advantage of extended gas length?

[NCHillbilly]

Ok, right up front, I'll admit I have very little luck using the search function on here, so if this has been addressed, I apologize.

I am looking to buy an lr-308 barrel in 6.5 creedmoor, and would like a better understanding of the effect the gas length has on the barrel or cartridge performance.

I have found some good deals on what I think would be decent barrels, but they all have rifle length systems. The only way to get a +2 system, which I've read is preferable, is to get a pricey custom tube.

let me hear your thoughts.

 

[Milf Dots]

It's a non-issue with piston based rather than Di based rifles.

In a 5.56 Di rifle, my understanding is that longer length gas systems result in better reliability and durability, and smoother recoil impulse.

 

[psmith23]

I'm not the expert either but it has to do with gas port pressure. In 308 if you're using 147 grain bullets and a fast-medium burning powder for 308 then rifle length is fine. My 308 is using 175 and 185's and I had to use an adjustable gas block and add some weight to the carrier to keep everything in check using either varget or 4166. On my 260 I went with a +2 since using h4350 and wanting best velocity it would have had gas port pressure way out of the original design parameters. Bore diameter also plays in as well as 308 is larger than 6.5 so there is more room for the gas to expand in the barrel before it gets to the port.
In short, if you want to run the same or similar pressures to the bolt guns then a +2 is pretty nice. Either way you will need an adjustable gas block and with rifle length gas you will have an easier time with the lighter bullets like 120's and 130's Vs the 140 and heavier class ones.

 

[NCHillbilly]

I should have mentioned, I am planning on getting a 22"-24" pipe, and will eventually run a can, so will also be using an agb, either sa or slr.

 

[Evintos]

The longer the gas length, the lower the system pressure ultimately returning into the BCG. The higher the system pressure, the cyclic impulse increases, which places increased wear and tear on your BCG, particularly strain on the bolt lugs.

However! This is dependent on gas port position and gas port size relative to barrel length (dwell time). The longer your dwell time, the smaller you can make the gas port, which would decrease system pressure.

The shorter your dwell time, the larger your port has to be in order to function properly. Both works long and short dwell time works provided the gas port is sized properly, but one has longer duration (talkin about milliseconds) of lower pressure onto the overall system versus shorter duration and higher pressure.

There is a minimum amount of gas flow necessary for either to work well, it just depends on how fast you deliver it.

The smaller the gas port, the less the combusted gas pressure is used to make the system cycle and the more gets put into pushing the bullet. Ultimately how much FPS loss occurs depends on gas port size and efficiency of your bolt carrier. The gas efficiency of your bolt carrier depends on how tight the tolerance stack is between your bolt and bolt carrier. A efficient bolt carrier with an oversized gas port would put just as much strain on bolt lugs as a properly sized gas port with short dwell time efficient BCG.

Reliability and durability (generally) favors longer dwell time and lower system pressures. However, tolerance stacking between parts ultimately plays an aspect in reliability and durability. The workable solution is an adjustable gas blocks in which you can compensate for tolerance stacking. The standard solution to this however is buffer weight. Adjust the overall mass of a system. More mass = more pressure necessary to move the mass.

Realistically your bolt (if made properly and assuming gas system is running optimally) should last the life of the barrel and should be replaced with the barrel (along with gas tube, gas block, gas key, firing pin, heck just replacing the entire BCG would be cheaper than buying the individual parts).

Edited to add - the higher the pressure relative to gas port position and chamber position, the faster gas port erosion occurs (dependent on barrel material quality). Ex. Two barrels with a 0.060" gas port size, the one closer to the chamber (carbine length vs rifle length) will have faster gas port erosion. Gas port erosion = gas port growing in size = more gas flow is occuring. Excessive gas port erosion can cause bullet shear in high velocity bullets. A good example is the development of the Russian AK74. Russians found out that the standard 45 degree drilled gas port in an AKM (7.62x39) would cause bullet shearing in the higher velocity 5.45x39. 90 degree gas ports solves that issue. A 45 degree gas port looks oblong much like an eroded 90 degree gas port.

 

[NCHillbilly]

The longer the gas length, the lower the system pressure ultimately returning into the BCG. The higher the system pressure, the cyclic impulse increases, which places increased wear and tear on your BCG, particularly strain on the bolt lugs.

However! This is dependent on gas port position and gas port size relative to barrel length (dwell time). The longer your dwell time, the smaller you can make the gas port, which would decrease system pressure.

The shorter your dwell time, the larger your port has to be in order to function properly. Both works long and short dwell time works provided the gas port is sized properly, but one has longer duration (talkin about milliseconds) of lower pressure onto the overall system versus shorter duration and higher pressure.

There is a minimum amount of gas flow necessary for either to work well, it just depends on how fast you deliver it.

The smaller the gas port, the less the combusted gas pressure is used to make the system cycle and the more gets put into pushing the bullet. Ultimately how much FPS loss occurs depends on gas port size and efficiency of your bolt carrier. The gas efficiency of your bolt carrier depends on how tight the tolerance stack is between your bolt and bolt carrier. A efficient bolt carrier with an oversized gas port would put just as much strain on bolt lugs as a properly sized gas port with short dwell time efficient BCG.

Reliability and durability (generally) favors longer dwell time and lower system pressures. However, tolerance stacking between parts ultimately plays an aspect in reliability and durability. The workable solution is an adjustable gas blocks in which you can compensate for tolerance stacking. The standard solution to this however is buffer weight. Adjust the overall mass of a system. More mass = more pressure necessary to move the mass.

Realistically your bolt (if made properly and assuming gas system is running optimally) should last the life of the barrel and should be replaced with the barrel (along with gas tube, gas block, gas key, firing pin, heck just replacing the entire BCG would be cheaper than buying the individual parts).

Edited to add - the higher the pressure relative to gas port position and chamber position, the faster gas port erosion occurs (dependent on barrel material quality). Ex. Two barrels with a 0.060" gas port size, the one closer to the chamber (carbine length vs rifle length) will have faster gas port erosion. Gas port erosion = gas port growing in size = more gas flow is occuring.

Thanks for that explanation. Don't know if you saw my last post, but it is my intention to run a 22"-24" barrel with an adjustable gas block ( superlative arms or slr) for future use with a suppressor. By using the agb, would gas length matter as much?

 

[psmith23]

My .02 is that on a 20" tube you can make rifle gas work in a 6.5 variant if that's easier to find at the moment. Any longer I would really want a +2. My +2 .260 tube is a 23.5 so right in between what you're looking at. Craddock Precision did my +2 and it's been great. JP used to have some +2 barrels ready to go back in the day but I haven't checked in a couple of years for availability.

 

[theLBC]

Thanks for that explanation. Don't know if you saw my last post, but it is my intention to run a 22"-24" barrel with an adjustable gas block ( superlative arms or slr) for future use with a suppressor. By using the agb, would gas length matter as much?

not one of the experts, but it's about timing as much as volume or port size.
pinching off or increasing the gas isn't going to help if you're trying to eject the round before the pressure in the barrel is low enough.

 

[Evintos]

Thanks for that explanation. Don't know if you saw my last post, but it is my intention to run a 22"-24" barrel with an adjustable gas block ( superlative arms or slr) for future use with a suppressor. By using the agb, would gas length matter as much?

Nope, provided that the gas port is not undersized. If it's undersized, you'll have to drill out the gas port or lighten the system weight (BCG or buffer).

 

[psmith23]

An agb will help to get pressures correct but with a 22-24" tube and a small effective gas port you will have a sharp peaky pressure rise to the carrier which will be less tolerant of fouling and gunk so I would expect reliability to be less than that of a longer has system. For a range only gun probably won't matter Mine gets pretty dirty after about 100 rounds with the can. Also overall length gets pretty long with a can so even more reason to keep it at 20" max

 

[r.tenorio671]

....enough time has passed that the majority of barrel producers making 6.5C barrels have a pretty good idea on port diameter size needed. Seeing as how you intend to use an adjustable gas block, you should be able to control gas flow on a rifle length gas system IF it is over-gassed from the get-go. The AGB should also help once your suppressor is attached as well. Modifying buffer weights and/or springs MAY NOT be necessary, but as needed can be used in conjunction with the AGB.

...there have been more reports of folks encountering issues with +2 gas systems than of folks reporting it was G2G from the start (search the WOA ODIN posts). Keep in mind that increasing the buffer weight or buffer spring rate will incur addition "recoil" impulse as it cycles whereas AGB's moderate the gas pulse driving those components and can be more precisely metered. IMHO, you can't go wrong with a rifle length gas system as opposed to the possibilities of a +1 or +2 gas system. Be aware that the powder you use DOES have an effect on QUANTITY of "gas volume" generated.

.....go with what you desire, "buyers regret" is unforgiving but passes quickly especially when sold and replaced...

...if you are purchasing a complete rifle, then you always have the manufacturers warranty to make it right if something isn't working as advertised.

 

[NCHillbilly]

No, this is for one I'm building from the ground up. I have used superlative arms adjustable gas blocks in the past, and like them. I want a longer barrel just for the velocity. This thing, when finished, will be a range gun, so I'm not concerned with extra length.

 

[Clayman]

No, this is for one I'm building from the ground up. I have used superlative arms adjustable gas blocks in the past, and like them. I want a longer barrel just for the velocity. This thing, when finished, will be a range gun, so I'm not concerned with extra length.

If you are planning on using a suppressor your dwell length absolutely matters. An adjustable gas block and buffer weights will help a rifle length but it will never be as good as a system with a shorter dwell length. +2 rifle length and tuned with your suppressor is going to be much better than just a rifle length.

Find a +2 or +3 barrel for 22-24".

Proof Research stainless 22" lr308 (6.5 creedmoor) camgas is a rifle length +2 and comes with a gas tube.

These barrels are around $489-509.

Midway has them in stock for $509.

https://www.midwayusa.com/product/1019039079?pid=807540