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Gas ports... uh oh.


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I'll try to be brief. I went to the store to find a .076 drill bit to open up my (four) ports. All they had was .098 and .059. I bought them both anyways. Went to my friends house to find the correct size bit. His dad had a massive drill bit box from the smallest to the largest, and I grabbed a .076 and .073.

 

Just drilled two holes and thought they looked rather large. I took my .098 bit that I had, and it fit in fine. So, his drill bit collection was mislabeled...

 

What would you do at this point? I've got two drilled at .098 and two that are in the .065 range.

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Invest in a dial caliper...

 

You can find some decent inexpensive ones around, and they come in handy for all sorts of uses besides firearms.

 

As for the ports... I would think they will be fine with two slightly oversized, and two slightly undersized.

 

Do the math - .076 X 4 = .304" of area

 

.098 X 2 = .196"

.065 X 2 = .13"

.196 + .13 = .326" of area

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Put your gas block back in and go test. If it needs any more, only increase the small ones.

 

It might gunk up more than others. If it were mine and it gunked up more often, I would be disapointed, but live with it. You still ought to be able to shoot several hundred rounds between cleaning. That's better than many popular duck guns. Cleaning the gas block only takes a minute or so anyway. If it is so bad that it plugs up at the range, I think I would also get an autoplug or adjustable gas plug that I could quickly remove by hand, and keep a piece of wire with the gun to poke into the ports. Autoplugs also can theoretically help to vent some of that gunk outside the gas block.

 

If it is getting so much gas that the 3 dram loads run solidly on setting #1 with the stock plug, then it is a bit overgassed. Not too big a deal to solve though. An adjustible plug or autoplug will deal with that.

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FYI, I've picked up a digital caliper off of Amazon for ~$7 and it works great. It is also handy because it can read in metric or imperial. Incredibly handy tool. It must be good enough for the big boys, because I saw the exact same model and brand in several pictures that Tromix uploaded for prototypes a while ago. The point is that while some cheap crap is cheap crap, some of it is pretty good.

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FYI, I've picked up a digital caliper off of Amazon for ~$7 and it works great. It is also handy because it can read in metric or imperial. Incredibly handy tool. It must be good enough for the big boys, because I saw the exact same model and brand in several pictures that Tromix uploaded for prototypes a while ago. The point is that while some cheap crap is cheap crap, some of it is pretty good.

 

Make? Model?

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Put your gas block back in and go test. If it needs any more, only increase the small ones.

 

It might gunk up more than others. If it were mine and it gunked up more often, I would be disapointed, but live with it. You still ought to be able to shoot several hundred rounds between cleaning. That's better than many popular duck guns. Cleaning the gas block only takes a minute or so anyway. If it is so bad that it plugs up at the range, I think I would also get an autoplug or adjustable gas plug that I could quickly remove by hand, and keep a piece of wire with the gun to poke into the ports. Autoplugs also can theoretically help to vent some of that gunk outside the gas block.

 

If it is getting so much gas that the 3 dram loads run solidly on setting #1 with the stock plug, then it is a bit overgassed. Not too big a deal to solve though. An adjustible plug or autoplug will deal with that.

 

I just put it all back together and am heading to test shoot tomorrow. I'm not too concerned about it gunking up. I always clean it after every time I shoot. I don't put much more than 150 shots per range trip.

 

Thanks for the input. I don't know why it's so hard to find odd sized drill bits locally. No luck at Lowes or Home Depot.

 

Invest in a dial caliper...

 

You can find some decent inexpensive ones around, and they come in handy for all sorts of uses besides firearms.

 

As for the ports... I would think they will be fine with two slightly oversized, and two slightly undersized.

 

Do the math - .076 X 4 = .304" of area

 

.098 X 2 = .196"

.065 X 2 = .13"

.196 + .13 = .326" of area

 

I did think of that, just wasn't sure of the downside of not having them all equal sized. Guess it'll just gunk up more.

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Staggered port sizes will have no effect on function.

 

I agree with Gunfun, go run it and see how it acts. My money says with those two large ports having a very large cross sectional area, it will run anything you put thru it. Check your carrier and rear trunnion for excessive contact (damage). Inspect it before you shoot so you know if it has been damaged or changed/deformed any, after you fire it.

Edited by Mullet Man
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As for finding odd size bits, go to an industrial supply. Every town has one. The numbered and lettered bits fall in between the sizes by thousandths or fractions. Then there is metric. Between all of those choices you can always find about what you want. Since you are really looking for "a little bit more, but not too much more." exact size isn't critical. i.e if someone have 4 ports that fit .065" bit, and they are not quite running 3 dram ammo, they could go buy 3 or so bits in steps so that they can start step #1 and go test and stop if it is good, or go home and step up to the next size as needed.

 

In such case, it hardly matters whether your first step is 0.0775" or 0.0780"... How steady your hand is, or how balanced your drill press is, probably makes a bigger difference than the measured size of the bit. The next step could be something near 0.080" and so forth. the bits should be less than a dollar each, so I just bought a series of bits in steps of about 0.002" apart from my base line through .093" I think it was less than $6 for all of them, and I have used the bits fr other projects too. For 4 ports, the concensus is that .078" is a good starting point and .093" is as large as you want to go. I am not sure exactly where the base line size would be for a 5 port build, but there is no harm in starting small other than work and trips to the range.

Edited by GunFun
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FYI, I am mostly filling in info for other people in similar binds.

 

It was this model, but I had found it cheaper. Often when I find something I want on amazon, I search by the model number through amazon and also by google and find the same thing cheaper and often on amazon. They don't always know what they have. The same item could be listed under industrial tools, hobby supplies.... and their search engine would think of them as unrelated items. I am by no means saying this is the best or only brand to buy, and if I worked as a machinist, I would probably want to have a starret caliper and micrometer for the critical measurements, but I have found that this is good for many basic machining projects and reloading, and the gauge is less fragile than dials. I would be willing to bet that the same thing is sold under many different brand names. http://www.amazon.com/Neiko-01407A-Extra-Large-SAE-Metric-Conversion/dp/B000GSLKIW/ref=sr_1_1?ie=UTF8&qid=1365894798&sr=8-1&keywords=digital+caliper (Another perk of having a cheapo in your tool apron for the every day jobs, is if you drop your cheapo on concrete or something, it is N.B.D. I would be having a bad day if I dinged a $150 Starrett measuring tool.)

 

A note on the math. The sum of port area is good as a rule of thumb, but remember that fluid dynamics are not straight forward you have laminar flow issues and so forth. There is not a simple linear relationship of area to flow. For example a pipe with twice the area of i.d. of a smaller pipe has more than twice the flow of the smaller pipe. So the simple math up there should give you an idea if you are in the right neighborhood, but it is not exact. I am not smart enough to calculate exactly what size for 5 ports would equal the flow of 4 ports at .078", especially without knowing viscocity and pressure, but I can use this basic math to calculate the surface of 4 at .093" and consider that to be a "do not exceed" number.

Edited by GunFun
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Invest in a dial caliper...

 

You can find some decent inexpensive ones around, and they come in handy for all sorts of uses besides firearms.

 

As for the ports... I would think they will be fine with two slightly oversized, and two slightly undersized.

 

Do the math - .076 X 4 = .304" of area

 

.098 X 2 = .196"

.065 X 2 = .13"

.196 + .13 = .326" of area

It is true that you could calculate the math, but that math is off. To find the area of the circle, you have to use the formula is π R ²

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Thanks GunFun you always have really good responses. Last stupid question, what should I do about the burring from the two new holes? Someone on here said just to fire a few slugs.

 

@ Mullet man: I hope you're right. First I uncovered the gas ports (three were half covered by the gas block). That slightly improved performance. This is my second attempt at making this S12 run better. I'll take pictures of the trunnion/carrier before I go out there.

 

 

The cheap ammo I had before that failed 100% is: Winchester Universal 1-1/8oz #8 shot (1200fps), Remington Game Loads 1oz #7-1/2 shot (1290 fps), Winchester Super-Target 1-1/8oz #8 shot (1200fps). #7 of the Super Target failed 5% of the time. I'll be testing it Monday and will make a post with pictures of my conversion and my results.

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Burring: use a stiff cleaning brush. the Cyclone style are good for that if I got the name right. Odds are that shooting any ammo through there will clean them out too. I doubt that the slug will take much off that a standard plastic shot cup wouldn't. Some people say to put polishing compound or some other abrasive onto a cleaning rod and spin it with a drill. I am not inclined to do this to my chrome when a brush or some casual use will accomplish the same task.

 

 

The ammo you listed is all 3 dram , the standard base line, so that is a good testing choice. The Win AA Super target is better quality stuff. You didn't list the weight of the remington ammo, but it should be enough to run in a properly tuned saiga as long as the weight is heavier than aprox 3/4+ oz. See here:http://i236.photobucket.com/albums/ff176/_DMC/DEP12.jpg. Win Universal is known for very poor QC and most likely using whatever powder is handy at the time it is not very consistent, I suspect it is somewhat lower velocity than labeled, based on it's performance compared to other ammo of the same rating. However, it is possible that it develops the same stats at a slightly lower pressure due to slower burning powders. I still want my gun to run it, since there is a good chance a friend will bring it to the range. Plus I just want to maximize the versatility of this extremely versatile platform.

 

 

 

I recommend saving that chart for reference when you purchase or reload ammo.

 

 

And good catch Mr. Pedal. I assumed the math was good without even checking the formulas.

Edited by GunFun
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FYI, I've picked up a digital caliper off of Amazon for ~$7 and it works great. It is also handy because it can read in metric or imperial. Incredibly handy tool. It must be good enough for the big boys, because I saw the exact same model and brand in several pictures that Tromix uploaded for prototypes a while ago. The point is that while some cheap crap is cheap crap, some of it is pretty good.

 

Make? Model?

Harbor Freight has some decent cheapo calipers: http://www.harborfreight.com/4-inch-digital-caliper-47256.html

 

These are featured quite often in their monthly/weekly ads with a coupon that knocks them down to under $10.

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Totally his port area would be the same as if he had one port drilled 0.166".

 

That's not too bad considering my Ithaca Model 51 has two ports big enough to easily run a pipe cleaner through, and it's piston is larger diameter than the S12.

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That's really not that bad. I have a factory 4 port I opened to .093" and it ran great there but I plan to cut 3.125" off the barrel so I added a 5th at .078" to be proactive and, with the Auto Plug, I still have no abnormal wear in my gun and it still shoots the same as far as I can tell. It does get a bit dirtier but I have the time to clean my guns after every range session so it's a non issue. I'm sure I could still fire over 590 rounds without it failing because of being dirty. Probably over 1000 to be honest.

 

Burring: use a stiff cleaning brush. the Cyclone style are good for that if I got the name right. Odds are that shooting any ammo through there will clean them out too. I doubt that the slug will take much off that a standard plastic shot cup wouldn't. Some people say to put polishing compound or some other abrasive onto a cleaning rod and spin it with a drill. I am not inclined to do this to my chrome when a brush or some casual use will accomplish the same task. The ammo you listed is all 3 dram , the standard base line, so that is a good testing choice. The Win AA Super target is better quality stuff. You didn't list the weight of the remington ammo, but it should be enough to run in a properly tuned saiga as long as the weight is heavier than aprox 3/4+ oz. See here:http://i236.photobucket.com/albums/ff176/_DMC/DEP12.jpg. Win Universal is known for very poor QC and most likely using whatever powder is handy at the time it is not very consistent, I suspect it is somewhat lower velocity than labeled, based on it's performance compared to other ammo of the same rating. However, it is possible that it develops the same stats at a slightly lower pressure due to slower burning powders. I still want my gun to run it, since there is a good chance a friend will bring it to the range. Plus I just want to maximize the versatility of this extremely versatile platform. I recommend saving that chart for reference when you purchase or reload ammo. And good catch Mr. Pedal. I assumed the math was good without even checking the formulas.
Just shoot the gun, that'll clean the burrs off the easiest.
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Totally his port area would be the same as if he had one port drilled 0.166".

 

That's not too bad considering my Ithaca Model 51 has two ports big enough to easily run a pipe cleaner through, and it's piston is larger diameter than the S12.

 

Yeah, but where are those ports located and how much mass are they moving? apples and oranges.

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I'll try to be brief. I went to the store to find a .076 drill bit to open up my (four) ports. All they had was .098 and .059. I bought them both anyways. Went to my friends house to find the correct size bit. His dad had a massive drill bit box from the smallest to the largest, and I grabbed a .076 and .073.

 

Just drilled two holes and thought they looked rather large. I took my .098 bit that I had, and it fit in fine. So, his drill bit collection was mislabeled...

 

What would you do at this point? I've got two drilled at .098 and two that are in the .065 range.

You'll probably be fine. I over drilled a port on my last s-12 intentionally. The newer 030's have a "self regulating" system which just means over gassed. Get a recoil buffer and don't forget to switch to proper settings.

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There are very real and appreciable differences between two guns on the same assembly line, much less two different weapon designs. The difference between resistences met by saiga gas system is also different. Each Saiga is a unique, albeit similar, system.

 

Port sizing is an art. Very complex math may get you close, but it comes down to guess-and-check in the end.

 

*OP - Do as Gunfun suggests.

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I had decent results but not sure where to go from here. The Remington target loads ran 100% (except when my skinny friend handled it like a bitch). The winchester universal and #8 super target jammed so hard it i had to use both hands to pry it out. The Remington would FTF every time before I opened the two holes though, so that's a decent improvement. It is also about $1.50 cheaper than what I usually run.

 

What I don't understand is how the #7 Winchester super target runs 100%, then the #8 fails 100%. Same brand, 1-1/8oz, 3 dram, 1200 FPS.

 

Thanks for the replies dubya, S12KS-K and Nephilim7. Slowly but surely I'm getting where I wanna be. I''ll just incrementally drill the other two larger, probably to .075ish and then go test again.

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I had decent results but not sure where to go from here.

 

Start with the DRAM chart like GunFun suggested in post #13. It may give you the answers on why your shotgun doesn't like what you're feeding it.

 

Second, invest in a digital caliper.

 

Third, figure out the total area of your open ports and compare it to what it should be. Are you over or under gassed? This calculation should tell you that information.

 

A = π × r2

 

A = Area

pi = 3.14159

r = radius

Edited by HighPlainsDrifter
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