I am considering purchasing a M&P 15, but am a bit confused by all the different models, 15T, 15Sport, 15PS, etc. I really like the 15MOE in flat dark earth color. Is there a preference between 223 or 556 ( I thought they were basically the same round )? Any help appreciated! I am AR newb. I have looked at all of them of the SW website just not sure which accessories are worth buying.

AFAIK they are all the exact same rifle, the difference is only how it is accessorized.

They will all shoot .223 or 5.56. While the specs are the same for .223 and 5.56 as far as size, the 5.56 NATO standard allows for higher pressures of military rounds.

You can shoot either .223 or 5.56 with out any problems. If you ever get into reloading you will only reload to .223 specs as you are unable to load to 5.56 specs as there is no consumer tool to crimp primers

So with that said, buy which ever model you like! You are making a great choice to buy the S&W. It's an awesome, extremely well built, extremely reliable rifle. Oh and S&W warrents it for life!
HTH

-Masta

Incorrect information on the differences between 223 Remington and 5.56mm. To the OP, this has been discussed to death, a google search will provide you with lots of good info.

Buy whatever model and accessory package you like, the base firearm for all the carbines is essentially the same.

This was true until recently. They have introduced the M&P15 Sport:
Product: Model M&P15 Sport?

It is 5.56. But it:
- Has no forward assist
- The barrel is not chromed
- No dust cover
- 1 in 8 " twist, not 1 in 9 like my M&P15OR.

Note this retails for less than 1/2 of a M&P15T. I think there's a thread about it. Yes, here:
http://smith-wessonforum.com/smith-w...ith-223-a.html

And then there's the M&P15PSX - it's piston operated, rather then direct impingment, like traditional AR's. I've never even shot one of these piston operated S&W M&P15s.

Anyhow, for a basic AR, I am very pleased with my M&P15OR. They can be found on sale for great prices.
I had said "If you want a quad rail, look at the M&P15T. It's more money, but the Troy quad rail and sights are first class IMHO", but now the 15T now longer uses the Troy components. But I think the price went down quite a bit.
If you like the Magpul MOE stuff, well then M&P15 MOE.

S&W has a lot of M&P15 Models these days. I just counted 24 models on thier site (just on the Tactical Rifles page)! And that dosen't even include one of our favorites, the 15-22 (we've got one of those also).

Oh yeah, the new Sport. S&W copying Taurus with the Governor, and apparently also copying makers like DPMS with a cheap junk version of the M&P 15....... Sad.

If memory serves me correctly, and I have not gone back to check, basically, its a difference between the SAAMI and NATO standards for .223 vs 5.56. There are two standards: SAAMI (Sporting Arms and Ammunition Manufacturers' Institute) and NATO

5.56 chambering (NATO mil-spec chambers) have a longer lead, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 chambering ("SAAMI chamber") has a shorter lead. The 5.56 has a higher pressure than the .223. I think the confusion between the use of 5.56 in a .223 only weapon arises from the fact that the .556 Nato round uses, as its parent case, the case of the .223, and that some of the .223 history involves the .223 Remington being standardized as the 5.56x45mm for the M-16 in the early 60's but people don't realize that when it was introduced as a sport round later that it was actually a little different and used a different standard for sport use manufacturing.

Using commercial .223 in a 5.56 chambered rifle should work reliably, but generally will be a little less accurate due to the excessive (5.56) lead. Using 5.56 in a .223-chambered rifle can cause excessive wear and stress on the rifle and even be unsafe (the SAAMI standard recommends against the practice).

If the rifle is marked for 5.56 then you can use .223, but if its marked for .223 only then its really not a good idea (in my opinion, and that of SAMMI) to use 5.56. Although, some people do get away with it as some commercial rifles marked as ".223 Remington" are suited for 5.56.

If you use 5.56 in a .223 only rifle, you should consult the manufacturer first before doing such. Some signs that the 5.56 might not be acceptable for that .223 only rifle, aside from a weapon malfunction (if your lucky in some cases), or possible injury as a result, is excessive pressure signs such as flattening or gas staining of the primers.

A weapon is like any tool, if you use it within its intended specifications then it works for its intended use. One of the primary gun safety rules is to always use the correct ammunition for the weapon.

Here is one brief reference, there are many more via Google: http://fulton-armory.com/faqs/AR-FAQ...3-Chambers.htm

Here is the SAMMI site as well: http://www.saami.org/specifications_...tion/index.cfm > where in the "Unsafe Firearm - Ammunition combination" standards (http://www.saami.org/specifications_...mbinations.pdf ) On page 7, it states:

In Firearms Chambered For:

.223 Remington

Do Not Use These Cartridges:

5.56mm Military
222 Remington
30 Carbine

(The "5.56mm Military" is the 5.56 Nato)

So it comes down to this:

If its chambered for 5.56 you can also use .223
If its chambered for .223 only then do not use 5.56

There are some marked .223/5.56, these are generally actually chambered for 5.56 but the .223 will work. However, I personally only look for marked as 5.56 chambered and don't pay attention to the ".223/5.56" marked rifles because some of these places may have marked for "advertising" purposes (but are probably correct in their marking as to use of the .223 in the weapon) and although thats not bad I just feel more comfortable knowing what the weapon was built for exactly and besides a single specific chamber designation is required for my use. However, I think the real reason some are dual marked as .223/5.56 is for legal reasons.

If I scewed this up I apologize, going from memory here as it never concerns me as I use the round the weapon is actually chambered for which in my case is 5.56.

Note: Now I had to correct my above post because the M&P15T no longer uses Troy Quad Rail, nor the most excellent Troy back up sights. They have done quite a few changes to the M&P15 line this year, which I was not aware of.

Good .223 vs. 5.56 info here.....................

http://le.atk.com/pdf/223VS556.pdf