I am wanting to build my first SBR.  I have shot a 12.5" SBR that a friend has and fell in love.  My question boils down to, what is the shortest barrel length I can go with, and make effective hits on a Larue sized sniper target at 300m.  I am not a HSLD operator, but I do play one on the internet.  My local tactical/3-gun matches have the majority of their rifle targets at 50m or less.  The SBR is perfect for those.  But, they also have a 300m range that they through a full size or LaRue sniper target on as well.  
I will be using this SBR as my all-around HD/SHTF/tactical match gun.  I hand load as is, and will be shooting Hornady's 75gr Match bullets.  I currently shoot with a 16" carbine with an Aimpoint and can make effective hits on the 300m targets.  I would not be opposed to moving to a 1x4 scope if that will make the longer shots easier.
Thanks for you insight.

Well, it will accurately get out to 300m no problem.  But - from what I've read - the effective range against a person out of a 12.5" 5.56 is like 75 yards.

If anybody has different info, please chime in.

I've got a 10.5 , your telling me the effective range on a person is only 75 yards?

I'm not sure about that.

As far as fragmenting ammo, the ranges for reliable fragmentation are very limited in 5.56 short barrels. 10.5 is well under 50 yards I believe (m193, etc). How effective it will be at 100 yards is something I can't guess at, but what the smart folks have been telling us is that past these fragmentation ranges, the bullets will likely be much less effective than their intended design.

Well that hurts.  So I can effectively make hits to 300m, but the bullets will not be fragmenting.  Will still ruin someones day, but it will not be as devistatingly effective as I would like.

this is one I found on a GOOGLE search:
Q:

I understand if we talk about a 16"+ barrel length and the maximum temporary cavity that would perform a lot more better than a soft point, but if we talk of SB in 223/5.56 like a 12.5-10.5 SB it would/should be better since you don't have to worries about the frag point.

A:

A 75 grain 5.56 TAP load will fragment out to ~100 yards from an 11.5" barrel. A 12.5" even further. There has never been a successful LE shooting or civilian self defense shooting in the borders of the United States in the past 50 years that has exceeded the range a 12.5" barrel will maintain fragmentation with 5.56 TAP. Consider that in your ammo and barrel length selection. Also past that range the 75 grain bullet will still yaw and present a full 1" profile as it tumbles and produce an almost identical wound to an AK-74 round at ranges past the fragmentation range.

http://www.223reloads.com/home/223-5-56-info/223-tests-performance/223-ballistics-effectiveness

Don't despair too much. According to the Ammo Oracle, M193 out of a 20" barrel has a fragmentation range of 200m. According to these fragmentation charts, M193 out a 20" barrel has a fragmentation range of 130 yards. Those same charts show 75gr Black Hills to fragment at 125 yards out of a 14.5" barrel, so with a 12.5" barrel, I would guesstimate under 100 yards.

Buy some Gold Dots or 5.56 tap 8162n or whatever the hell it is.  Why run ball ammo out of an SBR for defense?

A civilian is unlikely to need to be shooting people at 200-300 meters anyway.  

I will be running Hornady's 75 gr OTM bullet that is available as a component.  It is the T1 version of their TAP offering.

Cool.  Trust me... if you hit someone center mass at 250 yards, you're going to dump them.  We ring the steel at those ranges and there's plenty of thump left in the round.

Don't get too wrapped up in the "oracle" world.

Your making those shots with an SBR?  What length barrel?

Good choice

I ring the steel at 200 yards with surplus 5.45x39mm out of my 11.5" SBR every time.

Well that is making me feel better about going down this path.  I just didn't want to get it built, and then find out it won't perform like I need it to.

EOD guys were hitting steel at 250 easy with 10.5 inch barrels overseas. My understanding is if the baddies are any father than that move regroup and dump em.

There has never been a successful LE shooting or civilian self defense shooting in the borders of the United States in the past 50 years that has exceeded the range a 12.5" barrel will maintain fragmentation with 5.56 TAP.

Don't mean to "harsh your buzz" but examine that statement for a few seconds and it takes on the form of a self-fulfilling prophecy.........oh, yeeeeaaah, they weren't successful because they exceeded the optimal range for bullet upset.

So you say it sucks that the loss of velocity caused by the use of a shorter than original barrel results in an unacceptable reduction of the range at which a projectile will still enthusiastically upset and yaw upon striking a gelatinous target? Is that your problem, bubbie?( I think I just dated myself) So you may only achieve a through a through-and-through past that distance? Don't despair for the news is not all bad. There are many bones, big ones, in the kind of targets that concern themselves with fragmentation, to cause all sorts of weird internal ricochets, and even a t&t will still ruin someones day and make them late for breakfast at Mickey D's. Plus , that reduced velocity induces instability at a greatly reduced range as well, the kind of wobbling that causes upset.  

So you're GTG, hell on wheels out to 75, a paper puncher to 300, then tack 'em and stack 'em out to 400 where you're really out of steam. The obvious solution is to let 'em get real close so you can hear the bullets break apart, and you'll conserve on ammo, too!

Just havin' fun

And what range is that exactly?

The Sheriff's Office Rifle instructor here in Maricopa County had an example of a Cop Dumping a dirt bag on a manhunt  in the desert from a long distance.  I've forgotten the specific distance... so I won't make that part up, but the guy was shooting at cops and they were green lighted to let the air out of the joker.

Generally, however, I agree with you that people needlessly obsess on frag a distance and put way too much effort into obsessing on this oracle stuff.

There have been kills made at 300m using CQBRs.

I've personally met an officer that killed a man at 150M with an iron sighted rifle.

Only sure way to find out is stand out there at 300 yards and let someone shoot at ya. Then you'll know for sure.

Dude... who ya gonna be shooting at 300m, even in SHTF?  In short, you won't.  Even in SHTF, 99.99% of time, if you have to pull the trigger, then it will be inside 50 yards.  Use a quality round, like 75 or 77 grn OTM, or a quality barrier-blind round like Mk 318, and you will be effective at any range you're ever gonna shoot at.

There is hypothetical and there is practical. Hypothetically, you could conceivably need to engage someone out past 100m. Practically speaking, however, that ain't ever gonna happen unless theres an invasion on.

I have a 10.5" barrel for HD, and thats it. From one room to the next, and it'll work for that just fine.

Hpothetically, any joker with a 30-06 hunting rifle now has the drop on you.

But if we're taling SHTF, you should have some type of 6.5G, AR-10, .308 on your back anyways Moot point.

If it's SHTF, I ain't humping 2 rifles. in all likelyhood, if I need a rifle, I'll carry an SBR and a supressor.  It'll be useful fro virtually every situation.  If we get invaded (yeah... right.....) I have some longer barrel AR's to use.

Look at it this way, you don't have to worry as much about collateral damage from shoot throughs

In that case, I should get a Judge and load it with bird shot.  That way I would have a SBS with no chance of over penetration

+1

hahahahaha

From a 10.5" 1/7 twist, in general, in soft tissue, FMJ like M193 and M855 is unlikely to offer
much in the way of fragmentation potential even at close ranges; in
addition the yaw cycle is often too deep with FMJ.  Heavy OTM loads like
75 gr Hornady TAP and 77 gr Black Hills MK262 can continue to fragment
down to around 2000 f/s or so (a bit beyond 100 m); unfortunately the 77
gr SMK OTM  used in MK262 frequently has a deeper than ideal initial
upset point.  Lightweight JHP's have good fragmentation and early upset,
however, total penetration is often insufficient. JSP loads like the 55
& 62 gr Trophy Bonded and the 60 gr Nosler Partition demonstrate
early upset and good expansion out to 100 m or so, however they have
significant weight retention leading to deep penetration with frequent
exit.

Without question.

Sometimes they go down faster than the guys taking em in the chest point blank.

Hitting my range's Larue target at 250 yards with my Aimpoint equipped 10.5" LMT is pretty easy from a supported position. I can't imagine 300 yards being much more difficult.