Hello fellow patriots;

I just Picked up a new M&P 15 sport II today and I'm a little confused on the 300 ACC ammo.

From all the videos I've watched they say I have to change out the barrel to fire the 300 acc blackouts. The owners manual on page 9 says I can fire the 300 blackout if i have the 5.56 NATO stamped on the barrel which it does have. Do I have to change anything at all like the flash hider or is this riffle good to go if I want to use those rounds? I will most likely use .223 or 5.56 but I like the option for the 300 if it's available. I apologize if this has been asked a bazillion times, I did a search on google and it all came up that I would have to change the barrel.

thank you,
Gary

I don't have an M&P 15 but there's no way that is possible. You must be reading the manual incorrectly.

The bullets are a different size. Spend $200 on a new upper if you want to be able to switch between the 2 rounds.

let me figure out how to upload a pic and I will post it

You must change out the barrel or do a complete upper. The bolt, bolt carrier group, mags etc are all the same.

OK see if the atachement is there, if not I will try something else just let me know.

Attached Images

IMG_0260.jpg (140.8 KB, 103 views)

maybe it's just the ammo straight across from the caliber marked on the barrel, either way they need to be Much clearer.

What the instructions are saying is a barrel marked 5.56 can also shoot .223 and a barrel marked .300 Whisper can also shoot .300 AAC/Blackout not that a .30 caliber bullet can be fired through a .22 caliber barrel.

But, back to your original question, yes to fire .300 BLK you will have to change barrels.

DO NOT, repeat Do not try to fire 300AAC/Blackout in your Sport II. Round will chamber and go off, but you will cause some serious issues to you and the rifle.The Sport II will shoot either 223 Rem or 556 Nato.

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Peace was my Profession

Google "300 Blackout fired in 5.56 NATO photo." I cannot think of a faster route to disaster.

300 Blk WILL chamber in 5.56 barrels, WILL fire, and the results will be very ugly.

A 300BLK fired in a 5.56 or .223 barrel leads to a spectacular disassembly of the rifle with possible severe traumatic injury to the shooter.

Unless you like blown up rifles and emergency rooms, don't do it.

FWIW, I can't help but believe the OP is pulling our legs.

You are mis-reading that page.

Draw a line straight across from 5.56 NATO and you will find 223 Remington, both of which use the same 0.224 inch diameter bullets and feature the same case dimensions.

Now draw a line straight across from 300 Whisper and you will find 300 AAC, both if which use the same 0.308 inch diameter bullets and case dimensions.

BTW, the 300 Blackout (or Whisper or AAC) all start with a 5.56/223 case that is cut shorter and necked for a larger diameter bullet. Because of this with some loads of the 300 BLK featuring a "short" bullet it is possible to chamber this 30 caliber round in a 5.56/223 chamber. Doing this and then pulling the trigger will result in a BLOWN UP rifle, 100% OF THE TIME. I have both 223 and 300 BLK rifles built on the AR-15 platform and to insure that I don't ever make the mistake of using the wrong ammunition in a 223 is by using ONLY the light sand colored magazines for my 300 BLK. If it's in a dark magazine it's always 223, if it's a sand colored magazine it's always 300 BLK.

One further note on the 300 BLK. This particular caliber started with a Wildcat round based on a 223 case and featuring a 30 caliber bullet. This particular Wildcat caliber was called the 300 Whisper because it was designed specifically to produce a sub sonic velocity bullet that would supress well. A bit later the US Special Forces requested that a new 30 caliber subsonic round be developed for specific tactical uses and AAC simply purchased the rights to the 300 Whisper and re-named it the 300 AAC. A bit later they decided it might be somewhat popular on the Civilian market so IIRC they approached Hornady to produce this new civilian ammunition. At that point either AAC or Hornady decided to call the new loading the 300 Blackout. So, while there are some tiny differences in going from the 300 Whisper to the 300 Blackout they are really all the same cartridge that has simply been refined over the years. I will also note that the Whisper and 300 AAC were primarily subsonic loads with any supersonic ammunition being basic experimentation. The 300 Blackout has a much wider range of available ammunitions featuring a wide variety of bullets from around 100 grains up to 220 grains at velocities ranging from about 800-1000 fps all the way up to 2400 fps or possibly higher. This variety is one reason why the 300 BLK has become such a popular caliber.

I was shooting a preban colt a month or two ago and my gun got stuck. It would not fire. I cycled out the round in the chamber and a 300blk popped out. I am grateful it did not fire. Lucky Duck Day for dummy me.

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Just hanging with my dogs

Wow, just Wow!

Let me make it clear. If you are able to stuff a .300 blackout in a .556 chamber and barrel, when you pull the trigger, the gun will explode and it will be a most unpleasant experience.


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" . . . can be used in firearms so marked:"

Is your firearm marked ".300 Blackout?"

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Wisdom comes thru fear . . .

I shoot both calibers, and a couple more, in ARs but as the advice in the thread indicates you have to have a barrel chambered for the correct caliber to do it.

I'd also point out that it is safe to shoot .223 in a rifle correctly chambered for 5.56 but the reverse is sort of risky - you might get by with shooting a 5.56 NATO in a barrel chambered for .223 but it produces quite a bit more pressure due to the "throat" in the .223 chamber.

I have not found that the .300 BLK loaded round will readily chamber in a 5.56 barrel - but empty brass does and one might manage to get loaded rounds to chamber if he forces the issue - I agree, that would result in disaster!

I do take great care to mark my barrels, bolts and magazines with fingernail polish or colored tape so I don't get them mixed up.

Riposte

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"Adventure is the result of poor preparation"

The Internet is ripe with videos of people pulling an AR charging handle to the rear and then pounding on the forward assist when they don't get ignition when the trigger is pulled. I imagine that it would be rather easy to pound a cartridge into the chamber if the Blackout does not have the bullet crimped in place.

Yep.

One thing I've noticed about hanloading the .300 BLK is that it really does better with a hefty crimp but I had to get a Lee Factory Crimp die to do it as the regular roll or taper crimp wants to put a kink in the shoulder if the brass is just a tad long.

Jim

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"Adventure is the result of poor preparation"