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I'm new to it, but from what I have seen it's just the barrel is different. Same upper, BCG and mags since it's a .223/5.56 case.
 
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You need to take onto account what ammo you plan on shooting through it, subsonic or supersonic.
Buffer and spring might need to be changed and maybe an adjustable gas block to dial it in.
 
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I'm new to it, but from what I have seen it's just the barrel is different. Same upper, BCG and mags since it's a .223/5.56 case.

Yup. It's a .30 cal bullet shoved into a .223/5.56 case.

You can actually turn .223/5.56 brass into 300 blk brass by cutting it down and resizing.
 
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Yup. It's a .30 cal bullet shoved into a .223/5.56 case.

You can actually turn .223/5.56 brass into 300 blk brass by cutting it down and resizing.

I'm actually saving all of the military 5.56 brass I get, going to turn it into 300 blk to feed an AR pistol I will be building.
 
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So im curious about converting my 5.56 gun to a .300blk. What all is needed to do this? Can I just get a complete .300 upper and pin it to my AR lower?

you can do that, or you can just buy a new barrel for 200$ and reuse everything else... except if your current barrel is a midlength or rifle gas, you will need a carbine gas tube..

if shooting supersonic, I doubt you will need new buffers, subsonic is a totally different story
 
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You need to take onto account what ammo you plan on shooting through it, subsonic or supersonic.
Buffer and spring might need to be changed and maybe an adjustable gas block to dial it in.

no buffer or gas block changes needed. it was designed from the start to run both supers and subs just fine without any changes.

built several 300blk rifles and i reload extensively for it.
 
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Awesome thanks for all the info guys. So what I will look to do then is either get a complete 300 upper or buy a 300 barrel and put it on a new upper. That way I can keep my 5.56 setup also
 
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300blk is optimized for pistol length gas.

i repeat, 300blk is designed from the ground up to work without changes.

This and this:

Just be warned, you are going to want to turn that 300BLK into a SBR and add a suppressor real soon :s0155:

From reformed 5.56 brass:
IMG_20130818_120244.jpg

10.5" Rainier Arms/Noveske hybrid soon to be SBR:
IMG_20130818_181125.jpg

Accuracy not too bad:
IMG_20130818_095808.jpg
 
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Some day next spring I expect to receive the sbr & suppressor stamps.
I am now gearing up to start reloading 300 BLK subsonic for it.
 
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Any good places to find complete uppers for a decent price? It seems like they're all waaay more than 5.56 parts. I know the volume sold is lower, but really, it's just the barrel that is different.
 
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Any good places to find complete uppers for a decent price? It seems like they're all waaay more than 5.56 parts. I know the volume sold is lower, but really, it's just the barrel that is different.

I'm thinking of trying these guys. There were some complaints about slow orders during the panic, but from what I've read, most folks that got their stuff are happy with it.

RED X ARMS - AR15 RIFLES, PARTS, AND ACCESSORIES
 
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I built my .300 Blk from scratch with parts from Midway. Assembled it like any other 5.56 upper and it worked great after opening up the port on the barrel a bit. Mine came with a .093 port and I had to take it out to .120 to get it to cycle. I found out that the old 20 round vietnam era magazines work terrible with mine shooting the .300 Blk. Something about the indent on the side of the magazine and pushes against the side of the bullet. I found that Pmags work great and also the standard military 30 rd mags work great too. If I had it to do over again I would purchase an adjustable gas block and just dial it in from the get go.
 
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.300 AAC "Blackout" was designed by AAC to run on 10 inch suppressed barrels with subsonic ammo when engaging close in targets with armor or behind cover.

Key being subsonic, larger slower projectile, high end exit speed, quick drop off...

If you are not looking for an optimized SBR suppressed platform round then I would shy away from it as you have better options available.

The 300AAC in non sub sonic suppressed platform with longer barrels is actualy really sub standard on average as far as external/terminal ballistics to more average ammo.

It shines when suppresed in subsonic compared to other standard rounds when supressed, especcialy when reviewing close in penetration.

Perfect supressed upper, adams arms gas piston with a 10 inch 300 aac barrel and AAC cyclone supressor.

Serious low signature close in capabilities.
 

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