I have a 16" 556 upper that sits in my closet collecting dust. Can I just grab a 300 BO barrel for that upper and basically have an entirely new caliber? 300 BO and 556 use same BCG, right?

Yes, not something you're going to want to hot swap back and forth but it's definitely doable

I plan to leave it in 300 BO. I don't ever shoot the 556, as I have my 10.5

Yes, most of the parts are swappable.

Things to consider:

What length gas system is your 5.56 vs the 300BO you choose? Might need a different length gas tube.

What style gas block / handguard is the current upper? and will it work with the barrel you choose?

skelso said:

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Yes, most of the parts are swappable.

Things to consider:

What length gas system is your 5.56 vs the 300BO you choose? Might need a different length gas tube.

What style gas block / handguard is the current upper? and will it work with the barrel you choose?

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-556 is carbine length. I will buy a pistol length tube with the barrel

Current upper has polymer magpul hand guard and an atrocious quad rail gas block.

Barrel comes with a gas block, though I will probably get a SA adjustable at some point. Initially plan to run a heavier buffer to slow bolt down. Getting a hand guard from a fellow OB-er

Here it is currently

Might think of going shorter w a 300BO if you have the ability w all the atf stuff going on these days. For example if you had a multi cal SBR lower you could shorten that upper considerably, shorten your hand guard, and have room for a can and still maintain maneuverability.

Then you can just put that shortened upper on your SBR lower and you're ready to go. Seem to recall you were set up for that. The 300blk cartridge does pretty well out of 9-11 inch barrels. My 2 cents, but you may not want to do that much work on your carbine length upper.

You already have a low velo 556. A short 300 bo is redundant.

Keep the 16 I say.

Possible issue there... if it started life as a full rifle, it can't be changed to a pistol. Now, the odds of being caught with that are very low but it is a possibility. Start life as Pistol and go back and forth at will.

So that upper would only be able to be used on the 10.5 lower or on an sbr

SMM48 said:

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You already have a low velo 556. A short 300 bo is redundant.

Keep the 16 I say.

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Not redundant if he wants to say, pig hunt with subs and have a rig that's as quiet as possible and still run within AR15 mag constraints. Where the 300Blk (and 8.6Blk) really shine is being as quiet as possible while still firing a projectile that has reasonable terminal performance on critters within range limits. When you get into subsonic hunting, 5.56 simply cannot duplicate what 300Blk can do.

Ok, another thing after looking at the pics, kelso's concern is a thing. 300blk is generally pistol length and necessitate a different gas block and probably a free float handguard.

Im getting a new hand guard and gas block

Planning either 8.3 or 9. Have 2 SBR lowers; one with a 10.5 556 and one with a 12" grendel.

Will swap blk and 223 uppers on one of the lowers.

This gun's main job will be camera watching and sneaking up on pigs, suppressed and < 70 yards

Ok... now do some math. The only thing you are saving here is the upper. Make more sense to sell that for 150 and buy a new upper

He's be fine as long as he put the rebuilt upper w short barrel 300 blk on a SBR lower correct?

dr_boogs said:

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He's be fine as long as he put the rebuilt upper w short barrel 300 blk on a SBR lower correct?

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Yes.

At that point you might as well spend another 70-100 on a upper receiver and just have a full upper to swap.

TheEyeGuy said:

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Ok, another thing after looking at the pics, kelso's concern is a thing. 300blk is generally pistol length and necessitate a different gas block and probably a free float handguard.

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My concern is always a thing... Too often I see people give an incomplete answer to caliber swaps. Sometimes you can just swap barrels when swapping calibers but more often that not you end up swapping other parts as well. Especially with 5.56 to 300BO since 300 BO is designed to run pistol length gas and 16" 5.56 rarely, if ever, run pistol length gas.

TheEyeGuy said:

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Ok... now do some math. The only thing you are saving here is the upper. Make more sense to sell that for 150 and buy a new upper

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I agree with this. Part of why I brought up other things to consider. When you consider swappi g that many parts to get the config you want, you aren't saving any money. Build a complete upper. Sell or keep the one you have now.

Subs. Pffft

tandy miller said:

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Planning either 8.3 or 9. Have 2 SBR lowers; one with a 10.5 556 and one with a 12" grendel.

Will swap blk and 223 uppers on one of the lowers.

This gun's main job will be camera watching and sneaking up on pigs, suppressed and < 70 yards

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I have had 300BO from 8" to 12". My experience is under 10.5 (or 10.3 for BA barrels) gets into an area where some subs won't stabilize. At that point you run a higher rish or baffle strike and have gun that may be accurate but not precise.

I would recommend going at least 10.5. My favorite of all the 300BOs I've built is my 12" AAC barrel with a 15" handguard w/ large enough diameter I can tuck the suppressor underneath.

What twist rate?

I ordered a 9" 1 in 7" with a pistol gas system and have a 10" carbon handguard coming so I'll be able to tuck my can in a bit. Everything I've read seems as though that twist will be ok for supers or subs

SMM48 said:

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Subs. Pffft

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What do you dislike about subs?

Yeah, subs have major limitations, but if you already have lots of other guns in a multitude of chamberings, why not? They have their niche. I kill lots of pigs with subs in a suppressed rifle with thermal scope. I get a kick outta stealth night hunting with a CF rifle that's about as loud as a pellet gun.

Whatever else you choose, def make sure you get an adjustable gas block.

Correct me if I'm wrong, but I'm pretty sure that AR uppers, regardless of barrel length, are agnostic as to whether they are pistols, rifles, or SBRs.

It's the AR lowers that have to be designated as one specific type (with the appropriate NFA tax stamp if it's an SBR.)

And you can put an upper with a barrel >16" on a pistol or SBR lower, but don't put a short barrel upper on a rifle lower. Actually, if the ATF goons are feeling like d**ks, don't even have a short barreled upper around if you have a rifle lower and don't have either a pistol or SBR lower, because they might nail you for "constructive possession".

Smeghead4761 said:

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Correct me if I'm wrong, but I'm pretty sure that AR uppers, regardless of barrel length, are agnostic as to whether they are pistols, rifles, or SBRs.

It's the AR lowers that have to be designated as one specific type (with the appropriate NFA tax stamp if it's an SBR.)

And you can put an upper with a barrel >16" on a pistol or SBR lower, but don't put a short barrel upper on a rifle lower. Actually, if the ATF goons are feeling like d**ks, don't even have a short barreled upper around if you have a rifle lower and don't have either a pistol or SBR lower, because they might nail you for "constructive possession".

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You're correct. I have 2 SBR lowers so it will be a non issue

All correct. It's the serialized part (in this case the lower) that is the "firearm" and receives the NFA vs non NFA designation. Except an NFA receiver can later be put into non-NFA config with no problem, but not vice-versa.

Smeghead4761 said:

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Correct me if I'm wrong, but I'm pretty sure that AR uppers, regardless of barrel length, are agnostic as to whether they are pistols, rifles, or SBRs.

It's the AR lowers that have to be designated as one specific type (with the appropriate NFA tax stamp if it's an SBR.)

And you can put an upper with a barrel >16" on a pistol or SBR lower, but don't put a short barrel upper on a rifle lower. Actually, if the ATF goons are feeling like d**ks, don't even have a short barreled upper around if you have a rifle lower and don't have either a pistol or SBR lower, because they might nail you for "constructive possession".

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Depends on the state. Some states limit the barrel length on pistols.

Okay, I have the barrel, gas block, and tube on the way. Barrel is actually 8.3", the 9" was govt profile and I don't want the extra weight up front. So it will be inside handguard by about 1 3/4".

What I'm having trouble finding is a 1 3/8-24 x 5/8-24 direct thread mount for my suppressor.

Here's the handguard I'll be running. Barrel + can will be about 13.5". Going to a buddy's place first weekend in march, hopefully I have it ready to go by then.

Sounds you like to hog hunt like I do. I put together a 7in 1/5 twist faxon barrel upper thats a dozer with subs. I only shoot subs with it given the twist. It absolutely loves Sig 205gr hunting ammo. Getting up close and personal on the hogs is a rush in itself. Good luck with your project!

Thank you!

Just missing the barrel. Suppressor should tuck in about as much as pictured. Hard to believe how light this handguard is… like 6 oz including barrel nut

Sorry, just saw your reply. mine are all 1:7, except my 12" AAC which is 1:8.

No worries, I think I had ordered the barrel already. I have some 110 gn vmax and 220 gn SMK's on the way. Can't wait to get the barrel!