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Charlie

Another cause of short-stroking.

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I saw the "New 20" barrel, now it short strokes" thread from 2017 and wanted to share a recent experience:

Early this year I built a generic AR-15. Aero Precision upper and lower receivers, a DPMS lower parts kit, standard A-2 stock and a DI gas system on a 20" barrel. The parts went together without a hitch, but it would short stroke almost every time. I verified that the gas system was installed correctly and the gas port was clear. I also learned that the size of the gas port is directly proportional to barrel length: The longer the barrel the bigger the gas port, and that's because the amount of gas available to cycle the action decreases with barrel length.  So short barrel rifles have smaller gas ports and stiffer buffer springs. It seemed likely to me that my A-2 buffer was intended for a 16" barrel, and the spring was too stiff. There's a gun shop a few miles from my house that has a selection of buffer springs, so I picked one that had two less coils than my existing spring and; Voila! Problem solved.

Obviously this won't be a problem in a production rifle, but if you're going with a longer barrel (or you're building and your buttstock/buffer spring doesn't say what barrel length its optimized for) it might be an issue.

Charlie

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Thanks for the input.  A different buffer spring may be one way to balance the recoil system with the gas system.  However, @98Z5V is capable of calculating what size gas port is needed to give the proper amount of gas to reliably cycle a firearm.  There have been many times that drilling gas ports to a larger size is the correct solution to fix a cycling issue.

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3 hours ago, Charlie said:

 I also learned that the size of the gas port is directly proportional to barrel length: The longer the barrel the bigger the gas port, and that's because the amount of gas available to cycle the action decreases with barrel length.  So short barrel rifles have smaller gas ports and stiffer buffer springs.

That's not true, brother.  Longer barrels don't necessarily need larger gas ports, and barrel length really doesn't have anything to do with it.  Dwell Time and Gas Block Journal Size directly affect gas port size, though.  Dwell Time is the larger factor, then journal size, but they're both very, very important in determining gas port diameter.   Caliber/cartridge power is 3rd on the list, and important.

Ever seen a 16" barrel with a 0.105" gas port?  I have, and I know that's the only way that particular barrel will ever run, properly. 

Edited by 98Z5V

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On 5/26/2020 at 10:31 PM, 98Z5V said:

That's not true, brother.  Longer barrels don't necessarily need larger gas ports, and barrel length really doesn't have anything to do with it.  Dwell Time and Gas Block Journal Size directly affect gas port size, though.  Dwell Time is the larger factor, then journal size, but they're both very, very important in determining gas port diameter.   Caliber/cartridge power is 3rd on the list, and important.

Ever seen a 16" barrel with a 0.105" gas port?  I have, and I know that's the only way that particular barrel will ever run, properly. 

If you pull up the data on barrel length vs gas port diameter the trend is toward larger diameter gas ports as the barrel length increases. My experience seemed significant only to the extent that the poster who went from a 16" barrel to a 20" barrel and experienced short stroking _*MIGHT*_ have the same issue. But the post was 3 years old, and it seemed pointless to add anything to the thread.

I'm not a gunsmith, and I'm not trying to be a smartass, but I believe you will agree that in that situation it is something to consider.

Charles

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The BCG must have a certain amount of gas to move it backwards to cycle properly.  The biggest factor that affects the gas system is where the gas port is relative to the end of barrel.  The gas can only enter the gas system from the time the bullet passes the gas port until the end of the barrel.  The shorter that distance, the larger the gas port must be to let in the required amount of back pressure.

How long the barrel is will also play a part because of how far the gas must travel backward.  A 24 inch barrel will need more gas to create the same back pressure than a 16 inch barrel with the same distance from port to end of barrel.  Therefore, it will need a larger gas port.  However with a 16 and 24 inch barrel with the gas port the same distance from the BCG, the 16 inch barrel will need a much larger gas port because the 24 inch barrel has much more time to push gas into the gas port.

I cannot calculate the port size like @98Z5V, but I hope this makes sense. 

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20 hours ago, Armed Eye Doc said:

The BCG must have a certain amount of gas to move it backwards to cycle properly.

I hadn't considered the length of the barrel beyond the gas port. So given an 18, 20 and 24 inch barrel, all having a rifle length gas system, the 18 needs a larger port than the 20 or 24 to push the same amount of gas down the tube. That makes sense.

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3 hours ago, Charlie said:

I hadn't considered the length of the barrel beyond the gas port. So given an 18, 20 and 24 inch barrel, all having a rifle length gas system, the 18 needs a larger port than the 20 or 24 to push the same amount of gas down the tube. That makes sense.

^^^   That's it, right there.  That, my friend, is the "dwell time."  How long you give the gas to act on the BCG.  Gas port location to the crown of the barrel.  :thumbup:

If it has more time to build, before it gets to the gas port, it's usually a softer shooter.  That's gas system length.   How long you give the gas to build/dissipate, BEFORE it can act on the BCG.   Example - rifle length gas, versus carbine length gas.  That carbine gas system will have a sharper "felt recoil."

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Also, take it from someone who refuses to buy ammo, some powders have a lot more pressure during the dwell.

Example in 308 AR-Comp drops off similar to factory loads, similar port sizes work,  RL15 builds all the way to the end, exhibits over gass symptoms. Smaller port /less dwell would work fine

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16 minutes ago, Albroswift said:

Also, take it from someone who refuses to buy ammo, some powders have a lot more pressure during the dwell.

Example in 308 AR-Comp drops off similar to factory loads, similar port sizes work,  RL15 builds all the way to the end, exhibits over gass symptoms. Smaller port /less dwell would work fine

Reloading is a completely different animal, brother.  Completely.

My initial loads for the .260 Rem never failed.  147gr Hornady ELD-M projos and the RL-22 powder.  Ran that fucker like a champ. When I branched out into other loads, projos, - and even a factory load from NEXUS - wouldn't even cycle the gun.  So, ONE LOAD would cycle the gun...  and it was a stellar performer, so I didn't care.  That load - the one that I really wanted - worked. 

Once I tried out the Nexus 130gr Laupa Scenars  - that factory load, FIRST factory load ever through that gun...  and it didn't even cycle the action?!  I know something was wrong, WITH the gun.

Chased down some other previous loads I tried, looked at everything - gas port too small, when you crunch gas port numbers.  That thing was a 20" rifle-gas .260, with a 0.750" journal size, running a 0.070" port diameter.  Too little.  Punched that thing to a (real) 0.081" - runs everything now.  All of it. All of my previous failed "other weight" handloads, AND that NEXUS 130gr Lapua Scenars. 

As well as powders, and their burns rates/power, make a difference - if that gas port diameter isn't straight to begin with, you'll only find ONE combination or two that really work.  At that point, it's not the powders, or your loadings - it's coming down to the gas system itself. 

Never be afraid to punch a gas port up.  Ever.  If you fuk it up and go too high - then there's a metric shiit-ton of adjustable gas blocks on the market that can fix your over-sized drilling error.  :thumbup:

Edited by 98Z5V

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^^^  Wilson Combat now ships that exact same barrel with a 0.080" gas port diameter.  Wonder why?...  :popcorn:

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