When talking about accuracy several things need to be considered:
-Size of the target
-Type of ammo (match vs. ball)
-Distance
-Accuracy expectations
-Type of shooting you will be doing (ie. all prone shooting at static targets, running and gunning and shooting from unconventional positions, etc.)
There is a difference between “mechanical accuracy” and “practical accuracy”. If a majority of your shooting is going to be done with 55grain standard (not match) ammo with an Aimpoint or an ACOG are you really going to be able to see that barrel “A” is capable of shooting .5" better groups than barrel “B”?
If you’re shooting 6" steel plates at 400 yards with a 10x optic and match ammo, the difference in accuracy may mean a lot more to you.
Often times as shooters we don’t keep our accuracy expectations in the context of our real shooting needs.
03designgroup | [b]Which Carbine Is More Accurate 
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Stainless barrel is stainless steel and is a different material than a chrome moly barrel. Stainless steel 410 is a harder stainless steel than stainless 416. 416 is what most manufacturers make their stainless barrels out of.
Cold hammer forging is a process of making a barrel: http://www.rifleshootermag.com/gunsmithing/RSgunsmith1/
Generally a stainless barrel will be more accurate than a Chrome-Moly-Vanadium (CMV) barrel. Not by a huge margin. One example of many, take a look at the info Molon posted in the link below. Compare the group sizes of the Noveske 16" stainless barrel (0.92" average) to the Noveske 16" N4 barrel (1.24" average): http://www.ar15.com/forums/topic.html?b=3&f=118&t=467919 The difference is just slightly more than 1/3" (one third of an inch)
Generally a stainless barrel will have a slightly shorter service life. Don’t take this comment to the extreme.
Generally a stainless barrel will a heavier countour and be slightly heavier than a CMV barrel.
Weight comparisions:
03designgroup | [b]AR15 Upper Receiver Weight Comparisons [/b]
For most of us, practical accuracy is much more important than group size. I don’t have a problem hitting smaller than average targets out to 425 yards with a 3.5x ACOG or Leupold 1.5-5x with my standard chrome lined, cold hammer forged (chrome lined), or stainless barrels using XM193, Winchester Q3131A, Federal American Eagle 55 grain, Black Hills 55 grain, and other 55 grain ammo.
For a hard use, high round count, training gun I would select a chrome lined barrel over a stainless barrel.
I wrote this in another thread in reference piston guns:
As a frame of reference I have been hosting 2 to 5 tactical training classes a year, every year since 2001. A side from the classes I host I also take other training classes. I’ve been a full time Police Officer for 14 years, I’ve been a SWAT cop for 11 years, and I’m a Firearms Instructor for my agency, our Police Academy, SWAT Team, and SWAT Academy. Prior to that I was in the Marine Corps (Infantry / Security Forces).
In the early 60’s when the M16 first came on line there were several important people that wanted to see the M16 fail. So troops were told that they didn’t have to clean their guns, they used the wrong powder in the ammo (ie. they were suppose to use stick powder and they used ball powder), etc. By doing this the M16 got a reputation as being an unreliable platform.
In my opinion the reason that the direct impingement gas system (DI) has gotten such a bad reputation in recent years is because people go out an buy low end AR’s or they try to build a AR from parts from various manufacturers. They end up with a gun that is unreliable and this feeds into the myth that the DI gas system is unreliable.
When I was in the USMC the main malfunctions were caused from shooting blanks and magazine related. I had seen a hand full of other problems, but they were far and few between.
In the training classes that I host and take on my own and from the AR15’s that I see in training and qualification courses at work. Colt, LMT, BCM, Noveske, etc. run well. While CMMG, DPMS, Olympic Arms, Bushmaster, Stag Arms, RRA and others have a high number of reliablity problems.
Piston guns. I have seen a lot of piston guns that have had problems. I have never seen a POF make it through an entire class without problems. About half the Sigs that I have seen have had problems. About 1/3 of the LWRC guns I have seen have had problems.
From my experience piston guns
-have a sharper recoil impulse
-they are heavier
-piston system guns are more expensive
-the different piston systems are new and haven’t had the time to be as thouroughly tested as the DI gas system
-many of the piston system operate on a slightly different system
Piston systems on the AR15 is a fairly new concept (most within the last decade). The DI gas system in use on the AR15 has been in service for close to 60 years, this has given engineers time to work the bugs out of the DI system. Not the same can be said for the piston systems used on the AR platform.
In my opinion the piston system is not needed on the AR15 and it exists because guys buy lower end AR15’s, many of these lower end AR15’s are not reliable, and when a shoorter buys or builds an unreliable AR15 it feeds into the myth that the DI gas system is unreliable. DI gas system AR15’s are not created equal. There are different levels of quality.
A well known trainer has a DI gas system BCM upper that had 26,000 rounds through it before it was ever cleaned. Currently the gun has just shy of 29,000 rounds on it. Read this article for more info:
03designgroup | [b]BCM Complete AR15 Upper and Lower Receivers [/b]
In short I see no need to buy a piston upper. Buy a QUALITY AR15 with quality ammo, quality magazine, lube it, and it will run without any issues.
Hope this helps
