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Gas system lengths.

Discussion in 'Rifle Country' started by Demi-human, Mar 24, 2018.

  1. Demi-human

    Demi-human Member

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    While searching about for a barrel I was encountered with some barrels that were "plus two inches" gas systems. Referencing a "normal" rifle length system, what are some benefits or deficits to this option? Are they relative to barrel length?
    I am not a competitor, are the enhancements that noticable for humans like myself? Is this more for low mass carriers?

    Thank you for any experience, opinion or insight that may be provided.
     
  2. Mosin Bubba

    Mosin Bubba Member

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    I've never heard of that before. But just on an anecdotal note, I built a rifle length A1 clone the other day, after hearing about how much softer shooting the gas system is and all that. I didn't notice a difference between it and my carbine length gas system - maybe a little less kick, but I chalked the difference up to a pound in additional weight.

    If I can't even tell the difference between carbine and rifle, I doubt you would notice the difference between rifle and this rifle-plus-two.
     
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  3. mjsdwash

    mjsdwash Member

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    its all just gimmicks at this point.
     
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  4. jakk280rem

    jakk280rem Member

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    Gas system lengths are less about feel and more about the little things you can't see or feel. The the name of the game is dwell time. This is the amount of time the gas tube and carrier is exposed to chamber gas. The longer the barrel is in front of the gas port, the longer the dwell time.

    The +2 gas length you saw would only be advisable on very long barrels like 24 or more inches. Properly matched barrels and gas systems should have similar feeling recoil impulses because the force applied to the BCG would be similar.
     
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  5. NIGHTLORD40K

    NIGHTLORD40K Member

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    I don't think it makes much difference on felt recoil, but the biggest difference would be on stress applied to to the bolt/extractor. By starting the extraction cycle earlier, the case may not have fully contracted from the chamber yet, potentially leading to breakages or case seperations- although, in the real world both appear to be pretty rare even on short gas tube guns.
     
  6. Varminterror

    Varminterror Member

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    My 29” match rifle runs an “over length” gas system. The Proof “+2” system is the same idea - lower pressure further down the barrel, and on a long enough barrel, there’s plenty of dwell time left to still cycle.
     
  7. Demi-human

    Demi-human Member

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    What would you consider the lower limit for the barrel length? I see that JP has barrels as short as twenty. I assume these would need a lighter spring or low mass carrier.
    I was wanting a nicer rifle when I am done, but am apprehensive about being too custom. I don't need extra parts laying about from tuning this one rifle.

    I am mostly curious right now.

    I suppose the point is moot if one had an adjustable gas block?
     
  8. bfoosh006

    bfoosh006 Member

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    The +2 is far more prevalent in .308 / 6.5CM / 300WSM large frame style AR's ... those have considerably more volume of burned propellant gas then a 5.56 cartridge.

    The further down the barrel the gas port is... the lower the pressure at the gas port.
     
    Last edited: Mar 24, 2018
  9. jakk280rem

    jakk280rem Member

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    I'm not really sure I understand your post. Lower limit for barrel length? 5.56 AR's exist with barrel lengths between 7" and 30". Since you are posting this in Rifle Country and not the pistol or NFA forum let's say the lower limit is 14.7 with a permanently affixed flashhider or 16" with a standard threaded muzzle.

    No. 20" barrels are standard M16 length. No custom parts are needed. Just buy parts listed as for a rifle and not carbine or midlength and you will be fine.

    You say you want a nicer rifle when you are done. Why don't you give us an idea of your vision. What do you want from your rifle, tell us about it and we can help you come up with a list of compatible parts that will achieve your weight and accuracy goals.
     
  10. IndianaBoy

    IndianaBoy Member

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    I wouldn't buy a non standard gas system length for a typical 223/5.56 AR build.

    I have used carbine and mid on 16" and rifle length on 18" and 20". Whether or not you can feel a difference will often depend on gas port diameter, and many other factors.

    If I were making a recommendation I would suggest a mid-length for a 16" and rifle length for anything longer.
     
  11. GunnyUSMC

    GunnyUSMC Member

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    You should try a rifle gas system on a 16” barrel. This is my smoothies shooting AR.
    5211BB62-304B-4B2A-96DD-1CCFFFBA20A4.jpeg
     
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  12. Varminterror

    Varminterror Member

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    No low mass required. The port diameter is also in play as a variable. Need more gas? Open the port. Easy peasy.
     
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  13. Gordon

    Gordon Member

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    That why a couple years back people were buying Dissapater uppers which had rifle lengh gas on 16 inch barrel length .They do seem rather smooth opering to me. I use rifle length gas on 18 inch ar-10 , midlenth on 16 and 14 inch Ar15, and carbine length 14 to 10 inch and pistol length that.
     
  14. Gordon

    Gordon Member

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    Enough of posting from my phone n abar :(
     
  15. barnbwt

    barnbwt member

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    Fun fact; there are approximately two dozen different piston/gas system lengths across the entire AK47 family. All use the same diameter piston head. We should be happy the AR family gas system is only as confusing as it is.
     
  16. Demi-human

    Demi-human Member

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    It make sense to me that, to keep the dwell time the same, the port would be moved to the same position relative to the muzzle. Within reason.

    I am attempting to build a prairiedog rifle. I wanted a long barrel, but I don't wish to see the gas block. I have a fifteen inch hand guard already.

    I am new to building and though I want a custom to me rifle, I am anxious about going "too far" and having difficulty. I can assemble parts just fine. I would endure unecessary stress putting a drill bit to a brand new barrel.

    I have read two and own Sweeny's book on AR gunsmithing. They are mostly about the system as a battle rifle or general firearm. They covered the "whats" well, but this was more a system theory question.
     
  17. GunnyUSMC

    GunnyUSMC Member

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    I built this one to go groundhog hunting with my friend in PA.
    It has a 18” 223 Wylde 1/8 barrel and a 17” rail. Oh! I did go a little over and made it a side charger.
    9A123BAA-E953-477A-A7EA-BF6512AE37B2.jpeg DB21BC57-8361-4911-A5A9-8A41DA380BF8.jpeg 2FFD60EA-2646-4432-9E9F-50524968ADB8.jpeg
     

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  18. Jackal

    Jackal Member

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    Built a couple dozen AR's, all the different gas lengths. I've never been able to tell a difference while shooting.
     
  19. Demi-human

    Demi-human Member

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    That is a very cool receiver. What brand is that? That would make a very nice SBR!

    Oh, no. I feel it. Not again...
     
  20. Varminterror

    Varminterror Member

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    Dwell time is one aspect of the gas system. Port diameter is another - not many guys remember to consider the gas port diameter, because barrel makers don’t offer it as a selection, just length. Guys forget each maker is reaming a different port diameter according to each gas length. It’s all about the gas impulse. A longer impulse at a lower pressure will cycle the action just as a shorter impulse at a higher pressure will - obviously the feel between the two will be different. A tiny port right off of the chamber (pistol length) will cycle just as reliably as large port at 12” (rifle length).

    I run AGB’s on all of my rifles, and also prefer to have my gas blocks covered. I really dig the aesthetic of the new 18” handguards on 20”+ barrels.

    For a dedicated prairie dog rifle, my personal choice was a 22” 204Ruger, rifle length gas, 15” handguard. I also have a 24” 5.56 rifle in 1:9” meant for running the 69SMK into prairie dogs. I don’t know that I’ll ever buy another barrel with anything but a 0.750” gas block, unless I found a deal on very deep fluting to reduce the forward weight. The 0.836” and 0.920” muzzles are just ridiculously heavy and don’t really offer any justifying improvement on performance. If I’m adding weight out front, I’d rather do it as barrel length to gain velocity.
     
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  21. GunnyUSMC

    GunnyUSMC Member

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    Last edited: Mar 25, 2018
  22. Demi-human

    Demi-human Member

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    Thank you, Gunny. Very neat.

    Yes. And the rest will be further away where I like it. Parallax has a nice seventeen, but I received a great deal on this Aero set.
     
  23. Geno
    • Contributing Member

    Geno Contributing Member

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    Gunny:

    Permission to download your photo and share it with my SF nephew?

    Geno
     
  24. Demi-human

    Demi-human Member

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    Thank you for helping me to further understand the AR gas system. I appreciate the insight provided and the time taken to do so.

    In the end I will most likely be chicken and order a regular rifle length.
     
  25. IndianaBoy

    IndianaBoy Member

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    Unless you are going beyond 24 inches for the barrel, I wouldn't recommend going with the +2 in any case.

    Depending on the distances you plan to attempt with this prairie dog rifle, you would probably be well served with a 20 inch barrel and save yourself some weight.

    The velocity gains of 223 drop off pretty quickly after 20" and really quickly after 22".


    If you are already looking at JP for barrels, if you paired it with one of their muzzle brakes and used their LMOS carrier and an adjustable gas block, you will be able to watch your hits on varmints through your scope.


    Necessary for proper function? No.

    Significantly improved recoil characteristics? Yes.
     
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