Port pressure in a M1a

[Bart B.]

Not trying to be a smartass at all or get into this pissing contest, but I would like to know how to mitigate overexpanded cases. Seriously.

I don't know. Maybe reduce the powder charge.

The 7.62 cases fired from Garands I've used will not drop back into the chamber. They stop half an inch or more short. Obviously larger body diameters.

 

[Bart B.]

The design engineers at Springfield armory in Massachusetts worked very hard on the gas system and operating dwell of the M14 to eliminate the abrupt operation of the M1.

Easily seen in the data I posted earlier. Check bolt unlock times and bullet positions down range.

Any link to these rifles full cycle time would help. I cannot find any. Might use M14 lock time and full auto rate to start which calculates to a .080 second cycle time at maximum 750 rpm rate. A .072 second reload cycle time plus .008 second lock time

 

[riflewoman]

I don't know. Maybe reduce the powder charge.

The 7.62 cases fired from Garands I've used will not drop back into the chamber. They stop half an inch or more short. Obviously larger body diameters.

It doesn’t take much expansion to do that. Even a Remington 742 will have that issue.

 

[Uncle Ed]

Not trying to be a smartass at all or get into this pissing contest, but I would like to know how to mitigate overexpanded cases. Seriously.

If you would have read the link I posted by Mr. Zediker he states the resized case should be .003 to .005 smaller in case body diameter than its fired diameter. This allows the case to spring back from the chamber walls and extract reliably. He also covers the powder burn rate and port pressure and the effect it has on extraction.

What he is saying is the higher the port pressure and case spring back from the chamber walls governs how hard the bolt yanks on the fired case. Meaning the case being stretched and the case shoulder being blown forward.

I have two of Mr. Zediker's books and I find them very informative and the link I posted explains how the ejected case can end up longer than the chamber. Meaning port pressure, the resized diameter of the case, brass hardness and brass spring back after sizing.

If you want more information then go to the m14forum.com reloading forum or thecmp.org forums. The m14 forums is where I first read about the ejected cases being longer than the chamber long before I bought the books by Mr. Zediker. But it gives meaning to the saying "The cartridge should fit the chamber like a rat turd in a violin case".

Below Forster explains their three different .308/7.62 resizing dies.

Its amazing what you can find to read on the main page of accurateshooter by Mr. Zediker.

Ogives, Meplats, Boat-Tails and Other Bullet Design Elements
http://bulletin.accurateshooter.com...-boat-tails-and-other-bullet-design-elements/

 

[cfreeman]

If you would have read the link I posted by Mr. Zediker he states the resized case should be .003 to .005 smaller in case body diameter than its fired diameter. This allows the case to spring back from the chamber walls and extract reliably. He also covers the powder burn rate and port pressure and the effect it has on extraction.

What he is saying is the higher the port pressure and case spring back from the chamber walls governs how hard the bolt yanks on the fired case. Meaning the case being stretched and the case shoulder being blown forward.

I have two of Mr. Zediker's books and I find them very informative and the link I posted explains how the ejected case can end up longer than the chamber. Meaning port pressure, the resized diameter of the case, brass hardness and brass spring back after sizing.

If you want more information then go to the m14forum.com reloading forum or thecmp.org forums. The m14 forums is where I first read about the ejected cases being longer than the chamber long before I bought the books by Mr. Zediker. But it gives meaning to the saying "The cartridge should fit the chamber like a rat turd in a violin case".

Below Forster explains their three different .308/7.62 resizing dies.

Its amazing what you can find to read on the main page of accurateshooter by Mr. Zediker.

Ogives, Meplats, Boat-Tails and Other Bullet Design Elements
http://bulletin.accurateshooter.com...-boat-tails-and-other-bullet-design-elements/

I did read that post and appreciate all the extra information you just posted. I have all Zediker’s books and articles printed out for about 10 years now and I go back to them a lot. I was asking rammac in case he was using some other method than described above. Maybe I read too much into his response. These posts by engineers and folks that have been around for a very long time disputing what Zediker wrote made me think there was something I was missing.

 

[Bart B.]

How does Forster's 308 Match FL die know the headspace of a fired case so it always bumps its shoulder. 003 inch?

Most standard 308 FL dies have about 1.6255" headspace to start with. Very easy to measure. All of mine are 1.626" to 1.625". Used a GO headspace gauge and a set of feeler guages.

 

[Uncle Ed]

I did read that post and appreciate all the extra information you just posted. I have all Zediker’s books and articles printed out for about 10 years now and I go back to them a lot. I was asking rammac in case he was using some other method than described above. Maybe I read too much into his response. These posts by engineers and folks that have been around for a very long time disputing what Zediker wrote made me think there was something I was missing.

And yet we have someone saying Quickload is used for writing reloading manuals in this posting. "BUT" my Hornady and other manuals say they use a strain gauge glued to the barrels of the vast majority of its reloading data. In fact if a specific firearm is listed in a manual a strain gauge is used for the pressure data if a universal receiver and pressure test barrel are not used.

Before the internet if you wanted to learn more about firearms and reloading you read reloading manuals, books and magazines. And now we have people criticizing the very people who write books on the subject. I'll take the printed information in books, magazines and reloading manuals before faceless screen names in forums.

 

[Uncle Ed]

How does Forster's 30gauges. FL die know the headspace of a fired case so it always bumps its shoulder. 003 inch?

Most standard 308 FL dies have about 1.625 inch headspace to start with. Very easy to measure. All of mine are 1.626" to 1.625. Used a GO headspace gauge and a set of feeler guages.

Forster doesn't know the headspace of your rifles, thats why they make three type dies. And since chambers do vary in size it lets brilliant people like you chose the type die that fits your need best.

And the Forster website and their catalogs do not have oversized egos.

Have a nice day.

 

[riflewoman]

Before the internet if you wanted to learn more about firearms and reloading you read reloading manuals, books and magazines. And now we have people criticizing the very people who write books on the subject. I'll take the printed information in books, magazines and reloading manuals before faceless screen names in forums.

And even before the internet people published all sorts of misinformation some of which became legend and lore. Most of this was by people who should have known better. “...but it says right here in this book...”. doesn’t make it so. That’s why scientific and engineering books get a peer review.

 

[Uncle Ed]

And even before the internet people published all sorts of misinformation some of which became legend and lore. Most of this was by people who should have known better. “...but it says right here in this book...”. doesn’t make it so. That’s why scientific and engineering books get a peer review.

And I have never read any reloading or firearms books written by Bart B or riflewoman. And I have seen monumental screw ups done by Engineers.

Bottom line, asking a question in a forum about Quickload port pressure guesstimates is absurd. If you don't believe me then ask the same question in the CMP forums with far more M1 and M1A shooters and reloaders.

 

[Bart B.]

I'll take the printed information in books, magazines and reloading manuals before faceless screen names in forums.

Should we feel the same about your posts in spite of all the erroneous data in print?

Meanwhile, can you explain how Forster's 308 match die sets back all fired 308 cases' headspace. 003" when their headspace ranges from 1.623" to 1.638" as fired in different chamber sizes with different loads?

 

[Bart B.]

If you don't believe me then ask the same question in the CMP forums with far more M1 and M1A shooters and reloaders.

Then we won't be taking the printed information in books, magazines and reloading manuals before faceless screen names in CMP forums.

 

[Bart B.]

Forster doesn't know the headspace of your rifles, thats why they make three type dies. And since chambers do vary in size it lets brilliant people like you chose the type die that fits your need best.

And the Forster website and their catalogs do not have oversized egos.

Have a nice day.

I don't think you grasped the simplicity of my question about that match die.

Then there's reports from top military team members that reloaded cases fired in M1 and M14/M1A match-grade rifles don't shoot as accurate as new cases. I've observed that several times.

 

[rammac]

Not trying to be a smartass at all or get into this pissing contest, but I would like to know how to mitigate overexpanded cases. Seriously.

I suspected that nobody would even come close to mentioning the easiest way to mitigate case-expansion issues in the M14 platform. Most people think of changing the cartridge but you can actually tune the rifle.

It's a process of adjusting dwell time - the amount of time that it takes to move the bolt out of battery. If you increase dwell time you allow the case to cool and return to it's original dimensions more reliably. A side benefit is that your cases will experience far less damage and they will look pretty much like the brass that comes from a bolt action rifle.

Dwell time is adjusted by moving the operating rod forward either by taking metal off of the gas piston or the gas plug. You have to be careful because you can remove too much material and get in to unhardened material but it's a fairly well known process to people that have worked on the M14 for any length of time. I usually take material off of the gas plug because if I mess it up then it's a pretty cheap part to replace.

 

[Dusty Stevens]

I suspected that nobody would even come close to mentioning the easiest way to mitigate case-expansion issues in the M14 platform. Most people think of changing the cartridge but you can actually tune the rifle.

It's a process of adjusting dwell time - the amount of time that it takes to move the bolt out of battery. If you increase dwell time you allow the case to cool and return to it's original dimensions more reliably. A side benefit is that your cases will experience far less damage and they will look pretty much like the brass that comes from a bolt action rifle.

Dwell time is adjusted by moving the operating rod forward either by taking metal off of the gas piston or the gas plug. You have to be careful because you can remove too much material and get in to unhardened material but it's a fairly well known process to people that have worked on the M14 for any length of time. I usually take material off of the gas plug because if I mess it up then it's a pretty cheap part to replace.

Can you post a picture of a modified gas plug and simply explain how to get the right dwell time? Im guessing you just take some off til it gets like you like it?

 

[riflewoman]

Be careful with these modifications. The gas system on a 14 is a cutoff & expansion system. There’s a hole in the piston that has to line up with the gas port in the barrel. Move the piston too far forward and the rifle may not get enough gas. Take too much off the rear of the piston and the gun might get sluggish OR peen the face of the operating rod.

Some drilled a small hole in the gas plug for cleaning perposes and this was found to make the gun “smoother” but I always felt it was hooey.

 

[rammac]

Be careful with these modifications. The gas system on a 14 is a cutoff & expansion system. There’s a hole in the piston that has to line up with the gas port in the barrel. Move the piston too far forward and the rifle may not get enough gas. Take too much off the rear of the piston and the gun might get sluggish OR peen the face of the operating rod.

Some drilled a small hole in the gas plug for cleaning perposes and this was found to make the gun “smoother” but I always felt it was hooey.

I agree to an extent, but not totally.

The piston is not a perfect fit in the cylinder and gases move around the entire length of the piston so while you could take too much off the inside face of the plug (moving the piston forward) but it wouldn't prevent the gases from operating the system. The hole in the piston doesn't really have to line up perfectly with the gas port, in fact, most don't line up perfectly ( you can tell that by looking at powder burn marks on the piston, you can see that the burn mark isn't perfectly aligned with the piston hole). If you take too much off of the piston tail then you will end up with a space between it and the op rod which will eventually peen one or both contact faces over time due to the piston slapping the op rod. This is another reason why I prefer to take material off of the inside face of the gas plug. The gas plug can also become peened if you remove enough material that you expose the unhardened metal.

I suggest taking no more than a few thousandths off the gas plug and then test the system (.003" at a time would be fine). I don't recommend using a grinder, it makes the metal hot and can anneal it. I use a knife sharpening stone (hard Arkansas) because it's easier to keep the face perpendicular and flat to the center axis of the plug. You wont perceive much of a difference except that the recoil will seem slightly different and the brass will come out less distressed and closer to it's original dimensions - it wont be perfect but it will be better than before you adjusted the dwell. The head of the cases will usually show far less damage and the ejection pattern will change a little. Imagine looking down at yourself when you shoot with the muzzle pointing toward 12 o'clock, your brass should eject somewhere from 1 o'clock to 5 o'clock.

A Marine Corps armorer explained to me his process for adjusting dwell:

• Mark one of the flats on the gas plug with magic marker.
• Turn the gas plug until it just contacts the op rod.
• Then pull the op rod to the rear so that it doesn't touch the gas plug.
• Count the number of turns that it takes to tighten the gas plug until it contacts the gas cylinder.
• The acceptable range is between 1/2 turn and 1 1/2 turns. Shorten the gas plug until it will do that.

This process works and it's the fastest way to achieve good results but it also results in taking the most amount of metal off of the gas plug. My process is a lot slower but it's a bit safer to sneak up on the best gas plug length. Again, this is why I like working on the gas plug, it's cheap to replace so if you want to dive in and use the armorer's technique then have a spare gas plug ready in case things don't work out.

 

[fatelvis]

Very interesting. Thanks guys and gals.
Does a slotted piston also affect dwell time, or only weaken the op rod action?

 

[riflewoman]

What is being described is adjusting the base volume in the gas cylinder. This will require more time for the gas to get to pressure to move the operating rod thus delaying openin a fraction of a millisecond. The adjustment in volume available here isn’t enough to matter. The gas plug is about a half inch in diameter and the thread pitch is 32 TPI if I recall (.032/rev) correctly. So using round numbers, the area of the piston is 3/16 sq inch and the length removed from the plug is .050. But the plug has a hole in the center and this would reduce the volume of material by about half. So, this means the volume increase is about .005 cubic inches.

If you want to increase the volume, ream the bore of the gas plug a little. Reaming to .325 for an inch will do the same thing as you described.

 

[Bamban]

• The acceptable range is between 1/2 turn and 1 1/2 turns. Shorten the gas plug until it will do that.

That's how my 14 types are set up for competition. Worked well when I was shooting the big sticks. To Bart's point, I only shot virgin brass from 600 to 1000.