300 AAC and 5.56 AR 15s Are "Interchangeable" Fact or Fiction

We pretty much can assume the 300 AAC or Black Out can not be fired through a .22 caliber bore such as the the 5.56.

What about the opposite? Some believe a 5.56/.223 round can be chambered and shot from a 300 AAC barrel, rendering that gun a "copycat" of the scary AR 15 and in being so, must adhere to guidelines(heavy barrel) set by FSA 2013.

I say bunk!

There are many here among us who disagree and push the idea that 300AAC ARs must have heavy barrels. Here is my evidence to the contrary.

Here is what I found in just a short search. I would love for those who believe and espouse the notion that a 5.56/.223 can be fired through a 300 BO barrel, post some proof this is at all possible. I've held that at the very least, is inadvisable and more so, impossible.

Many people will falsly assume the size of the bullet is smaller thus it would fit when its the cartridge that comes into play mainly here.

I am pretty certain being a rimless and necked case, the 223/5.56 and 300AAC will headspace on the neck.
The neck on the 300AAC is further back. Thus unless you were to dramatically deform the 223 case, its not going to allow the bolt to go forward.
I was looking for pics to illustrate this on the interwebs to help support the first video.


Why not throw the 300BLK in a 7.62x39 here also. I bet that has a better chance of firing but I dont' know the safety issus other than a split case and blowby

I remain firmly in the 300blk barrel doesn't need to be an HBAR camp.

No one has said that you can fire the wrong round in the rifle.

What some people contend is, the .300 BO is a copy, under the MD AG opinion, in that if you swap ONE part (the barrel), you still have a functioning firearm. Yes, using the proper ammunition.

Yes, I can see the argument that it is not functional with the same ammo.

A COLT HBAR if you swap the barrel with a gov profile becomes banned, does that mean the COLT HBAR is banned?

THere are 300AAC/BLK (website will say .300) on the MD firearm list that says not banned.

Anderson AM15 .300 BLK is listed as not banned

Melnic said:

A COLT HBAR if you swap the barrel with a gov profile becomes banned, does that mean the COLT HBAR is banned?

THere are 300AAC/BLK (website will say .300) on the MD firearm list that says not banned.

Anderson AM15 .300 BLK is listed as not banned

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Colt HBAR is specifically not banned though.

Pinecone said:

No one has said that you can fire the wrong round in the rifle.

What some people contend is, the .300 BO is a copy, under the MD AG opinion, in that if you swap ONE part (the barrel), you still have a functioning firearm. Yes, using the proper ammunition.

Yes, I can see the argument that it is not functional with the same ammo.

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Swapping the barrel nullifies the argument. The barrel makes the gun. It can't be a 300 AAC without a 300 AAC barrel. Swap out any part of a 300 AAC and it will not shoot a 5.56 cartridge.

The problem in the specific case of 300AAC AR-15s is 100% parts interchangeability with a banned rifle (non-HBAR AR-15s). On a 9mm AR, the BCG, barrel, magwell, magazines, etc. are different parts. Swap _one_ of those, and your gun breaks and will not function correctly. It is not 100% parts interchangeable.

With 300AAC, though, it is literally just the barrel. If you swap it out with a 5.56 barrel, you still have a working gun. The law - and the MSP memo - are unclear about whether a caliber change breaks interchangeability. Trying to chamber a 5.56x45 round in the gun is missing the point, I think.

ericoak said:

Colt HBAR is specifically not banned though.

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Sorry, I meant to quote this:

What some people contend is, the .300 BO is a copy, under the MD AG opinion, in that if you swap ONE part (the barrel), you still have a functioning firearm. Yes, using the proper ammunition.

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The fallacy of the statement pinecone was saying that other people say. (in other words, I'm dissagreeing with the statement that Pinecone is also dissagreeing, thus agreeing with pinecone).

The diff from the HBAR to the non HBAR is obviously the barrel, so anyone saying a swaping a 300 black out barrel with a gov profile for instance makes the 300 banned because its swapping one part. Horse Hockey. the rifle is not banned until you do the swap.

To say it another way.
Goofball mall Ninja says : 300BLK is banned because if I swap out the barrel with an M4 profile it becames a copy of a banned rifle.
I say : So, that means Colt HBAR would be banned right now if I apply your same logic.

Melnic said:

To say it another way.
Goofball mall Ninja says : 300BLK is banned because if I swap out the barrel with an M4 profile it becames a copy of a banned rifle.
I say : So, that means Colt HBAR would be banned right now if I apply your same logic.

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Except that's not how the law is written. The law establishes the HBAR carve-out right next to the AR-15 ban - on the same line - to avoid that scenario. Essentially, if you are a Colt AR-15, you are banned, except if you are an HBAR. Expanding that, all copies of the Colt AR-15 are banned, except for all copies of the Colt HBAR AR-15. There is no logical problem with that. The problem we're dealing with is the MSP interpretation of the pre-amble text to the named assault long gun list on something that MIGHT be a copy of the Colt AR-15 with their interpretation.

Also, the mall ninja name-calling is insulting and unnecessary. Some of us have spent a lot of time looking at these laws so we and others can figure out how not to land in jail and still get what we want. I was warning people on here months before the AWB about the potential legal problems with new SBRs post-AWB, and everyone told me I was wrong - until I was right, and the MSP banned SBR copycats.

Perennially attempting to explain the illogical with logic.

If I put the ATI folding stock on my mini 14 ranch rifle, is it identical to the banned folding stock model? Maybe yes, maybe no (because the 1.5" shorter threaded barrel and flash hider on the production folding stock model makes it more dangerous, lets save the children)***.


The only real way to get this question answered is to get arrested and prosecuted, likely for some other crime in which you were using said questionable rifle. So, do you really want to know the answer that badly?


*** or maybe the folding stock model is the one as seen in the A-team, which they dont make anymore.

I think the key here is "100% interchangeability" That means what works on one rifle must also work on the other. If I swap barrels between rifles they must both work. A .30 caliber round and a .20 caliber barrel aint gonna jive.

I also don't think that fact will keep you from being arrested and prosecuted.

Does the law state "100% interchangeability"? Lunch is over and I don't have time to look.

I would say the approval of light weight 300 barreled AR-15's by MSP for sale within Maryland's borders sets a precedent - as with the HBAR version.
But IANAL....

MULE-JK said:

I think the key here is "100% interchangeability" That means what works on one rifle must also work on the other. If I swap barrels between rifles they must both work. A .30 caliber round and a .20 caliber barrel aint gonna jive.

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Jury's out on that interpretation. Maybe literally if it goes to court.

Does the law state "100% interchangeability"? Lunch is over and I don't have time to look.

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No.

outrider58 said:

Swapping the barrel nullifies the argument. The barrel makes the gun. It can't be a 300 AAC without a 300 AAC barrel. Swap out any part of a 300 AAC and it will not shoot a 5.56 cartridge.

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The AG letter on a COPY, says interchange one part between two firearms and both are still fully functional.

Swap the barrel between a 5.56 AR and a .300 BO AR, and you still have to fully functional ARs.

Magnumite said:

I would say the approval of light weight 300 barreled AR-15's by MSP for sale within Maryland's borders sets a precedent - as with the HBAR version.
But IANAL....

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Except that MSP says that their list is not actually a list of legal or illegal rifles.

Evidence is that they have reversed several original listings.

erwos said:

Except that's not how the law is written. The law establishes the HBAR carve-out right next to the AR-15 ban - on the same line - to avoid that scenario. Essentially, if you are a Colt AR-15, you are banned, except if you are an HBAR. Expanding that, all copies of the Colt AR-15 are banned, except for all copies of the Colt HBAR AR-15. There is no logical problem with that. The problem we're dealing with is the MSP interpretation of the pre-amble text to the named assault long gun list on something that MIGHT be a copy of the Colt AR-15 with their interpretation.

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The other issue is, the law does not say anything about caliber for the AR-15 listing. It DOES for other rifles that are banned.

So there is really no legal basis for saying AR-15s in other than 5.56 are legal.

You’ll shoot yer eye out!

Yeah, this is a mess and something that I have addressed elsewhere. Look at the list of rifles in Public Safety 5-101 that are banned. Notice that some have cartridges listed after them, the CAR-15 and Colt AR-15 do not have cartridges listed after them. Therefore, the judiciary will assume that if the legislature wanted to limit the ban to a specific type of rifle that fires a specific cartridge, it would have enumerated it when setting forth CAR-15 and Colt AR-15. The legislature did not do this. However, if you look at Colt rifles, none of them are chambered in anything other than .223/5.56. Next, we have a the "Bushmaster semi-auto rifle" on the list, and MSP is taking the position that all Bushmaster rifles are banned. Bushmaster does make a rifle in .300BLK. Interesting question would be whether Bushmaster makes something in HBAR. Would the carve out in the Colt AR-15/CAR-15 line for the HBAR trump the ban of all Bushmaster semi-auto rifles and their copies?

The AG opinion, dated May 24, 2010, states on page 101:

"a copy of a designated assault weapon must be similar in its internal components and function to the designated weapon."

In the conclusion on page 109, it does on to state:

"in order for a firearm to be considered a copy of a listed assault weapon, and therefore governed by the regulated firearms law, there must be a similarity between the internal components and function of the firearm in question and those of one of the listed weapons. A determination as to whether a particular firearm bears such similarity is a factual question entrusted in the first instance to the Department of State Police. Douglas F. Gansler, Attorney General"

Then, 6 months later, the MSP issued Firearms Bulletin #10-2 on November 4, 2010 stating that a copy must "have completely interchangeable components necessary for the full operation and function of any one of the specifically enumerated assault weapons as defined in Public Safety Title 5 subtitle 1, section 101(P)(2)."

All of this crap is clear as mud.

What is a copy of a Colt AR-15 Sporter HBAR? According to the MSP, an AR-15 with a barrel marketed as HBAR or stamped as HBAR.

So, does the ban only apply to AR-15 rifles in .223/5.56? I would love to know this. If so, it would make things a lot easier on my .300BLK SBR build. Anybody have an answer? Personally, I think the ban applies to more than one single cartridge/caliber when it comes to the AR-15. Otherwise, the legislature would have said Colt AR-15/CAR-15 in .223. Then again, Colt never made an AR-15 in anything but .223/5.56. (Somebody correct me if I am wrong).

End of the day, I can make an argument both ways on this one, and then throw in Bushmaster semi-auto rifles in the mix too. Short of this going to court or the MSP/AG really giving us a definitive answer regarding the AR-15 as it applies to cartridges other than .223/5.56 and what constitutes an HBAR, I think we will be left wondering which way a court of law will go on this matter. Oh yeah, I guess we could always hope for the General Assembly to clear this up with .223/5.56 for the Colt AR-15 or "all calibers" for the Colt AR-15, with a definition for HBAR like "a Colt AR-15 that has a barrel diameter of at least .750" through the entire length of the barrel, except for the threading at the end of the barrel for a muzzle device."

Yay, Maryland law is so fun.

Pinecone said:

The AG letter on a COPY, says interchange one part between two firearms and both are still fully functional.

Swap the barrel between a 5.56 AR and a .300 BO AR, and you still have to fully functional ARs.

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How is it possible to fire a .300 BO through a 5.56 barrel? It seems like it might function for one shot, but not as intended.