Barrel Length & Gas Systems
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  1. #1
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    Barrel Length & Gas Systems

    This is a post that I'm hoping people can go really slow on for the slow guy here. I seem to be either misunderstanding, or just not getting a lot of the conversation out there about barrel length with respect to a weapons gas system.

    Let's say you have two rifles that are exactly the same, but one with a shorter barrel. Say both guns have the gas block at 10 inches, with one having a barrel of 16 inches in length, and the other 12.5 inches in length. From what I've been reading, many folks are saying the 12.5 will be snappier or have greater felt recoil. Since it has less dwell time, and will have less gas cycling the weapon, wouldn't it have less felt recoil? Let's say the weight between the two is also the same. The 12.5 inch has a light on it or whatever that makes up for the missing steel from the barrel. Not really looking for personal experience here, but a conclusive answer based on the physics happening.

    Thank you all.

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    as i understand physics and under circumstances you describe shorter barrel should have less muzzle velocity, less felt recoil, more dwell time less energetic (reliable) cycling, less distance on case ejection pattern BUT more muzzle blast and concussion as some of the propellant wasted on making big boom outside the barrel instead of pushing on the projectile inside the barrel.

    from my experience i find that a lot of gun-owners are rather dim types who don't really have an education or understand physics and/or mechanics of a firearm on the level beyond 'where bullet goes out and where mag goes in'. Moreover many tend to parrot hearsay and some other ridiculous nonsense they've herd from some guy at the range. you won't believe what things people ask say at the shop often with having posture of authority and patronizing stance like they are Moses Browning or Mikhail Klashanikov re-incarnate.

    short barrel rifle concepts are even less well understood and people tend to pile up different things from different sources regardless whether its correct or applicable in the instance. SBRs are far less common than non-NFA guns thus there is plenty misunderstanding and wrong info as the result.
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    Dunno about the "snappier" part nor the physics.
    Only personal experience.

    For the physics I recommend that you use the searh engine 'Google' and query " barrel length effect on recoil in a firearm".
    Last edited by AKBLUE; 03-25-2019 at 12:09 PM.

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  5. #4
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    a lot depends on the position of the gas block AS WELL AS firearm itself + particular round/loading in question. Lots of variables especially when it comes to dual-purpose cartridges like .300BLK which instantly makes equation even harder to decipher/understand.
    you can have gas port too close to the chamber and stress gas system by blowing lots of un-burnt propellant into it and/or drawing gas too close to the pressure curve peak. you can have it slightly closer or farther than 'standard' and have no noticeable difference. pressure curve for different cartridges are different. Best place to draw gas for reliable yet not too violent cycling is usually empirically determined during the design and/or revised as needed during the evolution of the weapon system for the specific weapon and specific round loading. there is no on rule fits all type of scenario.
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  6. #5
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    Quote Originally Posted by AKBLUE View Post
    Dunno about the "snappier" part nor the physics.
    Only petsonal experience.

    For the physics I recommend that you use the searh engine 'Google' and query " barrel length effect on recoil in a firearm".
    Been there, done that. Plenty of opinions that almost always (and quickly) devolve into "MY rifle feels this way, oh and I have a 22" precision AR that shoots like a dream". Rather than just a clear breakdown with all of the variables the same stating "X length of barrel should have more/less felt recoil than X+N length barrel".

    Thanks @dnepr0mike, that was my understanding as well.

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    sounds like some of the AR-15 SBR lore made it's way into the discussion and skewed somebodies understanding. With ARs it's a somewhat different than with AKs and also somewhat counter-intuitive.
    the shorter gas length system is in AR the closer you are drawing to peak pressure curve. look a the table below and listed pressure values for different gas system legths.
    AR-15 Gas System Gas Port Location Pressure (PSI)
    Pistol 4.7″ 48,300
    Carbine 7.8″ 33,000
    Mid-Length 9.8″ 26,500
    Rifle 13.2″ 19,600

    Since AR bolt is also a piston and connected directly to the pressure vessel (that is the barrel during firing) the closer to the chamber you draw the more thrust bolt carrier will experience during unlocking, thus faster it will travel backwards, thus harder it will impact the back of the receiver extension harder, thus greater felt recoil at the shoulder. Add giant muzzle blast that short barrels are known for and you get a high 'shock value' on the shooter who instantly perceives it as harder, more violent recoil.

    second variable beyond the length of the gas system and pressure curve of the round is a proximity of the gas port to the muzzle. the closer it is - the less time gas pressure can act on the piston. it doesn't need much time but it still needs long enough to overcome the inertia of the stationary BCG and long enough to apply sufficient force to ensure that BCG has ample energy to move backwards with enough force to completely cycle the action and do it reliable under all temperatures.

    since you mentioned 12.5" barrel i conclude you are thinking about ak-102/104/105 and it's us-legal 16" counterparts sold by arsenal.
    my understanding is that arsenal sell these 16" versions with intent that folks who buy them will chopped them down to 12.5" length and use them with muzzle boosters. in it's 16" config those guns are terribly over-gassed even by AK standards. people are constantly heaving problems trying to run those 16" guns. is that where you questions come from? it would be easier to answer questions i we can understand what specific situation these questions relate to.
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    Quote Originally Posted by Theophilus View Post
    Been there, done that. Plenty of opinions that almost always (and quickly) devolve into "MY rifle feels this way, oh and I have a 22" precision AR that shoots like a dream". Rather than just a clear breakdown with all of the variables the same stating "X length of barrel should have more/less felt recoil than X+N length barrel".

    Thanks @dnepr0mike, that was my understanding as well.
    There is detailed information in the Google search. Complete with scientific numbers and data.
    Much more than you will find on gun sites including this one.
    Not many physics or ballistics majors or experts on gun sites.
    Do not use the gun site discussions for answers.
    Last edited by AKBLUE; 03-25-2019 at 12:53 PM.

  9. #8
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    Back when I was in the British Army if we were training recruits on the range we'd turn the gas off on the SLR and give them a surprise. I t kicks like a bolt action rather than the push against the shoulder when the gas was dialed in.

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    Math and science are wonderful things.
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