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Subs that cylce



Has this been hashed out before and i am duping?



Edit for link. Tried to embed and did not work.

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I believe you must be a team member to embed videos.

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No  I only use subsonic .308 and .223 with my bolt actions, if I want a quiet semi auto in .30 I use my 300 AAC Blackout.

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That makes since. So Mr. dryflash, has this been discussed before?

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No  I only use subsonic .308 and .223 with my bolt actions, if I want a quiet semi auto in .30 I use my 300 AAC Blackout.

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X2
 

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Subsonic 308 that cycles a semiauto requires either a modified gas & spring/buffer system or extremely heavy bullets. The modified gun route will not function (safely) with normal ammo, the heave bullets will need to be made with a very dense metal (tungsten, uranium etc) not something you'll find in the reloading section of your local gun store.

Any details to go with the video?

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In the comments section he states that he used Hornady 220gr round nose with IMR 4759. I dont think the hornadys are made of tungsten or uranium.

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In the comments section he states that he used Hornady 220gr round nose with IMR 4759. I dont think the hornadys are made of tungsten or uranium.

This is a piston gun, it has an adjustable gas block which allows us to make easy changes weather we are shooting subsonic or regular ammo.

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Then he used a modified gas system:

As I said either very heavy bullets or modify the gas system. Based on the limited info they must have opened up the gas port to get functioning with the low gas pressure subs generate and use an adjustable gas block to get supers to work safely.
I did something similar with my 300BLK, pistol length 0.12 in diameter gas port with an adjustable gas block. I get 150 grain subs to function semi auto but can still shoot full power supers.

There was/is a company that offered very heavy 223 ammo (120 grain?) that would function in an unmodified AR15.

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Why use a pistol gas system?  What am I missing here?  Thanks for eh clarification.

Getting the 300 BLK to shoot both subsonic and supersonic rounds without an adjustable gas system is not trivial and necessitated some compromises. To understand why you need to understand the dynamics of a gas driven gun.  This link does a good job of explaining the AR gas system.

Link

In short you need a pulse of gas pressure sufficient to function the bolt. The pulse is a combination of a pressure of X psi for a duration of Y seconds. The combination of these two defines the pressure pulse seen in the bolt chamber. For a given gas port size the pressure is determined by two factors, the peak pressure of the cartridge and the location of the gas port. The duration of the pressure is determined by the length of the barrel remaining after the gas port.  The further the gas port is from the chamber the lower the pressure available to operate the bolt. This also shortens the amount of time for the pressure to act with a given barrel length.  Changing the gas port size effectively changes the pressure seen in the bolt .

Back to the 300 BLK, subsonic vs. supersonic.

To get a subsonic bullet you need either a very short barrel or a low pressure load.  Since we don’t use a 3 inch barrel we reduce the load to get subsonic rounds.  To get high velocity we load the rounds at or near the maximum pressure rating for the cartridge.  The difference in peak pressure between these loads can easily be 2-3 fold.

The consequences of this are a 2-3 (or more) fold difference in the force applied to operate the bolt.  Most gun/cartridge combinations are not designed for such a large difference in operating pressure and this explains why you don’t see 223 or 308 subsonic ammo that functions the action in AR platform guns.

The AAC 300 BLK compromise was to narrow the difference in pressure between sub & supers by using heavy bullets in subsonic loads. Since a heavier bullet needs greater pressure to accelerate it they were able to close the pressure gap between the two extremes. Consequently AAC recommends using 220 grain or heavier rounds for subsonic loads. This is still a compromise and depending on the gas port size you are either over gassed on the hot loads or under gassed on the subs.  To complicate matters the 300 BLK was intended to be suppressed. A suppressor effectively adds length to the barrel equation which equals a longer gas pulse.  This is why some people report good results with subs suppressed but problematic functioning unsuppressed.  It is a narrow window for getting the port size/ pressure pulse correct.

You can go to a shorter gas port but the basic equation still applies. The only advantage I can see is if by using a pistol length gas port you may have a smaller difference in the pressure between subs & suppers since you are now porting closer to the peak cartridge pressure vs. being on the far end of the pressure curve at the carbine port.

If you want the most versatility from this cartridge you will need an adjustable gas system. This allows you to set the pressure pulse appropriate for the load being used. I am not saying you need one if you want to shoot both subs and supers but an adjustable system gives you a lot of dynamic range.

I have a 16 inch 300 BLK barrel with a 0.12 inch diameter gas port at the pistol position. With an adjustable gas block I can run 150 grain subsonics up to full power supers without a suppressor and not be over gassed. That is not possible with a fixed gas port. Of course I do need to make adjustments each time I go from subs to supers, that is the tradeoff.

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I think i am going to abandon this project. It seems like its more of a headache than what the outcome is worth. If i want a sub that cycles, looks like i will go 300blkout. Right now though, i am going to stick to the main 4 calibers of the world.

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pumbaajk

You can easily load subsonic 308 rounds they just don't cycle in semiautos without modifications to the gun.

If you want a nice load I have used 10.5 grains of trail boss with a Hornady 180 grain flat base round nose bullet. Loaded to  2.675 OAL these shoot at ~ 1000 f/s from my 20 inch barreled AR10. They are very quiet and very accurate but must be cycled by hand.