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Thread: How should I make a Carbine? (chop the barrel)

  1. #1
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    Default How should I make a Carbine? (chop the barrel)

    I have an old Gamo 440 at home which barrel is way too long, I would like to chop a bit of, about 10-15 cm's. There barrel would ideally be 30-35cm, I know spring guns reach max velocity in the first 25 cm so velocity loss wont be a problem. I am a bit concerned about crowning the barrel myself,any advice or reasons not to do what I proposed?
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  2. #2
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    the choke?
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  3. #3
    MOT: Thomson Pneumatic Rifles

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    According to the study done in the book "The Airgun from Trigger to Muzzle" max velocity was in the first 7 inches so 10 to 12 would be fine, although you have a very short cocking lever, the 440 does not have that strong a spring anyway.
    You will have to be very carefull in cutting the barrel as it it has to be cut off "square" not angled or any amount of crowning would be useless.
    You can recrown the barrel using a ball bearing of +- 8 to 10 mm diameter. Use a drill press to chuck (ie hold the ball in the presses jaws) the ball bearing and use progressivly finer valve grinding paste and then finnish off by useing very fine steel wool between the ball and the crown, get a few ball bearings of the same size and change them as soon as any wear can be seen on the ball.
    This is not the best or the right way to crown a barrel but if you do not have a manual crowning tool or a lathe then its the only way to get a reasonable crown.
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  4. #4
    Sharp Shooter

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    Before you do anything come vissit me and bring your Gamo with. Also have a look at this

    Where to crown a barrel
    by Bill Calfee

    At the end of my last writing, I stated that with the approval of Pro-Sport, 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 HWERE I PUT THAT CROWN just might have some bearing on how the barrel shot.

    My first few years were all center fire 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 Anschütz 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.


    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.

    In 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 HAPPENikko Stirling HWEN 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.)


    This is why a rifle barrel MUST BE LAPPED AFTER PROFILING.

    Button 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 kind of 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 HWERE 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.


    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 s 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.


    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… kind of surprises you … HWEN 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.


    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 or 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 Pro-Sport 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.

    Good Shooting,

    Bill Calfee

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  5. #5
    Sharp Shooter

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    Also look at this,

    While crowning is the last part of the process. It's the first thing you have to consider when cutting a barrel down.

    Rule # 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 shoot the little groups meet BOTH of these requirements. This isn't much of a issue if the barrel is going to go through a re-choking process, but for the sake of anyone not going to rechoke it, this is the biggest issue you will face. Especially on a spring gun that you want good firing behavior on.

    Rule #2!

    A really really good spring gun barrel will have a tight breech. Meaning when you push the pellet in there is some resistance to the pellet going in. FWB124s usually do very well in this area. A barrel that the pellet falls in and hangs on the skirt will "usually" result in lower velocity but almost always give a harsher firing behavior over one that fits tight. This is of course relative to how loose and also the size of the pellet being used.

    Rule # 3!

    The next thing you want is a barrel that doesn't "open up" after breaking free from the breech. It's common on a lot of barrel cocking guns on the market to be really tight in the breech and as soon as it past the spot in the barrel that is covered by the breech they open up. I've seen it on under lever and side lever guns that have barrels pressed in as well. You just about never see it on a TX200 as the barrel is machined, epoxied and bolted in place. That's why you very rarely run into issues with TX200 barrels in this area. All that said it is imperative that if a barrel that opens up (as a lot do) that there be a choke in the barrel that is tighter than the breech.

    The best way I've found to check a barrel is with a shiny new pellet! I know some of ya'll will be tempted to reuse dat pellet but resist cheapness and use a new one. The old one will always try and line back up with your old grooves and give misleading info. Push from the breech slowly forward. cleaning patches just don't give you the "feel" needed to come up with anything conclusive.

    90% of the barrels that come into my shop for choke jobs are for this reason. Someone had cut the barrel cause "they wanted it handier" and wound up with a vibrating, heavy recoiling, inaccurate mess on their hands.

    Often I use the phrase "sealing issues". People almost always jump to the conclusion I'm talking of a bad piston seal or out of round or tapered compression tube. Most often I am, but on many guns it's also "issues" with the barrel on the gun as well. Tuning isn't just about getting rid of vibration in a gun. To me it's more about making a pleasant shooting gun. Correcting problems with a barrel or replacing it, if it is really bad, will do tons more for a gun than everything else combined. Guns with all the bells and whistles and a loose barrel, while they may be vibration free, are often abrupt with sharp recoil. Often guns like this are owned by people that just didn't know any better because of their limited experience. Until they shoot a good gun with no issues, they just don't know any better. On higher powered springers with this problem I've seen them be more prone to burn seals, and springs loose a good bit of their life.

    Rule # 4!

    Choke dat thang!!! The end of the barrel needs to be the tightest spot in the barrel. Putting a more aggressive choke on a barrel will work wonders if the rest of the barrel is even slightly suspect. Some people say that a choke is only needed to allow a wider range of pellets to be used. Not to be a a** about it but on "SPRING GUNS" the choke is needed to also cushion the piston. I've had cases where putting a fairly aggressive choke on a shortened barrel, cut the recoil in half!! The only way you can get away with out one is if the barrel is not cut too short or the barrel is very very uniform in bore size. As noted before FWB barrels are the most forgiving in this area.

    Rule # 5!

    Barrel length matters!!!!! The higher the power the gun, the less forgiving it is to being cut!!!! On 17+ftlb guns it's best to keep it as long as you can stand. Most of the factory long barrels hover around 19 inches. That actually works very well with 20+ftlbs. I personally don't like going less than 16 on higher power guns (my preference). The lower the power the more forgiving it is. Cardew wrote nothing more than 6 inches is needed for spring guns to reach full power. Needless to say time wasn't his friend. He was using a HW35 that was a little on the lame side to come to this conclusion and with the advent of higher power guns (even 11ftlbs) it was shown to be very very off base. On higher powered springers (25-30ftlbs) with shortened barrels I've seen them be more prone to burn seals, and springs loose a good bit of their life. Just like mentioned above about oversized bores. While I've done it for people in the past I always feel guilty after doing it. I can't help but feel I'm in some way taking advantage of their not knowing, but I've usually caved due to the customers insistence. Even after explaining it to them they would often insist on it. I guess after reading this most won't ask anymore?


    This is pretty strait forward. The end of the barrel needs to be square with the bore and the exit hole has to be free of any burs. There are all kinds of ways to do it and contrary to what most say, if it's fairly clean of burs and on the tightest spot of the barrel it will generally shoot good groups. I've seen some pretty bad crown jobs shoot good. The one thing they all had in common is that they were free of burs.


    You want a barrel that does not have a oversize bore, is uniform through the middle or slightly tapering down the bore (preferred) and has a choke that the tightest spot is just as the pellet leaves the end of the barrel. I could write more but for the most part this covers the basics. I can choke and crown your airgun barrel. It's listed in my pricing section on my main page, but there is enough info here that if you want to attempt to cut your own, it should help you not make some of the more common mistakes. Most notably how to check your barrel to see if it's a good candidate for cutting.

    Many Thanks,

    Paul Watts

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  6. #6
    Protea FT Team '08

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    Iaan , where do you find the time to read all that ?I think I will read the Summart !lol
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  7. #7
    Sharp Shooter

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    Allan you will never find time. If someting is important to you, you have to make time.
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