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bulletsponge13
Apr 28, 2010


Yeah, that math went way, WAY over my dumbass head. I just figured you would do like I would "start small, move up", which is a terrible, terrible idea. In reality, I would just pay someone ten dollars to do the math for me.

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shalafi4
Feb 20, 2011

another medical bills avatar

bulletsponge13 posted:

Yeah, that math went way, WAY over my dumbass head. I just figured you would do like I would "start small, move up", which is a terrible, terrible idea. In reality, I would just pay someone ten dollars to do the math for me.

I kind of ruled out stepping up the sized a little bit at a time because that would require pressing pins in and out to remove the gas block each time

Doctor Grape Ape
Aug 26, 2005

Dammit Doc, I just bought this for you 3 months ago. Try and keep it around for a bit longer this time.


shalafi4 posted:

I kind of ruled out stepping up the sized a little bit at a time because that would require pressing pins in and out to remove the gas block each time

Could just make a second gas block with a large threaded hole on the top to give you access to the port. Once you find the right size hole then swap in the final GB.

shalafi4
Feb 20, 2011

another medical bills avatar

Doctor Grape Ape posted:

Could just make a second gas block with a large threaded hole on the top to give you access to the port. Once you find the right size hole then swap in the final GB.

That's true, I hadn't thought of that. The blocks I have are probably way too thin to modify for that though :/

Flatland Crusoe
Jan 12, 2011

Great White Hunter
Master Race

Let me explain why I'm better than you


Doctor Grape Ape posted:

Could just make a second gas block with a large threaded hole on the top to give you access to the port. Once you find the right size hole then swap in the final GB.

Yeah this definitely independently came to mind for me.

I would approach it as doing simple proportional calculations. Running a computer simulation with CFD software if I had access to a student computer lab. And ultimately just using those numbers to start small and open it up on the prototype. I'm not super familiar with AK's but I am a MechE PE and that's how I would do it. I have a friend from school who is an engineer in the firearms industry and a lot of what they do with designing on existing platforms involves a lot trial and error plus mag dumping for reliability statistics.

Flatland Crusoe fucked around with this message at 14:36 on Dec 24, 2016

shalafi4
Feb 20, 2011

another medical bills avatar

Flatland Crusoe posted:

I would approach it as doing simple proportional calculations. I have a friend from school who is an engineer in the firearms industry and a lot of what they do with designing on existing platforms involves a lot trial and error plus mag dumping for reliability statistics.

Check out the effort post towards the bottom of the last page. There's a bunch of handwaving involved but that's what I walk through. Honestly as simple as the proportion calculations are I should just setup a spreadsheet to calculate ranges of them.

Flatland Crusoe
Jan 12, 2011

Great White Hunter
Master Race

Let me explain why I'm better than you


shalafi4 posted:

Check out the effort post towards the bottom of the last page. There's a bunch of handwaving involved but that's what I walk through. Honestly as simple as the proportion calculations are I should just setup a spreadsheet to calculate ranges of them.

No I saw those calcs but I'm saying that my experience is that hand calcs are going to get you an order or magnitude calculation compared to needing a 3mm vs 4mm hole and that any sort of more complex modeling is both easier and more accurate. Hand calcs tend to minimize variables a lot of little variables that add up to mattering a lot when talking about expanding gases and varying combustion rates that need to then expand around (2) 90 degree corners. The surface of the barrel and the clean of the gas hole are going to net different results that saying clean 4140 steel into a hole that has a less that perfect radius (or lack there of) because those things affect pressure and flow of gases which will be dynamic during the acceleration of the gas piston.

A great example would the physics of motion used in classical physics vs control systems where acceleration includes the inertia of rotation and acceleration rate and air resistance or large multiple variable equations.

Fluid dynamics which you are dealing with combustion of powder really are much better at a small level using computer simulations than hand calcs.

At the end of the day you are going to play around with the hole size unless you pour the resources of nasa at the issue.

It's one of those issues where I calculate the spring size I think I need and then order 2 sizes harder and weaker to try in addition to exactly what I think I need.

Shaocaholica
Oct 29, 2002

Fig. 5E


And then the weakest/strongest still work making you rethink your calc.

Flatland Crusoe
Jan 12, 2011

Great White Hunter
Master Race

Let me explain why I'm better than you


As much as anything I'm telling you applied engineering largely revolves around adapting known or similar solutions (often in unrelated applications) rather that what engineering professors drill into your head in trying to learn to derive everything from scratch basic equations. Quick load or similar software would probably be the most useful if you are using different powders or situations.

People spend whole careers learning the nuances of specific systems and tapping that knowledge versus deriving the theory of everything is much more efficient and also has more simplified ratios and assumptions. I'm usually going to make machine design decisions based on manufacturers literature instead of rereading Shigley's machine design.

In the case of a drilled hole it's easier to make a hole bigger than smaller. So start on the side of undergassed and work up. Cartridge pressure, powder volume and barrel cross sectional area are the only variables I would consider not to be held constant.

Shaocaholica
Oct 29, 2002

Fig. 5E


Why not just make the hole towards the big end of hole sizes and make your gas block adjustable? If you're going to be loving around with different loads and barrel lengths and suppressors anyway.

Flatland Crusoe
Jan 12, 2011

Great White Hunter
Master Race

Let me explain why I'm better than you


Shaocaholica posted:

Why not just make the hole towards the big end of hole sizes and make your gas block adjustable? If you're going to be loving around with different loads and barrel lengths and suppressors anyway.

Because in the long run over gassed semiautos break stocks and batter receivers.

If you are going to sell the barrels optimizing the gas port should be a thing you have figured out at least to some extent.

darnon
Nov 8, 2009


You could do something like the Govnah gas block for testing and have a plate with different size gas ports you slide through.

Javid
Oct 21, 2004

My sole partiality is to that delectable spiced meat. Any additional confederation of vegetables shall not compromise the pie as I see it.

You could also hit up these guys and see what size port they used.

MrBusiness
Jan 19, 2016


The ideal gas law also makes the assumption that the actual gas molecules have such a minuscule volume compared to the container that it can be effectively ignored. This holds for pressures around atmospheric where the mean distance between gas molecules is on the order of 30 angstroms (or about fifteen times the axial length of a Nitrogen diatom) but at very high pressures the molecules are much closer together and the amount of volume they occupy will be significant.

This is handled by subtracting a correction term (usually b) from the volume,
P(V-b)=nRT
so
P1(V1-b) = P2(V2-b)

This won't influence the V2 portion of the equation much because the gas will have expanded to the point where the correction term is insignificant again but a lot of the initial chamber pressure probably comes from the fact that all of that powder that was taking up free volume in the cartridge is now a gas which takes up roughly the same amount of free volume in the cartridge (P1 is large because V1-b is small and nRT is constant). I wouldn't be surprised if the b term reduced the effective initial volume by as much as a half (and so increased the initial pressure by a factor of 2). Of course, this all gets a lot more complicated when you factor in burn rates and supersonic fluid mechanics, but the takeaway is that the final pressure will likely be considerably lower than you're predicting using the ideal gas law, meaning you'll need a larger gas port.

I'd suggest what others have, figure out some way of doing it by enlarging the hole incrementally from a best-guess starting point.

Edit: the sensitivity of chamber pressure to things like powder compression and bullet seating depth is strong evidence for the idea that b takes a significant chunk out of V1 in firearms.

Here are some PV curves for the ideal gas law (purple), b = V1/4 (orange) and b = V1/2 (green). The red vertical line is your unadjusted cartridge volume and the black line is your barrel volume at the gas port.

MrBusiness fucked around with this message at 20:45 on Dec 27, 2016

Shaocaholica
Oct 29, 2002

Fig. 5E


Flatland Crusoe posted:

Because in the long run over gassed semiautos break stocks and batter receivers.

If you are going to sell the barrels optimizing the gas port should be a thing you have figured out at least to some extent.

Just trying to save the effort of taking a gas block off and putting back on for each test iteration. Not suggesting he sell barrels with an oversized port. This would strictly be for testing with an adjustable gas block..

shalafi4
Feb 20, 2011

another medical bills avatar

MrBusiness posted:


Edit: the sensitivity of chamber pressure to things like powder compression and bullet seating depth is strong evidence for the idea that b takes a significant chunk out of V1 in firearms.


That's possible, however the powder compression and bullet seating depth affecting how the powder burns probably is significantly higher.



thread posted:

Ideal gas law doesnt work chat

Yup I'm well aware of the limitations using the ideal gas law and I know it breaks down BADLY at high pressures when you're trying to do direct calculations. That's why I'm planning on taking several data points and using the same set of assumptions on both halves of the equations. Are the numbers for the pressures wrong? probably. however if they're all off the same way it should put it in the ball park.

Just out of curiosity I ran the numbers assuming I'm going to a 300 blk from a 5.56 doing the same thing. Numbers are taken off this website. I know the gas systems between an AR and AK are totally different but comparing like systems should be similar.

http://www.tacticalmachining.com/learn/ar-style-rifles/ar-15-gas-port-sizes.html

I'm going with 16" barrel and carbine length systems. Cartridge pressure is off of wiki for a 5.56 (although the way these work I could use any pressure value and get the same result, it's just the assumptions would be even less valid)

5.56 ports ranging from 0.0625~0.065

P1 = 380 MPa
V1 = 1850 mm^3
P2 = ?
V2 = V1+ 4505mm^3

P2 = 107MPa

1.65mm ~0.065" so this is on the high side

F= P2*A
A= 2.137mm^2

F5.56 = 228.7


for a 300 case

P1 = 380MPa
V1 = 1575 mm^3
P2 = ?
V2 = V1 + 8546mm^3


P2 = 59.13

F = P2*A
A= F/P2

A= 228.7/59.13
A= 3.8677

A--> diameter 2.218mm or 0.0874 in

They have 300 blk ports ranging from 0.093~0.096" for the supersonic so that's not a huge difference


although the jump to ~0.12 for the subsonic ones is a much larger jump.

MrBusiness
Jan 19, 2016


So I guess the hard part would be collecting data on the port sizes of AKs in different calibers and gas system lengths.

If you get this working, put me down for an AK kit.

shalafi4
Feb 20, 2011

another medical bills avatar

Also I think I just solved my removing the gas block problem.


Undersized shoulder bolts exist

Shaocaholica
Oct 29, 2002

Fig. 5E


shalafi4 posted:

Also I think I just solved my removing the gas block problem.


Undersized shoulder bolts exist

Please to be diagram. No understand meaning.

shalafi4
Feb 20, 2011

another medical bills avatar

Shaocaholica posted:

Please to be diagram. No understand meaning.

Meet Mr. Dr. Shoulder bolt




So that ground surface is done like a precision pin They're typically on size but some are a few thousands of an inch undersized. Basically instead of pinning the gas block and front sight block on. I run a couple of undersized pins through and tighten a nut on the other side. Probably would rattle loose long term but for basic function testing it should hold up fine.

Shaocaholica
Oct 29, 2002

Fig. 5E


shalafi4 posted:

Meet Mr. Dr. Shoulder bolt




So that ground surface is done like a precision pin They're typically on size but some are a few thousands of an inch undersized. Basically instead of pinning the gas block and front sight block on. I run a couple of undersized pins through and tighten a nut on the other side. Probably would rattle loose long term but for basic function testing it should hold up fine.

If its just temporary, how is it any better than a standard bolt?

shalafi4
Feb 20, 2011

another medical bills avatar

Shaocaholica posted:

If its just temporary, how is it any better than a standard bolt?



Couple reasons.


First the shoulder takes all of the load. This would prevent the threads chewing the inside of the holes out (I'm looking at like 100~200 rounds at a minimum for a function test)

Second, normal bolts tend to have tolerances that are all over the place (comparatively) so the block may have a chance to slide slightly. This running start combined with the recoil coming down on the threads could be bad news bears.


aka I don't wanna take the chance to mess up my shiny gas block and barrel

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shalafi4
Feb 20, 2011

another medical bills avatar

Quick update all.


I'm hoping to get enough of a tax refund from Uncle Sam to be able to put the barrel order in. Anyone interested in getting in on it shoot me a PM or username @gmail.com. I'll be getting in touch with the barrel manufacturer to figure out how they would want to handle the group buy.

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