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Old November 25, 2008, 02:28 PM   #1
eth77
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vz 58 custom barrel questions

I'm thinking of changing to a fluted barrel on a vz 58. I'd purchase a barrel with a gas port drilled in it. So the questions:

1. Is there some kind of guideline for the slant of the gas port into the barrel - say 45 or 60 deg angle?
2. If I'm going to go from a 16" barrel to 20", should I keep the same port point or move it to keep the relative delay before gas piston actuation?
3. Any ideas about piston configuration, since it becomes a free variable wrt shape, concavity, etc.
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Old November 25, 2008, 07:56 PM   #2
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The main purpose of gas vents in a rifle barrel are to drop the pressure behind the bullet just before it exits the muzzle. Any pressure remaining when the bullet exits the muzzle accelerates the propellant gas into a jet that adds its impulse to the recoil of accelerating the bullet. It is called rocket effect, and in high power rifles it is a very significant portion of the total recoil.

Opening vents at the muzzle bleeds off the pressure rocket effect needs to work. This is why a multitude of holes that are uniformly drilled all around a muzzle brake and venting perpendicular to the barrel axis will reduce recoil; it does not depend on having the holes slanted or pushing the muzzle down to counter a recoil moment. For accuracy, an even surround of holes has the jetting gas ports neutralize each other's push, and thus avoids contributing to barrel vibration. It is, however, louder for the shooter than vents that slant forward.

Vents that slant forward still produce some rearward thrust. Slanting forward is a compromise, therefore, and you have to make a personal choice about where the balance lies for you? Other slots are cut only on top to help thrust the muzzle down against recoil. Whether that helps more than it adds to barrel deflection and vibration is another personal call. Certainly, in short, rigid guns, such as handguns, there isn't enough barrel length to vibrate all that much, so it usually works out well in that application.

You could talk to the Magnaport people about it.
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Last edited by Unclenick; November 25, 2008 at 08:03 PM.
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Old November 25, 2008, 09:46 PM   #3
eth77
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Ooops not asking about a compensator

I'm interested in the port to operate the gas piston which in turn drives the bolt back on a gas piston operated action like that of the vz 58.

I'm sure that the Czech designer had a computation that he used when he designed the weapon, I'd love to know it, but failing that I thought that there might be a knowledge base in this group which could help.
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Old November 26, 2008, 09:43 AM   #4
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Ahh, Sorry about that. It's unusual for someone to inquire about that level of detail on gun design.

All gas ports are a compromise because the powders and bullet weights and therefore the pressure and pressure dwell times on the barrel side of the port have some range. The gun has to run with the lowest port pressure impulse and not be damaged by the highest. So, the first thing you would need to know is what the barrel pressure at the port location is over the time between when the bullet base passes over it and when the bullet exits, and to know that for the full range of loads expected to operate the gun properly?

The next thing you would need to know is how much force needs to be applied over what period of time to do the work of running the operating mechanism through a cycle? The gas available will start out entering the port fast and wind down after the bullet exits and gas starts to bleed backward through the gas port (assuming no valve is present as the M14 cylinder creates)? In the case of gas impingement systems, how much force is needed to impart enough inertia to compress the springs and still drive the bolt back all the way? That's your starting data.

Next you need to calculate the area the gas pressure works against so you can divide it into the required force to find the pressure needed to work the mechanism. Once you know how much pressure you need, calculate the volume of gas needed to create that pressure in the volume of your gas system.

Once you know how much gas you need, you can calculate the size of the hole. Treat it as a short pipe. Calculating the gas flow through the port isn't too bad with all the on-line calculators available. This one may help? You will have to guess at gas temperature here.

The easiest thing, if you just want to replace a barrel, is to buy a set of wire gauges and measure the existing gas port diameter. Drill something a bit smaller initially to see if it works OK with your ammo? Buy a set of number drills so you can enlarge it in small steps from there.
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Last edited by Unclenick; November 26, 2008 at 09:49 AM.
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Old November 26, 2008, 05:32 PM   #5
eth77
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Many thanks for the info!

That's what I was looking for, but actually wanting to avoid calculating.

I figure that if I have to do it, I'll look up the pressures for some reloads on AmmoGuide Interactive. That will give me some gas pressures that I can work with. What I won't know, is where the bullet was when that pressure was reached. Or is that the pressure as the bullet exits the barrel?

Next I need to calculate the volume of gas I'm going to need to tap. start with the min volume for the tube - put a piston into a gas adapter after filling the adapter with water, then measure the remaining water after the piston hits bottom. That gives me my starting point, then measure the piston chamber volume just as the piston reaches the bleed ports - that's my max volume.

I'm going to guess that the port sized was picked so that the weakest load would cycle the action, thus a good start would be a light bullet at relatively low velocity.

But I really want to just use an equivalent of the current gas adapter on a port which would be the same size and 1" farther forward. I need a new adapter, as the barrel will be 0.800" and the current barrel is 0.765" in diameter. I'll also need a new piston, longer by 1". While I'm at it, I'll make it a smidgen sturdier - make the shaft between the two ends slightly thicker. This will make it heavier, so it may take more gas, but that can be finessed with a slightly larger port.
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Old November 27, 2008, 12:20 PM   #6
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The pressures given for loads are their peak pressures. That happens when the bullet is only one or two inches down the barrel. How high it will be when the bullet base passes over the gas port depends on the powder and bullet combination used and on how far down the barrel the port is. For charges that produce the same peak pressure, a slower powder will create higher port pressure while a fast powder will create a lower one. I think your best bet is to start with the undersize gas port and open it by incrementing the drill sizes using the load you want to shoot routinely in the gun. That lets you start with choosing the bullet and rate of twist and powder rather than adapting those choices to the gas port. It also lets you minimize the gas port so the gun has the least violent operation and drops the cases nearby rather than trying to launch them into orbit.

Something I've done with the Garand is disable the gas system and develop a load in single-shot mode, running the op-rod manually. That let's me find what the chamber and barrel like before involving the gas system disturbances. You might want to try the same thing, setting your barrel up with no gas port at all at first. Develop some candidate loads, then start drilling?

Nick
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Last edited by Unclenick; November 29, 2008 at 10:57 AM. Reason: typo correction and clarification
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Old November 28, 2008, 12:36 PM   #7
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Thanks for an excellent idea. This should make things much easier.
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Old November 29, 2008, 11:31 AM   #8
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I'd like to know how that works out for you? PM me when you get done, if you remember to?

I took a quick look at Enco's site and found they had stock on one reasonably priced jobber length (the short, stronger size) high speed drill set here. That set includes #60-1 (0.040" through 0.228"), plus all the letter drill sizes and has a set of common fractional sizes in it, to boot. If you needed drills still smaller, I believe you can still get a #80-61 (0.0135" - 0.0390") set from Exacto, but those will probably all be too small. My thought is just to use the drills themselves as the hole gauges to find the existing gas port size and start at maybe 2/3 that diameter (half the area) and work up?
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