Suggestions or warnings for M4 upper

6.8 SPC, a fairly common alternative AR caliber will give you 1,694 ft·lbf of energy from a 16" barrel, likely to still be over your req'd 1200 at 14.5.

.300 AAC BLK will give you 1,360 out a 16" barrel with a fast load, but requires only a barrel swap, no new bolt or mags needed.

Rock River Arms produces a complete 6.8 SPC upper for $550 but you'll have to add rails, either at the factory or aftermarket. The 6.8 SPC is getting quite the following from deer hunters, though I don't know if that's what you're planning on taking with it.

Quality 6.8 SPC hunting ammo (Remmy Core-Lokt) has a cost per round of about $1.32.

Here's a warning: Simple physics dictates the the 6.8 SPC is going to have more kick/bite than the .223, I'd recommend a mid-length gas system to soften that up a bit, it won't add any length (you're barrel + comp/flash hider must add up to 16" to be legal anyway) but may add weight. However you'll have more room for accessories. Remember that the original carbine length gas system was designed for the CAR-15 carbine that had an 11" barrel, for a civilian who doesn't want to SBR, a mid length is actually more suitable. Note that that when Eugene Stoner, the inventor of AR15 went back to revise the AR15 design for KAC (for a rifle that was designed around a 16" barrel) near the end of his life he moved it to a mid length gas system.

But my state requires 1,200 foot pounds of energy at the muzzle to hunt a number of things.

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this sounds very difficult to enforce

Yes it will come down, weapons like the Remington Defense ACR are designed around being able to easily switch to the cartridge, so the military is obviously interested. NATO Special Forces have been testing the round are pleased with it. As the AR platform gains popularity people who want to hunt with it will also invest 6.8 SPC platforms thus bringing the cost down and bringing the ammo into more shops. Unlike some other alternative calibers, 6.8 SPC has Remington (and thus the enormous defense conglomerate Freedom Group, which includes UMC ammo, Advanced Arms Corp., Bushmaster, DPMS etc.) behind it, combined they have epic pull with the military and other ammo manufacturers and retailers.

CmdrSlander said:

Quality 6.8 SPC hunting ammo (Remmy Core-Lokt) has a cost per round of about $1.32.

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Keep in mind that is CoreLokt Ultra Bonded, not just the standard CL.

Hornady also makes a good SST load that Midway is selling for $22 a box. http://www.midwayusa.com/product/26...n-spc-120-grain-super-shock-tip-sst-box-of-20

And Silver State Armory typically sets the bar with several offerings with Barnes or Nosler bullets. http://www.midwayusa.com/find?dimen...imensionId=9877&promotionId=&userSearchQuery=

For plinking ammo, well, it doesn't exist. Right now the 6.8 is more of a working cartridge than berm fodder. Tula has said that they will have steel cased ammo this fall though.

There is also an outfit that will load/reload ammo using your brass and has very good pricing. 36.25 per 50 loaded with 85gr Barnes frangibles. http://bridgercustomammo.com/

CmdrSlander said:

Unlike some other alternative calibers, 6.8 SPC has Remington (and thus the enormous defense conglomerate Freedom Group, which includes UMC ammo, Advanced Arms Corp., Bushmaster, DPMS etc.) behind it, combined they have epic pull with the military and other ammo manufacturers and retailers.

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It seems to have come down and the selection continues to improve. I would say that it is getting better, but this is in spite to Remington (et al) instead of because of it. Like many other cartridges, they brought the 6.8 to market then moved on to something else. Their current ammo offerings have some of the highest prices and worst field reports (internet fueled of course). They also have a new pony that they are showing off instead.

Bushmaster and DPMS 6.8 offerings are solid, and I know of 2 Bushmaster ACR conversions that made it into private hands. There are several non-Freedom Group companies that make very good options too. For ammo, look to Hornady, Barrett, Silver State Armory, Double Tap, CorBon, BVAC, and some others.

Take that $600 you'd spend on a marginal caliber upper and get a nice hunting rifle with scope.

Personally I'd stay away from 6.8spc. Most folks are going to 300 blackout now. I predict it will be "the" caliber for harder hitting AR's in the future. Mostly because you don't haft to change the bolt or mags. Whatever you get do go for a mid-length gas system.
Although I must admit I like the "just buy a bolt gun" idea the best. But to each his own. Good luck and have fun.

RAINS said:

Personally I'd stay away from 6.8spc. Most folks are going to 300 blackout now. I predict it will be "the" caliber for harder hitting AR's in the future. Mostly because you don't haft to change the bolt or mags. Whatever you get do go for a mid-length gas system.
Although I must admit I like the "just buy a bolt gun" idea the best. But to each his own. Good luck and have fun.

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Conflicting data here. If you are worried about a perceived "most folks" and want a .300, the developer recommends the carbine length gas for 16" barrels.

RAINS said:

Personally I'd stay away from 6.8spc. Most folks are going to 300 blackout now. I predict it will be "the" caliber for harder hitting AR's in the future. Mostly because you don't haft to change the bolt or mags. Whatever you get do go for a mid-length gas system.
Although I must admit I like the "just buy a bolt gun" idea the best. But to each his own. Good luck and have fun.

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The caliber for harder hitting AR's will remain the original AR caliber, .308. The biggest advantage of the .300 AAC is rifle power in a round that works very well with a suppressor, but your max velocity is 2200 fps or so which means rainbow trajectories. The 6.8 bolt costs $70 and the mags are $14, plus you don't have to revert to 1800's era trajectories.

What specific range and target will this gun be used for? Because of the hunting requirement, some cartridges got mentioned, none of which would be optimum if it's prairie dogs at 450m.

"I want something light and handy." doesn't really answer the question. From the toolbox perspective, it's going to be shot at a specific maximum range, at a specific target. Those parameters limit the cartridge choices quickly, and could even eliminate the AR15 as the optimum barrel carrier.

.308 is a capable round, but having used it and .30-06 over twenty years hunting, the disadvantages exist - it's more power or range than needed on a lot of game animals, almost too much for spindly little whitetails. If the North American Big Five were being considered, it'd would be almost too small.

One thing to consider is that military surplus ammo is the exception, not the rule, and thats why most premium hunting rounds are "pricey" regardless of caliber. Even 5.56 goes for a $1 a round in hunting grades - but hunters don't blast 1,000's of rounds of it into the dirt. Choosing a specific hunting caliber means it's really limited to a few hundred a year. Cheap milsurp is what any reasonable shooter would bumpfire into a gravel pit filled with beer cans, get it a bottlerocket prices.

Once the range and target are nailed down, pick the optimum caliber, and go from there. The rifle will likely be superior at the job simply because it's got better ballistics than trying to shoehorn a second best guess at what is needed. Don't start picking barrel lengths until the cartridge is chosen, because if anything, the example of a 14.5" 5.56 is exactly what NOT to do - it's a compromise that seriously impacts the use of the round, sacrificing speed and power when the better answer may have been going to 20". Until you specify the range and target, jumping ahead to barrel length may introduce some poor performance and needless waste of cash fixing it.

That target upper is a case in point - does what it does ok, but not what you'd like to have for other kinds of shooting. Maybe the decision making for that rifle was a bit more impulsive than you'd like.

Don't simply repeat the same thing, think about it, nail down the specific range and target you plan to use this upper on. Shooting paper vs. live game makes that much difference.