends can't hurt himself. Treat cannelures as more of guidelines than rules when you are using bullets designed for 308 win.
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Sithlord
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Re: Down the rabbit hole I go!

Post by Sithlord »

Early on in this forum, there were people having all kinds of issues getting a piston-gas driven 300 BLK in all 4 configurations (supers/subs/suppressed/unsuppressed). Similar issues were not had with DI systems.

That's not to say that it can't work, or won't work, or won't work for certain configs, which you may or may not care about.

Handguard tucked or untucked is a personal preference. You will probably want to stick with direct thread mounts rather than quick attach/detach if you're going to tuck. Something else to consider is that when you tuck the suppressor, depending on the handguard, you may not be able to put accessories if they require an MLOK or KeyMod rail added (the suppressor may interfere with the locking mechanism of the rail).
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B y r o n
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Re: Down the rabbit hole I go!

Post by B y r o n »

GunFunZS wrote:
B y r o n wrote: . . . A common thought is that while it looks cool to have your handguard surround your can, that's a great way to transmit heat to your handguard.

I have found with this caliber, that the main things you will find is that it is picky about bullet profile shapes, and most of the barrels have feed ramps that are designed for quarter inch bullets. Almost every "high end" barrel out there, would be improved with bubba level dremel work in that are. It's a shame. But once you get over squeamishness, and do the obvious smoothing and widening of the feed ways you will never regret it.

Another note. As you start reloading, make a dummy cartridge for any potential load. If you can get it to nearly chamber in a 556, then either use a different bullet profile or seat it out longer. I know you can make a lot of stuff work, but IMO, it needs to be .125 from fitting into a 223 case gauge, for it to be acceptable. Some of the M80 ball type bullets will mimic the shoulder of 223 at reasonable seating depths (for 300 BO), or be close enough that if someone beats on the forward assist, they will chamber. If you are a social shooter, it's a good idea to keep your ammo stuff separate, and make it so even if that safeguard fails, the dumbest one of your friends can't hurt himself. Treat cannelures as more of guidelines than rules when you are using bullets designed for 308 win.
It looks like the handguard over the suppressor issue has been solved for me since the enhanced receiver from Aero is not piston compatible. The Atlas S-One I then bought is only 9" and not big enough inside to handle a suppressor at only about 1.3" ID.

I appreciate the input on the bullet selection and dummy rounds, hopefully it will prevent a few head aches.
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Re: Down the rabbit hole I go!

Post by B y r o n »

Sithlord wrote:Early on in this forum, there were people having all kinds of issues getting a piston-gas driven 300 BLK in all 4 configurations (supers/subs/suppressed/unsuppressed). Similar issues were not had with DI systems.

That's not to say that it can't work, or won't work, or won't work for certain configs, which you may or may not care about.

Handguard tucked or untucked is a personal preference. You will probably want to stick with direct thread mounts rather than quick attach/detach if you're going to tuck. Something else to consider is that when you tuck the suppressor, depending on the handguard, you may not be able to put accessories if they require an MLOK or KeyMod rail added (the suppressor may interfere with the locking mechanism of the rail).
My plan is to get it assembled and then make sure it functions with subs without a suppressor and the gas unrestricted. Make any adjustment necessary to get it running and able to lock the bolt back on the last round. Then see how it handles super sonic's and bleed the gas as needed to keep it from beating itself to an early death. With any luck I may only need to remember 3 settings, sub's without a suppressor, super's without a suppressor, and then sub's and super's with a suppressor.

Adjustable gas blocks seem to be needed even with DI and the piston setup is adjustable so we'll see how it works.
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Re: Down the rabbit hole I go!

Post by Sithlord »

B y r o n wrote:
Sithlord wrote:Early on in this forum, there were people having all kinds of issues getting a piston-gas driven 300 BLK in all 4 configurations (supers/subs/suppressed/unsuppressed). Similar issues were not had with DI systems.

That's not to say that it can't work, or won't work, or won't work for certain configs, which you may or may not care about.

Handguard tucked or untucked is a personal preference. You will probably want to stick with direct thread mounts rather than quick attach/detach if you're going to tuck. Something else to consider is that when you tuck the suppressor, depending on the handguard, you may not be able to put accessories if they require an MLOK or KeyMod rail added (the suppressor may interfere with the locking mechanism of the rail).
My plan is to get it assembled and then make sure it functions with subs without a suppressor and the gas unrestricted. Make any adjustment necessary to get it running and able to lock the bolt back on the last round. Then see how it handles super sonic's and bleed the gas as needed to keep it from beating itself to an early death. With any luck I may only need to remember 3 settings, sub's without a suppressor, super's without a suppressor, and then sub's and super's with a suppressor.

Adjustable gas blocks seem to be needed even with DI and the piston setup is adjustable so we'll see how it works.
My 8" DI works without issue in all four modes without an AGB... Good luck!
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Re: Down the rabbit hole I go!

Post by gds »

In my personal opinion a piston gas system in an AR15 is a solution looking for a problem.

That having been said. I realize many like them in 5.56.

I can run all my AR15 300BLK DI guns all 4 ways without adjusting the gas block. Not saying it could not help in fine tuning to do so. I simply have not.

There is a reason you will have a hard timr finding anyone using a piston on a 300blk short barrel with subs. It is difficult to get running and keep running.. Subsonic factory loads will very likely not work. Maybe handloading will work. But you will be limited on powders.

Either way, I hope you are successful in yoiur endeavor.
Yes, I am a Baptist, and yes I carry a gun. You might think I carry a gun because I don't trust God. Well you would be wrong. I have complete faith in my Lord. It is mankind I have no trust in
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Re: Down the rabbit hole I go!

Post by GunFunZS »

^ That is exactly my thoughts.

One real benefit of the piston though is that with cast and a can, you aren't really able to deposit lead vapor right under your nose.
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Re: Down the rabbit hole I go!

Post by B y r o n »

Well, it seems most of my parts are in or will be in within the next couple of days. The exception is the barrel. Ordered it from Optics Planet because it was listed in stock only to find out later that it wasn't. I contacted Ballistic Advantage and was informed that the barrels were in production but at least 2 weeks out. That was for their site users and they wouldn't comment on my Optics Planet order. Hopefully I'll be back at the house around the same time as the barrel and I can get to assembly.
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Re: Down the rabbit hole I go!

Post by B y r o n »

During this build I've not had the greatest of luck with vendors, except the ones that post here.

Tried to order a carbine buffer tube end plate only to have the vendor cancel the order and refund since they apparently didn't have it in stock as listed.

Optic's Planet shipped me a 15" handguard instead of the 12" I had ordered. Barrel is still not delivered yet but the manufacture should have them available any day now I hope.

None of the receiver/handguard combo's I've ordered are gas piston compatible without some modification. May end up going DI unless I can swap some parts from one of my other guns and make things work.

Rifles and carbines seem to be much easier to build compared to pistols. One of my best finds though was a Vortex Strike Eagle 1-8x24 with mount on sale at PSA for under $300. I guess you have to put up with a few inconveniences every once in a while.
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