I have a few 80% billet lowers that once done I will make a few projects with.  I'm planing an SPR style/clone rifle for longer range shooting.  Which would you build and why?  I know pros of the .223 wylde would certainly be the ammo selection and availability and cost.  But from what I've read 300 AAC has better bullestics and stopping power at 200+ yrds as a 5.56 NATO.  Already have a few 5.56s and .223s in rifle (carbine) and pistol formats.  Thanks in advance for your insight.  

For an SPR i'm not sure I'd go with either one.

for any sort of distance shooting you need a combination of both velocity and bullet mass.

5.56 is fast but too light    easily detoured by wind.
300AAC is kind of like the .45 ACP of the rifle word.  Decent bullet mass for the cartridge size but trades velocity for bullet mass.   I like it for shorter range and hitting like a hammer but lacks the velocity for 400 yds+.

For the AR15 package, I think Id strongly consider the 6.5 Grendel or 6.8 SPC.

The long range ballistics are better. Although more special parts are needed.

If it had to be either 5.56 or 300AAC   Then have to go with 5.56.   Velocity trumps mass for long range.

I would go with 5.56 for your purpose and never look back.

I agree.



5.56 out of the two cartridges you said are an option.  6.5 if I was going for an alternate cartridge in the AR platform.



 

 

556/223 compares to the 300 blk........The same amount of powder pushing 2 different bullets. Without doing the math, you gotta understand the 300 blk with bullets that weigh twice as much doesn't offer the ballistics you think it does. It has a small niche in sub-sonic, suppressed, close-quarter builds where it offers more punch than a subsonic 556 round.

Not sure what you are planning on doing with your SPR but Wylde would definitely be the way to go. 69 to 75 gr Barnes TSX give decent game performance. Maybe 150 yards for deer, 250 or so for coyotes and maybe even out to 800 for prairie dogs and woodchucks....punching paper.....

If you already have some 5.56/.223 rifles just stick stick with that. Less hunting ammo and more
Shooting it

I think the 300 BLK is being sold short as only CQB/suppressed round and mediocre at all else.  I think it would be a great general purpose AR chambering for 0-400m if ammo was more readily available.  That said, for an SPR, the 5.56 will be better and cheaper to shoot with the 6.5 and 6.8 better still but more expensive and some different parts required.

THIS!

This thread has a chart comparing long range bullet drop for 5.56, 6.8 SPC, and 300 Blackout (9th post).

I've had this same thought. I'm going  with the .223/5.56. The only reason for my choice was because of the reloading stuff. I'm good to go with reloading the 223. But have nothing for the .300 AC

Not really. My 75gr BTHP load with a MV of 2540 out of a 14.5" will surpass the velocity of M193 @ 300yds out of the same barrel.

For longer ranges I would take the 5.56 over the 300BO. The 300 really shines inside 300yds, the perfect candidate for SBR's and suppressor use.

SPR, 5.56.

If you are looking for the Swiss-Army knife that will do everything well then the 6.8 is probably ballistically the best choice of compromise, with the 5.56 being the easiest to build and find parts for.  I went 5.56 but will likely be building a 6.8 in similar dimensions for exactly this reason.  The SPR was meant to do everything well, but the limitation to the platform is the 5.56 round.  The 77gr projos help, but the 6.8 is superior out of a similarly lengthed barrel.  Some may bring the 6.5 into the discussion but to get a complete powder burn at least 22" of barrel is needed.  The 5.56 needs about 18.25 and the 6.8 around 16"  The SPR was a compromise to many scenarios, the 6.8 is as well.

Great responses and insight, thanks.  I've thought about building an AR10 for some precision long range shooting as an option but that's getting into an entirely new platform so for now I've ruled it out.  I'll have to look into the 6.5 and 6.8 conversions and see what's involved and cost comparison.  

If I stay with a 5.56 or .223 wylde I'm looking for 20-22" barrel likely.  Full powder burn and better dwell time from what I've read.  
Other that weight savings, what is the advantage of the fluted barrels?  From the looks of it an SPR style build would have a threaded barrel with a muzzle device (brake).  Would there be an advantage with going with a slightly longer bull barrel instead of the traditional threaded barrel?   I know that would make it more of a varmint style from what I've been reading.

I've had my carbine for a couple of years (16' threaded barrel, 1 in 7 twist) and it's decent out to 200 yards but beyond that it seems to drop off quick.  Of course, there's always the "me" factor and I'm sure that could be a big part of it.  I've got a couple of AR pistols in various states of completion that still need to be shot.  Those will be for shorter ranges although I know that they are capable of being lethal out to 200 yards in .223/5.56 with the right shooter and situation.   And let's face it...adding another AR in a precision long(er) range shooter is always a nice to have.  It's such a versatile platform and I find it fun to go through the whole process.

If 308 was on the table, I would not hesitate to push you in that direction. Your already in-hand AR rifles are probably capable of great accuracy at the distances you are talking about so you might ask what is the point of adding another. The 308 is a truly new level of performance. And with Palmetto entering the Market, you can put together a 308 for a decent price. Good mags are available now from $13 to $20. Go for it. You may only have a 400 yard shooting but if you every find yourself in a situation to shoot further, you will be equipped.