Okay- A stiff barrel is  accurate and a long barrel is accurate. Now if,  lets say you have a BULL 9.90-1.10  diameter barrel AR15. A  long barrel 24'inch barrel,  although it is a "Super Bull barrel" will vibrate after a shot because of its length. Now if you have the same barrel but only 16'inches- to reduce barrel whip/vibration.will this cause the shorter BULL barrel to be more accurate dueo to it being stiffer & causing less whip & vibration?

If I remember correctly, long barrels arn't more accurate they provide more velocity making the bullet less suceptible to other forces such as the wind.  I'll go out on a limb to speculate that all other forces being constant (ie, wind, cartridge charge, bullet weight, temp, ect) that both length barrells would be equal in accuracy.  Of course I'd wait for a mechanical enginneer or someone more versed in physics than I am to make a sound decision.  

Also, on iron sighted weapons a longer barrel can provide a longer sight radius. Most of the match rifles built on the AR platform have their sights wayyyy on the end of the barrel.

ETA: With a scope, sight radius is irrelevant

Residual pressure (pressure when the bullet leaves the crown) will be higher on the shorter barrel.  The higher the residual pressure the more critical the bullet base (especially boat tails) and the barrel crown become to accuracy.

Some interesting points on barrel vibration/whip. A straight tapered barrel will have less vibration than a bull barrel where the starting diameter at the breach end are the same (weight towards the muzzle has a significant impact on barrel vibration amplitude).  A barrel become exponentially stiffer by shorting it.

I agree with the above posts, but I am unfamiliar with what stiles has mentioned.  Never mind that though, I don't know a whole lot about this subject.  I mostly replied because I like your name, DrDeath.    That was my name when I played paintball 12 years ago.  The others on the team named me after some wrestler they said I looked like!

Quoted: Residual pressure (pressure when the bullet leaves the crown) will be higher on the shorter barrel.  The higher the residual pressure the more critical the bullet base (especially boat tails) and the barrel crown become to accuracy. Some interesting points on barrel vibration/whip. A straight tapered barrel will have less vibration than a bull barrel where the starting diameter at the breach end are the same (weight towards the muzzle has a significant impact on barrel vibration amplitude).  A barrel become exponentially stiffer by shorting it.

what about barrels like a m4 with different diameters throughout the length?  or is the fact that you would probably be shooting a m4 style at less than 300yds be the closer vs any type of vibration?

Quoted: what about barrels like a m4 with different diameters throughout the length?

A M4 profile invariably has some weird ass vibration nodes with the M4 cut and being thin under the hand guards.  One thing I should say is everything I have said about barrel vibration assumes a free floated barrel.

Everyone is right here.... but for different reasons, and commenting on different things.

Long gives high velocity so yes, "tends" to be more accurate at longer ranges, but it's a function of lack of drift and drop, as noted.

Shorter IS stiffer so barrel whip IS reduced and can contribute to accuracy.  The jury is still out however if you are shooting at the same distance and the whip is "consistant", but that's really only practical in event specific competition rifles, not something where you'd be engaging at various distances.    Barrel harmonics and whip are also affected by bullet "jump" which is why HP shooters experiment with distance off the lands when handloading non-magazine length for the 600 yard line.  (Did cool work with this at a Jim Owens HP clinic last summer.  We started at .010 off, and backed up .005 for each batch of 5.  It was interesting how different rifles (all 20" Service rifles) prefered different jumps.

HP "Match" rifles DO have their front sights WAY out.  Even past the end of the barrel on an extension to increase sight radius.  Interesting though that the extension is "bored out" and not rifled so the bullet is passing through it clean at that point.   Implies that there is no additional advantage after about 24".

Stiles is also right about about pressures and bullet exit.  This is why some shoot 50 or 52 gr FLAT BASE rather than Boat Tail in Bench Rest competitions.  I'm thinking of trying it for 100 yard reduced course ala Zediker.  I'll let you  know how the experiments turn out.

Stiles is also right about the M4 cuts in the barrel.  That's why I don't use M4 cut barrels and do use FF handguards.  Sling pressures or leaning on barricades will inevitably change POI.

Lastly, some think that a shorter barrel contributes to accuracy regarding follow-through.  That is, the bullet is released from the barrel sooner, before movement will change it's path, between trigger pull and bullet exit.  If that extra millisecond or two matters, you're either REAL GOOD or REAL BAD with your position hold.

Check out the .22 silhouette rifles used in Sportsman competitions by the likes of David Tubbs.  VERY short barrels.  He subscribes to the trigger pull to bullet exit theory.

Quoted: Quoted: what about barrels like a m4 with different diameters throughout the length?   A M4 profile invariably has some weird ass vibration nodes with the M4 cut and being thin under the hand guards.  One thing I should say is everything I have said about barrel vibration assumes a free floated barrel.

if i do my part would this be an issue at a distance of 300yds or less?  mostly ~100yds?  i am assuming no but i would rather ask since there is a lot of good knowldedge here :)

Okay-I am talking about BULL Barrels with  OPTICS...DPMS BULL & Super Bull AR15s, Bushy Varminter, RRA Varmint , ETC....  Also- what is the optimum barrel length for accuracy & performance out of an AR15 for the 223/5.56 round?

Quoted: if i do my part would this be an issue at a distance of 300yds or less?  mostly ~100yds?  i am assuming no but i would rather ask since there is a lot of good knowldedge here :)

You are correct, it's no real issue to hit man size vitals at those ranges with a M4 barrel.

DrDeath, what I would choose for an AR rifle that can shoot lights out wouldn't be a bull barrel it would be a SPRish type of a profile.  It's hard to beat a 18" barrel with a rifle length gas system, it's a nice compromise IMO.  But a 20" barrel can shoot groups too.  To be honest you will get better groups by doing load development in comparison of choosing a shorter/longer/heaver barrel (well within reason a 5/8” light weight profile won't preform like a heavier contour for instance).

Quoted: Quoted: Quoted: what about barrels like a m4 with different diameters throughout the length?   A M4 profile invariably has some weird ass vibration nodes with the M4 cut and being thin under the hand guards.  One thing I should say is everything I have said about barrel vibration assumes a free floated barrel. if i do my part would this be an issue at a distance of 300yds or less?  mostly ~100yds?  i am assuming no but i would rather ask since there is a lot of good knowldedge here :)

At short ranges, it's almost too easy to surprise the Elmer Fudds with the accuracy of your Black Rifle. Even in stock form with all of the "bad" accuracy characteristics (non-free float, odd profile, abrupt gas system, short sight radius,stiff trigger) the M4 has no trouble at all in reliably doing what it was designed to do....
For competition accuracy, the limiting factors are eliminated with the common types of accessorizing seen on this site: FF handguards, straighter bbl profiles, midlength gas systems, red-dot/scoped optics, match triggers. With a competent shooter, an M4 type carbine can range from deadly accurate to unbelievably accurate as long as you stay within a realistic range, limited mostly by the ballistics of the 5.56.

S&W's MP15 was tested by Guns and Ammo. The FFHG Tacticool version did not show any improvement over the standard CAR-handguard version in terms of grouping size. Needless to say, I am not sold on the use of FFHGs for short barrels. If it does provide an improvement in accuracy, it wasn't much if any at all.

metroplex is that barrel is pretty thin under the hand guards like the original M4 profile? I have a Stag limited edition M4 upper and it measures at it's thinest point under the hand guards .600".   What I'm hinting at is it's not how short the barrel is but how thin it is that keeps it from improving by floating the barrel.  .700" is approximately the break point of a .224" barrel in terms of vibration performance.

stiles: I haven't touched a M&P 15 but based on its weight data, it appears to be a 16" M4 barrel (basically a 14.5" M4 but only 16", like Bushmaster's Patrolman's carbine barrel), thin under the handguards, with the M4 cut forward of the FSB.

This thread is getting side tracked- I am only talking  about BULL barelled carbines used for varmint , target , sniper/spotter roles. Not HBAR , M4s but BULL barrels and  the stiffness factor & length factor. THANKS

Any of the GURUs have anything to add.....

Quoted: Okay- A stiff barrel is  accurate and a long barrel is accurate. Now if,  lets say you have a BULL 9.90-1.10  diameter barrel AR15. A  long barrel 24'inch barrel,  although it is a "Super Bull barrel" will vibrate after a shot because of its length. Now if you have the same barrel but only 16'inches- to reduce barrel whip/vibration.will this cause the shorter BULL barrel to be more accurate dueo to it being stiffer & causing less whip & vibration?

Get the 16" bull barrel or 18" bull barrel.

I read about this a while ago on the Net. I was just trying to find it again for you (no joy).

It has to do with the vibration of the barrel when fired. And why MOST match/sniper (.30 cal) weapons max out at 24". Every barrel 'whips' when fired. It seems that 24" is an optimum length for accuracy (ie the whip effect on accuracy is minimized at the barrel length).

Some weapons have a provision to 'tune' the barrel whip with a set screw, to 'dampen' the vibration for optimum accuracy.