Hey guys. I was wondering what kind of groups can I expect from an Oly Arms pcr-4 with a 20" barrel? What about a 16" barrel? How big of a difference will the 4" make?

Thanks,
Clint

Believe it or not, no inherrant difference in accuracy from one barrel to the other. The difference will be in the longer sight radius of the 20" model. Longer sight makes it easier to be accurate.

What about the velocity of the round. Will this be affected? If so, wouldn't the bullet drop sooner?

Come on guys. I know that this isn't that tough of a question. Some of you have to know this.

You will have a different trajectory sure, but this wont effect accuracy.

Of course, when the bullet drops below the speed of sound (say 1050 fps) it will do weird things. And the 16" barrel will drop below that velocity sooner (maybe 900 yards) than say the 20" (around 1000 yards).

Short of that however, if the shooter does their part, the bullets, theoretically, will make the same size groups, just at different places.

*Thanks to Brou for his quick charts and velocity guestimates!

AR_Clint

Oly makes a pretty good barrel, I would have to say that honestly either one (16 or 20) will shoot better than you or I are capable of using iron sights.

Scope up and go long range shooting and that is a different story, but even their standard barrels will outshoot 99.9% of the nuts behind the trigger!

Yes, accuracy & velocity will both be better with the 20" barrel. You'll lose about 100fps per inch off of the 20" barrel so figure a rough estimate of 400fps less with the 16".

The accuracy measurement isn't quite so precise. Common sense will tell you that the longer barrel allows better stabilization of the rd as it goes thru the barrel. Is it worth the difference? Not to me, I bought the 20" version.

Just parroting here, but from what I understand, velocity will definately be less with the 16", but precison, if anything, will be improved on the 16", because of higher stiffness leading to barrel vibrations of higher frequency, but lower amplitude.

Edit: I should actually distinguish between accuracy and precision here. The precision should be improved, but because of the amplified ballistic trajectory, accuracy may be affected.

I think it is important to establish some definitions here.  Trajectory is the roughly defined as the flight path of the bullet. Accuracy is the ability of the shooting system (rifle, ammunition, shooter) to place multiple bullets along the same trajectory, ultimately striking the same place (or at least as close as possible to it).  

As others have stated, accuracy itself (theoretically) is not going to be that much different between the 16" and the 20" with the nod going slightly to the 16" barrel based on its increased stiffness.  Practically speaking, if you are using open sights the longer sight radius of the 20" will make it easier to be accurate (again, reflecting what other have said), but if you are using an optical site, this factor is irrelevant.

The other major difference is that the 20" barrel will impart a greater initial velocity to the bullet, giving it a slightly flatter trajectory.  This will (theoretically) slightly minimize holdover as a factor at longer ranges, but as a practical matter the difference will not be enough to make much of a difference at what constitute normal .223 distances for most of us.

Unless you are going to be shooting out beyond 400 yards or so, I would think that the 4" difference in barrel length would be pretty negligible.  Anyway, just my 2 cents.

Yes, definitions are important. Semantics moreso, though.

Accuracy is the deviation from true value (bull's eye) of the mean of the set (center of group of bullet holes).

Precision is the "spread" of the set, and by extension, the ability to place bullets along the same path.

If your shots group .5" at 100 yards, but a foot low, that's good precision and poor accuracy.

If your shots group 12", and the average of the "group" lands on the bullseye, that's good accuracy, and poor precision.

Because the barrel vibrations would be dampened, and fewer bullets would "diverge" from the muzzle, the 16" should theoretically have better precision.

However, because of the "compressed" trajectory (the result of lower horizontal velocity and constant vertical acceleration), it will be more difficult to determine at which point of the trajectory the bullets will impact; as such, the accuracy is adversely affected.

I have both 20" and 16" Oly heavy barrels.  Accuracy is equally good with both.  The difference is as stated by others, the sight radius.  With a scope, the difference is not noticable.

There is only about 200 fps difference in velocity between the 20" and 16" barrels.    

As others have stated the inherent accuracy will be slightly better with the 16" barrel.  BUT if you are using the A2 sights the extra sight radius will be a real aiming advantage with the 20" barrel, thus you should shoot the 20" barreleg gun better.

Personally I like the 16" barrel best, much handier, a bit lighter and makes it easier to store, travel with etc.  I have 3 AR's, all with 16" tubes.  One is a scoped DPMS Panther Bull Sixteen flattop with a good scope fitted it shoots 5-shot groups under 1/2" at one hunders yards with tailored handloads.  A great varmint rifle!  The others are iron sighted Colts one with a SureFire light/handgaurd for when things go bump in the night.

I would only go with a 20" barrel for shooting long range matches if you are into that stuff.  Take a good carbine class and you will see that lightweight short barrels ar the way to go (unless target shooting)

Yes the accuracy should be roughly same.

But let me throw in this one...initial velocity of bullet and bullet fragmentation is a different story. I too have 2 14.5s for light weight carbines but I DO feel need for 20" for longer shots. So if you are going for an only AR, get 16" for best of both worlds.

What 24" barrels like you would find on a space gun?

How much better is ti to have the 4" over the 20"?

my .02,  I learned a little about gun weight and barrel weight from skeet shooting.  I liked the heavier barrel to keep my swing slower and smoother.  I would like to guess that the longer barrel would be steadier at a stationary target and a short one more jittery.  The 30-40 fps you get really doesnt add up to much in trajectory imo
Edit:CUFF-II
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