I know the M&P15 Sport 2 comes with a barrel rated for 5.56 ammo, but what do most people shoot for target ammo ? .223 ammo is a little bit cheaper than 5.56. Is there any benefit in shooting 5.56 ammo over .223 or vice versa ?

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Depends on what you buy. If it's Federal, there's 55gr Federal American Eagle .223, Federal AE 5.56 and Federal 5.56 LC XM193.

From a 16in barrel, I can't tell any difference between 55gr Fed AE .223 and LC 5.56. And the specs show no difference either at 3240fps. I'll have to put them through my chronograph to see but my guess is that they are very close. The 55gr Fed AE 5.56 is rated with a lower muzzle velocity at 3165fps.

I've also got some PMC Bronze 55gr .223 that shoots incredibly soft... I mean... soft soft soft. It's rated 3200fps (which PMC reports from a 24in barrel) but the velocity tests out of a 16in barrel that I've seen show around 2700-2800fps which tells me why it's so soft shooting . On the other hand, 5.56 LC XM193 typically stays above 3100fps from a 16in barrel. I think most 5.56 is tested in a 20in barrel and is typically loaded to a higher pressure.

So... ya know... it's not too straight forward.

From a practical standpoint for the casual shooter, it won't make a meaningful difference.

Now, as soon as I have written that someone is going to come along and say that:
* X Manufacturer's 5.56 delivers more velocity, or
* Y Manufacturer's 223 has softer recoil, or
* Z Manufacturer's 5.56 will wear out your barrel overnight.
And if you're shooting in competition this might matter, but if you're just out at the range with friends punching paper, I doubt you will notice the difference. And if you think you would, buy two brands of ammunition (same bullet weight) and have a friend mix them up and put them in the magazine and see if you can tell the difference. I bet you can't.

I should point out that the only time I shoot factory ammunition is the first box through a new gun. I do that so if there is a problem, the manufacturer (or their attorney if it is a real problem) can't blame my reloads. After that, everything going through any gun I own (or my father owns, or my sons own), other than rimfire, is something I assemble.

Buy on price.

But, I urge you to give preference to American manufacturers. After all, someone has to buy American products if any of us want to continue to have jobs. It's only a few cents more and the job you save may ultimately be your own.

Check the rules at your range if it's public. This could drive your decision, but military 55 grain FMJ is hard to beat for price. As with any rifle, you'll need to test several types to see what your gun shoots the best.

About the only factory ammunition I buy in quantity is LC 5.56 XM193. The brass is well thought of and I will have plenty to reload if I ever run out of WW. Federal brass is know to be less than stellar. Seems strange Federal can load LC in excellent cases and their own headstamp in bubblegum soft brass.

I use the XM193 for plinking and training noobs. No reason to waste my handloads for those purposes.

I do admit to buying one box of factory for each new rifle I obtain. Like hdwhit, I want to have a baseline if there are any problems, find a baseline accuracy with the gun, and have chronograph results from a factory load similar to what I plan on loading.

Although many shooters use steel case ammunition without problem, I have no intention of ever trying any.

I can't really tell the difference in my ARs or Mini 14. I like Fiocchi FMJ in .223 and PMC Xtac or Federal AE in 5.56mm.

I try a few commercial loads and figure out what my gun likes accuracy wise and then when I find a good price on that ammo I try to buy enough to last me awhile. Sometimes even the same ammo from different lot numbers can be different,not real important for plinking but more so for hunting rifles.

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Hue 68 noli me tangere

I bought some AE 5.56 and some .223. You could tell which was which. The 5.56 had noticeably more recoil out of my sport 2. Let a friend shoot it and he said the same thing.

Not unusual for that to happen. Difference in velocity, powder being used and charge weight will cause the difference. I've got some yellow box UMC 223 that I bought back in the 80's, and it feels more like 5.56 than current 223.

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Peace was my Profession

Both 5.56 and .223 will feed fine in your S&W, so buy what is cheapest or what you like the best. I've shot a ton of Wolf Gold in .223 and a ton of M193 in 5.56 and both have performed equally as well with 55 grains. I just bought some surplus 62 grain 5.56 stuff but have not shot it yet (found a good price on it) but I fully expect that to perform well also, maybe even better.

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Still carrying my S&W 642

My new Colt LE 6940 will shoot either. I ordered a case of 62 gr. 5.56 for it.