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Blowback Rifle Formula
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  backbencher

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Posted: June 20 2017 at 9:20pm | IP Logged Quote backbencher

I think the little Beretta/Taurus .22"LR/.25" ACP pistols w/ the tip up bbls actually use a very strong spring to hold a lightweight slide in place.  Beretta had .32" ACP & .380" ACP versions as well, but I don't know if the larger ones were conventional mass-delayed blowbacks or spring-delay blowbacks like their smaller brethren.
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  Zuzzy

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Posted: June 21 2017 at 9:46pm | IP Logged Quote Zuzzy

Problem with Tanner rifle was in the false superiority thinking of his concieved gas delay that it is gonna work so good with that leather seal delaying, that you dont even need to think about the slide weight (we all asked about it and get no answers - not having any scale?), which was I suppose nearer smg range than rifle cartridge range.

Announcing of the new explanations of gas system means that thinking is still in him - which could be good if it is really now perfected system, but chances are slim.
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  backbencher

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Posted: June 27 2017 at 11:40pm | IP Logged Quote backbencher

Tanner Frisby wrote:


backbencher wrote:

Tanner, if I remember aright, your "gas-delayed" blowback 7.62x39mm had similar issues to the original Nazi "gas-delayed" blowback 7.92x33mm rifle in the 2nd WW, namely the gas port was WAY too far forward to provide an effective delay, unlike the HK P7 or Walther CCP.  Fortunately, the German rifle had a sufficiently heavy bolt that it functioned as a mass-delayed blowback - yours did not have a heavy enough bolt, and as it was a bullpup, you almost ended your engineering career precipitously early.  It beat itself to death b/c the bolt was WAY too light.  Post the weight only as a cautionary tale.


You are absolutely right about my lack of safety when I test fired my build, but I have to disagree with almost everything else in the quote.

I will make a thorough write up on how the gas delayed system works and how to design an action around gas delay, with a few theoretical examples worked out. I'll try to write and post this in about a week.


I've always been interested in the "VG 1-5", so I will let you argue w/ Gun Jesus:





http://www.full30.com/video/8cf42b984258db76bb96eb123aa52591
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  justin22885

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Posted: June 28 2017 at 2:58pm | IP Logged Quote justin22885

while the guy from forgotten weapons claims it operates as a straight blowback, id be hesitant to believe that to be true as the extraction should be too fast with too high of pressure as i doubt the bolt weighs anywhere near sufficient enough to operate as a blowback, granted the orions hammer calculations are grossly inaccurate

im wondering if the action actually begins to open, but is caught and stopped when the gas finally does reach the forward piston?.. like it begins extracting too fast, but is stopped in the process eventually when the bullet breaches the gas port

all indications suggest the VG1-5 should fail as a design, but it doesnt so it must be doing something
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  backbencher

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Posted: June 29 2017 at 11:49am | IP Logged Quote backbencher

Ian's actually a member here, though I don't think he's visited in some time.  My apologies for the autostart of the video, that appears to be a "feature" of Full30.  I am pleased I could get Full30 embedded, however, it's better than clicks going to the liberals at YouTube.

The movement of the bolt, however slow, begins as soon as the bullet begins to move.  Someone in the comments on one of the videos posited that the gas released acts as a buffer spring, preventing the bolt from slamming against the stops.

GunLab looks to be producing a few:  http://gunlab.net/vg1-5-preorder-now-available - whether they come to fruition is anyone's guess.  If someone wanted to risk their $4000 reproduction by plugging the gas ports and testing, after 72 years we would know exactly.

I'm not positive, but I expect by the time the "VG 1-5" was born all the 7.92x33mm ammo available was in steel cases; w/ the relatively strong rim and the flutes in the chamber, extraction might be rather early but still work.

I had thought this footage was in the top video, but I'll include it for clarity - you can see the gas vents on the bbl don't remain under the gas chamber for very long:


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  backbencher

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Posted: June 29 2017 at 12:04pm | IP Logged Quote backbencher

Just to be clear - it's not that I don't believe in gas delay as a viable delayed blowback mechanism.  Clearly the HK P7 mastered it, as did the Steyr and now the little Walther.  My point is the ports of the "VG 1-5" are too far forward to work effectively.

If Tanner can demonstrate mathematically that I'm incorrect, and that the Nazi engineers perfectly placed the ports for the "VG 1-5", I'll be happy to shut up & listen.
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  justin22885

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Posted: June 29 2017 at 3:09pm | IP Logged Quote justin22885

i just watched the second video, it is exactly as i thought it was with that gas port acting as a gas buffer, not necessarily a bad idea in an otherwise blowback weapon.. and 6lbs mass is actually enough for 7.62x39 so id imagine the almost identical 7.92x33 would be fine with that as well...

that aint bad when you consider the VG5-1 was lighter than a thompson SMG... much greater power and range working on the same unlocked breach blowback type of action and weighed less than a pistol round SMG
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  Tanner Frisby

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Posted: June 29 2017 at 7:29pm | IP Logged Quote Tanner Frisby

Here is my write up:

Gas Delayed Blowback

My build had the port hole drilled as close to the brass (or steel in my case) as possible. The original gas port was plugged with a set screw.

My build's failure was due to three distinctly different issues.

1. The leather o-ring which was recommended to me, and which I agreed with (I'm not placing blame on anyone else for this one), did not survive the first few rounds. After that, it was effectively non-delayed.

2. The low carbon sheet steel bolt plastically deformed at the breech. It might have been robust enough for normal blowback, but was overloaded when partially "locked" forward.

3. The load paths we're very poorly designed. It was disadvantaged from the beginning.



I agree with backbencher's assessment of the vg1-5.
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5 most recent builds as of this post:

.380 Blow Back
Vetterli Rifle
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7.62 x39 v1
Mosin Nagant

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  backbencher

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Posted: June 30 2017 at 8:03am | IP Logged Quote backbencher

Tanner Frisby wrote:
Here is my write up:

Gas Delayed Blowback

My build had the port hole drilled as close to the brass (or steel in my case) as possible. The original gas port was plugged with a set screw.

My build's failure was due to three distinctly different issues.

1. The leather o-ring which was recommended to me, and which I agreed with (I'm not placing blame on anyone else for this one), did not survive the first few rounds. After that, it was effectively non-delayed.

2. The low carbon sheet steel bolt plastically deformed at the breech. It might have been robust enough for normal blowback, but was overloaded when partially "locked" forward.

3. The load paths we're very poorly designed. It was disadvantaged from the beginning.

I agree with backbencher's assessment of the vg1-5.


My apologies, Tanner.  I had completely forgotten you had plugged the original gas port and drilled one close to the chamber.  I remembered you were using an SKS bbl @ the time.

ETA:  Excellent write up.  You might change the term "main spring" to "recoil spring", as typically the main spring activates the hammer  or striker - a recoil spring being a relatively new innovation in firearms.
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  ds1948

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Posted: June 30 2017 at 8:42am | IP Logged Quote ds1948

Great write up tanner. I agree with your analysis though ponder if the velocity is really impacted that much. Does the velocity of a gas operated rifle differ that much with a bolt action? Either way its too bad 7.92 kurz doesn't have an easy to find pressure vs. time curve. Otherwise we could compare expected from predicted values. Also does anyone know the weight of an gustloff sturmgewhr bolt? Ive heard 6 pounds and 3. The Russian documents ive read suggest about three but Ian and others have said 6. 6 just seems to high and my math suggests 3 but who wants to argue with gun Jesus? Does anyone know chucks he runs gun lab email? He might be able to settle this debate
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  backbencher

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Posted: June 30 2017 at 9:18am | IP Logged Quote backbencher

You can contact Ian directly via his website, I believe.  He is a member here but very doubtful he checks his PMs.
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  Tanner Frisby

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Posted: June 30 2017 at 10:05am | IP Logged Quote Tanner Frisby

ds1948 wrote:
I agree with your analysis though ponder if the velocity is really impacted that much.


For velocity calculations, I took a trapezoid rule definite integral of the pressure vs distance curves and compared them 1:1 to the original blowback curve. The pressure is acting on a constant cross sectional area (bullet circumferential area) and can be converted to a force. Force equals derivative of momentum with respect to time (commonly written as Force = Mass x Acceleration for constant mass bodies), The integral of force should equal Mass x Velocity.

Ignoring a lot of constants (I am comparing relative percentages instead of absolute figures), the trapazoid rule definite integral of the pressure vs distance curve is taken to be equal to a constant scalar of final bullet velocity. The only potential catch is that I am integrating a pressure vs DISTANCE curve instead of a pressure vs TIME curve. Converting distance into time domain would be more rigorous. I'm not saying that my numbers are correct, but I think they are at least close.

I modified my chart to simulate a 9mm carbine with a .15" diameter piston, 1.5" bolt recoil (similar to the HK P7) and a port location at max pressure. I got 92% bullet velocity and 85% Bolt Weight requirement.

My spreadsheet definitely has its limitations, but think of it similar to Orion's Hammer's Blowback article.

ds1948 wrote:
Does the velocity of a gas operated rifle differ that much with a bolt action?


No, they are about the same. Everything else being equal (barrel length, loading, etc) the bolt action rifle will have higher velocities. The gas operated rifle is a bolt action rifle until a small amount of it's pressurized gas is siphoned off a few inches from the end of its muzzle. The pressure will drop a little and not accelerate the bullet as fast as it was previously, but almost all of the bullet's velocity has been achieved by this point. It might be analogous to removing 1" of barrel length between two otherwise identical bolt action rifles.

Tanner Frisby

P.S. @backbencher I tried to respond to your PM. I'm not sure if it sent successfully. If not, send me an email and I'll respond that way.
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5 most recent builds as of this post:

.380 Blow Back
Vetterli Rifle
7.62 x39 v2
7.62 x39 v1
Mosin Nagant

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  ds1948

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Posted: July 04 2017 at 6:06pm | IP Logged Quote ds1948

Thanks for the indeoth answer. I wasnt questioning your math i was just surprised by the results. I'm glad we are getting some. more math in this thread
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