There is a substantial advantage of the midlength over the carbine. Everyone knows why. If lightness is preferred over accuracy potential the M4 profile is a great system but if you want to cash in accuracy potential go for heavier possibly fluted barrels. I never like a barrel where the mid section if of the same or less of the diameter of the gas block. I think the BH standard profile looks really nice. With the slimmest barrels a dissipation blocks helps address some of the flimness and heat issues, but again something might not be a great concern to the average user. At 18" I think mid length is pretty good but rifle will work too. you have to keep in mind the extra rail you need and if that is at any use to you. It could be optional but you do not want to leave the gas tube exposed to damage. For my taste the Grendel in 18" is marginal. If you really want to take more advantage of the round then one might want a little longer but it really depends on the intended purpose of course. Also if you are planning in short stroke gas system I find the carbine being the most accurate, the midlength next and the rifle last. An adjustable piston resolves some of the issues we have with the many times overcharged carbines. The wider the section the better and probably you might want flutes. The answer is back at the roots the AR that was designed to run with more dwell time and opening the bolt from the original bolt and not by a piston striking the carrier. so the longer that arm is the more effect it has in barrel flex and therefore in the accuracy. This defeats the original purpose of the AR but at the same time, the longer the AR is the best the impigement gets and it is where we normally want to maximize the accuracy with the 20 inchers and longer that are designed to reach further whether is target or hunting.
This doesn't mean that a piston cannot be accurate, in fact I have 3 of them that are pretty good but they are more 'picky'. They are good for other reasons.
I think that the midlength provides a perfect balance and you can always get a simple yet adjustable gas block to properly tune the gas also based on load playing. beware of the tuning screws in some of the popular ones, they are not that well built and need some custom work to make them 100% reliable.
I hope this helps you or someone else.
Last Edit: Dec 12, 2011 14:50:50 GMT -8 by Deleted
I have an 18" White Oaks Barrel with a 1:7" in 223 with a rifle length gas system, and I could not be happier with its performance.
When the AR-15 (M16) was designed, it was done with the 20" barrel and rifle length gas system in mind. By introducing short barrels variance we are messing around with the timing of the weapon's cycle of operation. One of the main problems of carbine length system is because the gas tube is so short, the returning gases start to excerpt pressure on the bolt key and begin to cycle the weapon before the bullet has actually left the barrel. At that time the pressure inside the chamber is at dangerous levels. The operator feels this as a sharp return of the bolt carrier group and enhanced recoil (not overwhelming, but a quite noticeable increase in recoil) and there might also be a failure to extract the empty casing. There are two solutions to this problem, the use of a piston driven system or a mid-length gas system; both will take care of the timing issue and will provide a smoother operation.
What in the World does this have to do with 18" barrel? The mid-length 18" barrels have similar length from the gas port to the muzzle as the carbine does. Although the problem is not as noticeable as it is on the carbines, you still would feel a sharper more violent recoil than if you were using a rifle system. Once again the solutions are use a piston driven system or use a rifle length system.
I remember reading some years ago that 6.5 Grendel were having some overpressure problems (not due to ammo), and it was discover that the gas port was too wide for that system.
I do agree with 1Shotzero, to really take advantage of either one of those 6.5mm calibers you should go with a at least 20" or longer, but it all comes down to personal preference.
"Stops Jihad on contact" - Noveske Rifleworks "Si vis pacem, para bellum - If you want peace, prepare for war."
I think letaz77 is giving some pretty good advice. If you do not mind the extra rails/handguard the rifle is probably the best and smoothest. Mid will work. You can also get a tube with a staging chamber that delays the pressure peak. Whatever you do avoid carbine and have the upper properly gassed. A properly gassed AR should be throwing the brass at 4 to 5 o'clock and about 30 to 40 degree angle.
I had the same question and the answers in this thread were very useful to me, thanks.
This may be a bit off topic, but the real issue is about properly gassing the system. My friend created his own adjustable gas block by drilling and tapping a hole into it, then tuned his ejection to avoid the brass deflector entirely.
Is anyone aware of someone having done this with a piston system gas block, such as that from Adams Arms? I imagine you'd have to tap horizontally into the block rather close to the ejection port on the barrel, before the gas reaches the piston above. A bit off the wall I know, but has anyone seen anything like this?
I have several adams pistons. the piston comes already with 3 positions. Full open, suppressed and cancel. The suppressed is perfect for the Nato loads while the full open is needed for the very mild loads like PMC bronze. If you shoot in suppressed you need suppressed. The piston by design itself is providing a delay to the dwell time as the piston cam needs to reach enough pressure to move the piston then strike. This doesn't mean that the piston might not need some tuning but this by design is pretty well timed. You also have the capability to further delay bu moving the block or using a stronger spring. Tapping the adams brake is not a good idea.
In the impingement a tuning screw is a good idea since it is the only alternative other than having a staged tube (blown) tube but then no real tuning. So the tuning either happens at the block or in the tube somewhere.
I hope this helps.
Last Edit: Dec 14, 2011 12:14:57 GMT -8 by Deleted
My ultimate goal is to tune it such that brass never hits the deflector. What you are saying about adjusting the spring makes a lot of sense. Do you know where to source springs of different strengths that would work for this? Is it possible to modify the existing spring in some way?
Also, by moving the block to you mean intentionally misaligning it slightly over the gas port to reduce the venting to the block?
Very interesting, but I just want to be sure what you mean. Thanks again.
I prefer the rifle length system on a DI rifle. Carbine length will give you a VERY FAST bolt. I forget the IPM measurement, but it's FAST! (heavy buffers and feedramps are just to try to aid feeding due to an overly fast bolt) Midlength is an improvement. The rifle designed by Stoner was made to work with the rifle length system. This is on a 5.56 chambered rifle. I hope a mid length is OK on the 264 because that's what I have.
Last Edit: Dec 19, 2011 16:08:36 GMT -8 by ctrmass
The 18" mid length is what I'm looking at as well, a black one.
I've been lusting over the Sampson Evolution 12.37" rail for this myself or maybe the 15" model. I really need to make up my mind as I'm guessing I'll be placing the orders in the next week or so regardless. LOL!