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PostPosted: Thu Mar 02, 2017 4:36 pm 
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Mil-Surp Psychosis
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Joined: Fri May 30, 2008 10:50 pm
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Location: Charleston, South Carolina
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My son in law picked up an AR-15 styled pistol from a pawn shop. Last weekend he went to an outdoor range to shoot, and found he could not load a cartridge. Took it to a gunsmith to have it checked out. Later, the gunsmith calls and tells him that the pistol is stamped 5.56mm/.223 but is in fact in .300 blackout caliber. Further, the barrel had an 'obstruction. Not sure what that obstruction was as it was a voicemail and so could not ask questions last night. My question to you guys who know more about AR-15s than I; is it possible a previous owner simply put a .300 barrel on a 5.56mm stamped receiver? Is that all such a conversion needs? What would happen if you then fired a 5.56mm round? I am betting the obstruction was part of a 5.56mm case. I expect .223 would fit down that barrel but the case would be unsupported at the shoulder. Gunsmith said barrel was fine once obstruction was removed.

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PostPosted: Thu Mar 02, 2017 5:39 pm 
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.300 Whisper ( backout,blackout,withouta doubt) is based on the .221 Fireball cartridge,not the .223 cartridge.
The blockage is prob a .30 bullet that was loaded to "sub-sonic" velocities that was too weak and stuck in the barrel.
The barrel is normal marked of its caliber and twist rate.
A simple look into the flash hider will tell you if the bore is .22 or .30.

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PostPosted: Thu Mar 02, 2017 6:50 pm 
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Mil-Surp Psychosis
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Joined: Fri May 30, 2008 10:50 pm
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Location: Charleston, South Carolina
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Yes .300 Whisper is based on the .221, but 300 AAC Blackout is a SAAMI spec cartridge whose origins come off the 5.56mm/.223 case. That was my understanding of why you can chamber both in the AR-15 without big modifications. I guess both would fire though,. just not sure about the pressures on .300 Whisper.

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PostPosted: Thu Mar 02, 2017 9:54 pm 
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.221 fireball is a shorted .222 Remington. .223 Remington/ 5.56 is a .22 Remington Special. A lengthened .222 ( not made from .222 Remington magnum)
The .300 Whisper is the same cartridge with another name,same pressures.
6.5 Sportsman is the same as the 6.5 Grendel. Just avoiding the money trail.

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