If there is one thing that gets attention in smokeless shooting it has be the breech plug. This small part of every rifle has such a divergence of jobs and possible ways to do those jobs I dare say it is worthy of its? own chapter in a book and that?s exactly what it is going to get here. As normal I will reference the common plug as being the one from the 10ML. However I will address matters that will apply to any plug of any design and make.
The plug is really a simple device but if you look at a 10-ML plug questions start from the very beginning. The first might be why it is 1 7/8? long but less than half of it has what seems to be the needed part: the screw threads? The second might be related: why does it have a long smooth portion after the threads? As many questions as it poses the plug is not a bad design though I don?t want anyone to get the impression I think it?s perfect.
Most of the questions can be answered with logic and the ones that are otherwise don?t matter much from a function point of view. To get started I?m going to provide a drawing of a modified plug. Most of you will recognize the difference between it and the factory plug in an instant but I?m not listing it to encourage modifications I simply want an easy reference point.
You will note that there is no difference between the regular plug and the drawing except the vent liner is recessed into the plug. We will consider why later. Here I want you to note the screw threads as depicted by the wiggly white line at the top. Some (who want to be authorities) have told me there is magic place where these screw threads must end for maximum strength. However true that may be in some sense I am not one to believe in magic.
What I?d like the observant to note is the threads must be long enough for strength. They are as it is but that length is not critical as long as the thread surface area is enough to make the plug threads as strong as the surrounding metal. There are many ways to express that but most machinists use a common rule that if the thread length is 150% the width of the screw the threads are stronger than the metal surrounding them. If you measure the length of threads on this plug you will find it to be about that length. So the threads are long enough.
However they could be longer and not change anything. The last time I wrote that a small firestorm started. A writer suggested any more threading of the barrel would only weaken it and the system. I hope if you look at the bottom of the drawing you can see the barrel is not the only part(s) in the system. You should see the barrel is surrounded by the receiver near the primer end then the recoil lug as you go forward and finally by the barrel nut as you near the right angle stop of the barrel and plug junction.
So the parts surrounding the plug are really a laminate of two layers of steel and as long as the screw threads stay behind the laminated parts a few extra threads don?t weaken anything. Indeed the weakest part of the system is the right angle junction of the plug (Point X) and barrel because it is relatively thin and there are no laminated parts. If you have seen the pictures of the one blown up 10ML this would be obvious to you as the rupture was never in the laminate but at the stop juncture.
So what, you say! The point is you can do things more than one way. And some improvements are possible even when an item works on its? own merit. The plug has a job to do and doing a better job is what it?s all about.
One of the jobs of the plug is to provide positive ignition. Over the course of years (if I was smart it wouldn?t take that long) it has become apparent that the length between the primer and the powder is an important point. This length must not be too short (too much gas on the primer causing blowback and bulging) or too long (a lot of primer crud and difficult to light some powders). The best length is about 1.1? to 1.5?. I think this allows adequate primer protection for proper load pressures and ample ignition for even slow burning powder.
Now you know why the plug in our image is modified. With the modification the distance between the primer and powder can be in the optimum zone.
Along with the distance one would want to take some time to figure the proper diameter of the vent liner. The vent opening affects ignition and primer pressure in much the same manner as the length. A bigger vent provides stronger ignition (but possible some blow back) a small vent may aid accuracy. The sizes of choice are .031? and .033? of these two it?s hard to make a difference but if I lived where it is really cold I?d shoot the bigger diameter.
The vent also represents the part of the plug that wears. To most that doesn?t matter much because if it last 100 shot that?s 5 years worth. But to those who shoot a lot it is a constant going out of accuracy, change the vent cycle that means there are going to be several occasions when your rifle won?t shoot for any apparent reason, until you remember to change the vent. This brings me to another image.
Again you are looking at a modified plug. Again the vent is recessed into the plug but this time the vent is not a replaceable screw but a pressed in cylinder with a small hole in it. The common name for this type of device is a drill bushing. I could discuss the workings of a drill bushing but unless you think the machinist handbook deserves a Pulitzer prize you'd be bored to tears. Let?s just say it?s very hard and has a precision sized small hole running the whole length. The bushing is so hard in fact it takes a very long time to show any wear from shooting. I?ve had several to 500 shots and see no wear so I?m thinking 1000 or more is possible.
Now when I make the special plugs for a custom rifle of smaller caliber I depart from the factory model and use what I like and know to work. My plugs are usually started from a 5/8? X 18tpi grade 8 bolt. The alloy steel is tough so I?m not worried about strength. The bolt allows me the use of fine threads and still not require special ?gun only? machine tools. I generally copy the 10ML because it works but in my own plug I can change things like over all length and vent line hardness and position of the vent. I also always use a hex head so the plug can be removed with regular tools and not a piece of steel useless for any other reason.
I guess you can say I?ve put a lot of though into the breech plug. The reward is I can get a lot of performance out. Generally I can have a plug that shoots to a reasonably high pressure without deforming primers while at the same time giving positive ignition in a system that lasts a long time.
The plug is really a simple device but if you look at a 10-ML plug questions start from the very beginning. The first might be why it is 1 7/8? long but less than half of it has what seems to be the needed part: the screw threads? The second might be related: why does it have a long smooth portion after the threads? As many questions as it poses the plug is not a bad design though I don?t want anyone to get the impression I think it?s perfect.
Most of the questions can be answered with logic and the ones that are otherwise don?t matter much from a function point of view. To get started I?m going to provide a drawing of a modified plug. Most of you will recognize the difference between it and the factory plug in an instant but I?m not listing it to encourage modifications I simply want an easy reference point.
You will note that there is no difference between the regular plug and the drawing except the vent liner is recessed into the plug. We will consider why later. Here I want you to note the screw threads as depicted by the wiggly white line at the top. Some (who want to be authorities) have told me there is magic place where these screw threads must end for maximum strength. However true that may be in some sense I am not one to believe in magic.
What I?d like the observant to note is the threads must be long enough for strength. They are as it is but that length is not critical as long as the thread surface area is enough to make the plug threads as strong as the surrounding metal. There are many ways to express that but most machinists use a common rule that if the thread length is 150% the width of the screw the threads are stronger than the metal surrounding them. If you measure the length of threads on this plug you will find it to be about that length. So the threads are long enough.
However they could be longer and not change anything. The last time I wrote that a small firestorm started. A writer suggested any more threading of the barrel would only weaken it and the system. I hope if you look at the bottom of the drawing you can see the barrel is not the only part(s) in the system. You should see the barrel is surrounded by the receiver near the primer end then the recoil lug as you go forward and finally by the barrel nut as you near the right angle stop of the barrel and plug junction.
So the parts surrounding the plug are really a laminate of two layers of steel and as long as the screw threads stay behind the laminated parts a few extra threads don?t weaken anything. Indeed the weakest part of the system is the right angle junction of the plug (Point X) and barrel because it is relatively thin and there are no laminated parts. If you have seen the pictures of the one blown up 10ML this would be obvious to you as the rupture was never in the laminate but at the stop juncture.
So what, you say! The point is you can do things more than one way. And some improvements are possible even when an item works on its? own merit. The plug has a job to do and doing a better job is what it?s all about.
One of the jobs of the plug is to provide positive ignition. Over the course of years (if I was smart it wouldn?t take that long) it has become apparent that the length between the primer and the powder is an important point. This length must not be too short (too much gas on the primer causing blowback and bulging) or too long (a lot of primer crud and difficult to light some powders). The best length is about 1.1? to 1.5?. I think this allows adequate primer protection for proper load pressures and ample ignition for even slow burning powder.
Now you know why the plug in our image is modified. With the modification the distance between the primer and powder can be in the optimum zone.
Along with the distance one would want to take some time to figure the proper diameter of the vent liner. The vent opening affects ignition and primer pressure in much the same manner as the length. A bigger vent provides stronger ignition (but possible some blow back) a small vent may aid accuracy. The sizes of choice are .031? and .033? of these two it?s hard to make a difference but if I lived where it is really cold I?d shoot the bigger diameter.
The vent also represents the part of the plug that wears. To most that doesn?t matter much because if it last 100 shot that?s 5 years worth. But to those who shoot a lot it is a constant going out of accuracy, change the vent cycle that means there are going to be several occasions when your rifle won?t shoot for any apparent reason, until you remember to change the vent. This brings me to another image.
Again you are looking at a modified plug. Again the vent is recessed into the plug but this time the vent is not a replaceable screw but a pressed in cylinder with a small hole in it. The common name for this type of device is a drill bushing. I could discuss the workings of a drill bushing but unless you think the machinist handbook deserves a Pulitzer prize you'd be bored to tears. Let?s just say it?s very hard and has a precision sized small hole running the whole length. The bushing is so hard in fact it takes a very long time to show any wear from shooting. I?ve had several to 500 shots and see no wear so I?m thinking 1000 or more is possible.
Now when I make the special plugs for a custom rifle of smaller caliber I depart from the factory model and use what I like and know to work. My plugs are usually started from a 5/8? X 18tpi grade 8 bolt. The alloy steel is tough so I?m not worried about strength. The bolt allows me the use of fine threads and still not require special ?gun only? machine tools. I generally copy the 10ML because it works but in my own plug I can change things like over all length and vent line hardness and position of the vent. I also always use a hex head so the plug can be removed with regular tools and not a piece of steel useless for any other reason.
I guess you can say I?ve put a lot of though into the breech plug. The reward is I can get a lot of performance out. Generally I can have a plug that shoots to a reasonably high pressure without deforming primers while at the same time giving positive ignition in a system that lasts a long time.
