Forum: Where is the proper place to crown a rifle barrel ?
Ein Bericht aus dem Magazin 'Precision Shooting' (Mit
Genehmigung des Verlagers)
Leider nur in Englisch, aber sehr empfehlenswert für Präzisionsfanatiker !
the end of my last writing, I stated that with the approval of PS,
I would do an article on how I listen to my rifle barrels tell me where
they want to be crowned.
I think I need to first tell of some of my observations about rifle barrels. When I first got into fitting rifle barrels, I would take a blank, find out how long the customer wanted it or calculated how long I could make it depending on the weight requirements of the class it was shot in, chamber it, crown it, send it out the door and pray. Sometimes they worked and sometimes they didn't. Some barrels are good and some aren't, right? It took a long time for it to dawn on me that WHERE I PUT THAT CROWN just might have some bearing on how the barrel shot.
My first few years were all centerfire work. One day a fellow brought in a 52 Winchester. He wanted it re-chambered and the barrel shortened so he could make a kind of a sporter. For some blessed reason I took that rifle to the range and shot it, just as I received it. I kept all the targets.
I pulled the barrel, cut the old chamber off, shortened the barrel down to where the customer wanted it, chambered it, crowned it and went back to the range to test this winner I had produced. Man, you talk about a wake-up call. That thing wouldn't shoot in a bucket. Not near as good as before I had re-modeled it. The customer was happy cause he had it like he wanted it and said he expected it to shoot worse cause the barrel was shorter…sounded fair enough to me.
Before I was through I messed up about a dozen 52 Winchesters, 37 Remingtons and an Anschutz or two. I got to the point that I was either going to figure out what I was doing wrong or get out of the business. (I probably would be way ahead if I would have got out of the business). I eventually learned that one of my problems was where I was crowning these rifles.
OK. WHERE IS THE PROPER PLACE TO CROWN A RIFLE BARREL?
Rule number one! The crown must be at the TIGHTEST and ROUNDEST place in the bore. If for some reason you can't have both of these requirements, the one you MUST have is the crown must be at the TIGHTEST place in the bore. But note: The barrels that turn the big scores and shoot the little groups meet BOTH of these requirements.
I have thought a bunch about how to go about explaining where to put that crown. I have decided I need to talk some about rifle barrel construction first, then proceed with just how I determine crown location. And I have got my fingers crossed that my lack of being able to express myself in writing doesn't get things too confused.
I have made some drawings. In the figures A, B, and C, I attempt to show how the size of a hole drilled through a bar of steel changes when material is machined from the outside of the bar.
Fig. A, I show a bar of steel, 1.250" in diameter with a .250"
hole bored and reamed lengthwise through it. This is meant to represent a
rifle barrel in the un-turned blank.
In Fig. B, I show the same piece of steel that has had half of it machined down to say, .750" in diameter. Notice that the bore in the reduced section has enlarged. (I have exaggerated the drawing to illustrate my point.)
In Fig. C, I show the same piece of steel that has been tapered, say like a rifle blank being reduced to a sporter profile. As the piece of steel is tapered, the bore opens up proportionately. THE SAME PHENOMENON HAPPENS WHEN A RIFLE BARREL IS TURNED DOWN FROM THE BLANK TO SAY, A SPORTER CONTOUR. We must deal with this somehow.
Now, if you will look at Fig. 1, I try to show a rifle barrel finished in the blank before any profiling. If this barrel has also been finished lapped, THIS BARREL, IN THIS STATE, IS AS UNIFORM AS IT WILL EVER BE. ONCE PROFILING TO REDUCE IT TO SOME USABLE CONFIGURATION STARTS, THE BORE BECOMES DEGRADED.
In Fig. 2, I show a rifle barrel that has been reduced to a sporter size from the finished and lapped blank. As you can see, the bore has enlarged proportionately as the outside is reduced toward the muzzle. (Again, I have exaggerated the drawing.)
PLEASE REMEMBER: A RIFLE BARREL MUST BE CROWNED AT ITS TIGHTEST (and roundest) SPOT IN THE BORE IF GREAT ACCURACY IS EXPECTED.
This is why a rifle barrel MUST BE LAPPED AFTER PROFILING.
Rifling Vs Cut Rifling
Button rifled barrels need to be made in a uniform sized blank. And a bunch of barrels are made like this so all I have described so far applies to this style of rifling. With this system the barrel is made in a uniform blank and then turned to some usable profile. So all profiled button rifled blanks have the bores degraded to some extent. Again, button rifles barrels MUST be lapped after profiling.
But cut rifled barrels offer another possibility. A cut rifled blank can be drilled, the ends centered, then the barrel profiled to the desired size, then the bore reamed and rifled. This means that the finished bore will not be degraded by profiling. I think the potential for great accuracy from cut rifled barrels is unlimited. But for every silver lining in the cloud there always seems to be a thunderstorm lurking, so it is with cut rifled barrels. Good cut rifled barrels must be made by artists. They need to be made by craftsmen who take the time to make absolutely sure each groove is cut to exactly the same depth.
A little story about cut rifled barrels. Some years ago I had a very good cut rifle barrel maker make me a couple of test barrels. Both of those fine barrels were extremely uniform in the grooves and they both shot extremely good. So I got kinda excited about them and put the word out and a bunch of my customers put in orders for them. Well, in filling those orders, quality went out the window. Those barrels came in with groove diameters out as much as .0004"… I tried fitting some of them with no luck so stopped ordering them. And I have been a little gun shy ever since. I do not tell this story to condemn cut rifle barrel makers nor am I condemning button rifle barrels with my comments. What I am trying to do is to point out the difficulties and pitfalls of barrel making to help make the point that WHERE YOU PUT THAT CROWN HAS A MAJOR BEARING ON ACCURACY.
See, the barrel is the engine of an accurate rifle but it is the part that we can do the least with to correct deficiencies. If we have an action that has the barrel thread mis-aligned, there are machining operations that we can perform that will usually cure the problems. So we have some control over the action. Same for the other parts of an accurate rifle, EXCEPT THE BARREL. The more we machine on that barrel, the more the bore becomes degraded.
LISTENING TO MY BARRELS TELL ME WHERE THEY WANT TO BE CROWNED.
When I get a barrel in my shop, the first thing I do is carefully clean the bore. I also make sure there are no checks (burrs) in the ends of the barrel i.e., caused by lathe centers running in the bore from profiling. If the barrel is properly lapped after profiling there will be no checks as a rule in the bore at the ends. If there are checks then the barrel must be set up and crowned on each end, or accurate readings of the bore can't be made.
Pure lead makes the best slugs for measuring bores but may be hard to get in the sizes you need so the next best thing to use is a lubricated lead bullet for your size barrel. You want the slug to be a couple of thousands larger than the groove diameter of the barrel. If there is any doubt about your slugs being big enough, you can bump them up to fill the grooves as follows:
Take a long polished steel rod that is a close fit to the bore diameter, that has the end slightly rounded off and polished. Insert a slug in either end, depending on which end you are wanting to measure, stand the barrel up on a hard clean surface, insert the long rod down the barrel and bump the slug up in the bore between the hard surface and the rod. Then you are sure it is completely filling the grooves. You will not hurt the bore by doing this as lead bumps up easily.
You need a ball bearing cleaning rod. You want the jag on the end of the rod to be rounded or flat with the edges rounded slightly and polished. You do not want to use a pointed jag because the point will try to spread the slug as you apply pressure and will give you funny "feels".
I now carefully insert a lead slug in the breech end. I push the slug in about 1 inch or so, turn the barrel around and push it back out and carefully catch it. I then measure the slug, which gives me the groove diameter of the barrel near the breech end.
I then do the same thing at the muzzle end. At this point I am praying that the groove diameter at the muzzle end is the same or SMALLER than the breech end.
I am also checking the roundness or the grooves at both ends. Remember, the perfect exit for the bullet is at the tightest and roundest place in the bore.
OK. Let's say the slug taken from the muzzle is .0002" smaller than the one taken from breech. So far so good.
I then push a slug completely through the bore and pray again because if this slug measures smaller than the one from the muzzle, we are in trouble. This means that at some place in the bore there is a restriction.
Now, when I use these slugs, I do 2 or 3 at each end to make sure I am not getting a false reading for some reason.
OK, let's say the slug that was pushed through the bore measures .0001" smaller than the one from the muzzle. We now have to find the restriction. I then insert a slug in the muzzle and push it about 2 inches into the bore, turn the barrel around and push it back out. If this slug measures what my first muzzle slug measured, then I insert another and push it about 3 inches into the bore, remove it and measure. At some point I will get a slug that shows me where that restriction is in the barrel. If the restriction turns out to be say, halfway down the barrel, well then the only option I have is to try to lap it out. If the restriction turns out to be, say 4 inches from the muzzle, then that is where I will make my crown. That is provided that this part of the bore is extremely round in the grooves.
The breech end of a barrel can have a little run out in the groove diameter and not kill accuracy too badly, provided that the crown is at the tightest place in the bore and is extremely round.
So by a careful process of elimination, you can determine just where to cut that barrel to put the crown. Does this take time? Yes it does. But if this is not done, your, success in making an accurate rifle will be determined by luck.
IT'S LIKE MIKLING A COW
Remember when you was a kid, the first time you tried to milk a cow. You grabbed a teat and squeezed. Not much happened. You had to learn to grab the teat up next to the udder with your thumb and side of your first finger, grab a slug of milk and progressively squeeze it down the teat past your middle finger, ring finger and little finger, all pressing against the inside of your hand until it exited the teat into the milk bucket.
Well, the perfect rifle barrel does the same thing to a bullet. That barrel encapsulates a bullet at the breech. Then as that bullet travels down the bore, it is being squeezed ever so slightly, just like your squeezing that slug of milk down that cow's teat, and this process of the bullet moving and being squeezed, moving more and being squeezed, moved and squeezed, moved and squeezed and at some point it passes through the crown.
When you push a lead slug through a fine barrel, there is a "feel" you get when you have a winner. You start the slug in the breech of the barrel. Start applying pressure on the slug with your cleaning rod. You can close your eyes and as you apply pressure the slug starts to move. The movement is smooth and you can feel a constant slight resistance. The slug is moving along and you gradually feel a little more resistance, it keeps moving and a little more resistance, tighter, tighter, tighter, smooth all the while with no lurching, then tighter and at some point, a point you don't know for sure when it's coming, but at some point as the resistance is building … BAM, the slug exits the crown … kinda surprises you … WHEN A BARREL IS PERFECTLY ROUND IN THE BORE AND GROOVES AND HAS THE "FEEL" I HAVE JUST DESCRIBED, YOU HAVE A WINNER. Gosh, I hope I have made sense here.
When you slug a bore, you are looking for the "feel" I have described above along with measuring the bore itself. One word of caution. When you are pushing a slug through a bore, a rough spot in the bore can have the same "feel" as a tight spot. If you are pushing the slug through the bore and you feel what seems to be a tight place, stop, mark the cleaning rod so you will know where this spot is, then insert the rod in the other end of the barrel and back the slug out and measure it. If it is tighter than the rest of the bore, then it obviously is a restriction but if it measures the same as the bore close to it then it is a rough place in the barrel. Again, have I made sense here?
Now I have included a page of rough drawings showing how I position slugs in a micrometer to properly measure different numbers of rifling. I give a little description of each drawing. When you measure a bunch of slugs you get to know how much you can rotate the rifling marks past where I show them in the drawings so you can get more readings across the width of each groove.
CONCLUSIONS AND THOUGHTS
You have done all the things necessary to produce an accurate rifle up to crowning it. You have carefully slugged and measured that barrel and know where the tightest and roundest place in the bore is. What if you were wanting to make a long barreled varmint rifle and you wanted a 26-inch barrel for all the velocity you could get but you determined the crown must go at 21 inches? Let me tell you right now. You must believe in your measurements. And you must have the nerve to pick up that saw and cut that barrel where your readings tell you to put that crown. Sometimes this is tough to do. It is easy to just say; a tenth or so ain't going to make that much difference, I want the velocity. Well, a tenth or two makes a big difference. One of the old time gun writers, it may have been Col. Townsend Whelen, although I ain't positive, said; "Only accurate rifles are interesting". Who ever said it was correct. When you take that new varmint rifle afield, with that 26" barrel, when your measurements told you to crown it at 21 inches and you miss the first 5 groundhogs you shoot at or worse yet, gut shoot a couple and have them crawl away, you will wish that you would have believed in yourself and crowned that barrel where it told you to.
I want to thank all the folks for their correspondence and phone calls about my last article. (Play It Again Sam) Gosh, there are a bunch of interesting people who love searching for rifle accuracy. I learned a bunch of things from those folks. I learned that folks love to read technical things and so do I. I learned that PS has long arms cause not all of the correspondence was from the United States. Thanks again.
I worry that this article on where to put that crown only hit some of the high spots. When I started writing it, I figured a page or two would tell the whole story but there is 10 times more I would like to say. Too much to put in one article. I am sitting here typing this now and part of me wants to go back and add a bunch of detail but I am going to stop.
What little success I have had as a gun worker, started for the most part, when I finally learned to let my barrels tell me where THEY wanted to be crowned. I hope this effort may help someone else.
LEAD SLUGS FROM BARRELS
In Figures 3,4,5 and 6, I show the position of the slug in the jaws of a micrometer. This shows how to position the rifling marks properly.
Fig. 3 shows the correct position for measuring the slug from a 6 groove barrel. Take a reading here, then rotate to the next groove, etc. (same for 3 groove barrels).
Fig. 4 shows how to position a slug from an 8 groove barrel. Two grooves need to be centered between the jaws.
Fig. 5 shows the slug position for a 4 groove barrel … now a 4 groove is a little tough to measure. In fig. 5 you need to center two grooves of the slug and as you can see the grooves are almost hidden by the micrometer jaws. This only gives you two diameter measurements so in Fig. 6, I have added some additional information.
Fig. 6. To really measure at several places around a 4 groove slug, you need to position one groove as I show in Fig. 6, which is the same position as one of the grooves in measuring an 8 groove barrel. Then you need to rotate that same groove over to the other position of an 8 groove. Am I making sense here? You need more than 2 measurements to properly check the groove diameter of a bore.
Odd rifling patterns like 5 or 7 grooves are somewhat difficult to measure as with them you have a groove and land opposite of each other … I do not work with 5 or 7 groove barrels personally, as I can't properly measure them.
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