How are barrels rifled? – Part 1

by B.B. Pelletier

Several readers have asked for this posting, and one reader asked about lapping a barrel, which is supposed to be part of the rifling process. It has all but been abandoned by modern barrelmakers, at least those who make large volumes of barrels. Actually, the first person to request this post asked me to explain how BARRELS are made, but because that answer is included in this discussion, I included it within the talk about rifling.

History
Rifling was discovered very early in the history of gunmaking. In the beginning, the grooves ran straight down the bore, but soon they were made in a spiral pattern, and immediately gunmakers discovered that a spinning ball was more accurate. There are records of shooting matches in the mid-1500s, where rifled barrels were NOT permitted, because of the advantage they offered. So, the effect of a rifled barrel was known a long time ago.

Three types of rifling
There are three principal ways rifled barrels are made today, and two of them start with a long tube of metal. They are the cut-rifled barrel and the button-swaged barrel. The other – hammer forging – is quite different, so I’ll cover it by itself.

The following process refers to both cut-rifled barrels and button-swaged barrels. To get the long tube of metal needed for the barrel, the maker starts with a tube or a solid rod. Some small makers of airgun barrels start with a seamless hydraulic tube that they rifle. If they start with a solid rod, the hole through the center must first be drilled. The task of drilling a deep (long) hole through a solid rod of metal is one of the toughest machining tasks known. In World War II, the M1 Carbine was redesigned to eliminate one deep hole in the side of the receiver, because too many receivers had to be scrapped when the drill broke out of the side of the hole.

Many barrelmakers drill this hole on a lathe, but the precision barrelmaker uses a vertical axis machine to eliminate the effect of gravity on the long drill bit. The drill bit itself has a special cutting surface to reduce the tendency for the bit to wander. Even so, no hole is ever drilled entirely true. The barrel maker has to use other means to true up the hole if he wants a quality barrel.

The hole is reamed and (possibly) lapped
The next step is to ream the hole. A trueness of about 0.001″ along the axis is possible with very careful work. If the process is speeded up or the reamer is dull, it will be 0.0015″ or even 0.002″ of variation along the entire axis. If the maker is a good one, the next step is to lap the bore.

Lapping does not increase the dimension of the bore. It’s purpose is to remove the tooling marks left by the reamer, just as the reamer also removed the larger marks left by the drill bit. The finest lapping is done with a lead slug that is cast right in the bore of the gun, so the fit is perfect. The cooled slug is broken free and pushed halfway out the bore, where fine abrasive power and oil are brushed on. This is called charging the lap. I have read in many places that lapping doesn’t use abrasives at all, but rather it uses polishing compounds. Well, Virginia, polishing compounds ARE abrasives! They’re just very fine. If they weren’t abrasive, they wouldn’t work.

The charged lap is run up and down the bore, recharging as required to keep polishing the bore. Because the lap is lead, the lapping powder sinks into it before it scratches the steel bore, so this is a laborious process. It’s not unlike using J-B Non-Embedding Bore Compound to clean a leaded barrel!

Cut rifling
After lapping, the barrel is cleaned. If it’s going to have cut rifling, it is now installed in a rifling jig or machine, which looks something like a lathe. A headstock holds one end of the barrel, which is held at the other end so that it can be turned easily. A rifling cutter is a very small tool that fits on the end of a rod long enough to pass completely through the barrel. The cutting rod is mounted on a fixture that causes the rod to spiral as it passes through the bore. Two hundred years ago, this fixture was a wooden positive of the rifling pattern desired. It had the same twist rate that was desired in the rifle. Today, a precision rotating fixture is used. Alternately, the barrel may be rotated and the cutting fixture held still, and the same result will happen.

When a cut is complete, the headstock is indexed for the next groove and another cut is made. When all the grooves have had one pass of the cutter, it is adjusted to cut deeper and another set of cuts is made. Each pass of the cutter will remove about one ten-thousandth of an inch, if the barrelmaker is a good one, so each thousandth of an inch takes ten full passes. If the rifling is 0.005″ deep, each groove took 50 passes of the cutter. If the barrel has six grooves, it took 300 passes of the cutter to completely rifle that bore. To speed things up, the cutter can be set to cut deeper, but that means more chances for burrs, gouges and associated tool marks.

Cut rifling has largely gone out of fashion today, though it’s still practiced. It does not result in a barrel that’s any better, but it does allow complete freedom over dimensions that button rifling does not allow. There is one more step after cutting the rifling, and it’s a final lapping. I will cover it after I describe button rifling in the next part.

29 thoughts on “How are barrels rifled? – Part 1

  1. Thanks for the post, BB. Real interesting and I’d love to see some video of the process.

    Off topic, I’m getting ready to plunge into PCP with a Talon SS. My shooting is about 85% paper punching at 30-50 yards. With that in mind, which caliber will give me the most shots per fill at a good plinking setting?


  2. I have a pair of RWs 34′s that have about 2,000 shots each through them . They have been home tuned / lubed and patch and felt pelet cleaned . What can I do to get the barrels as smooth as possible . I am thinking of running JB Paste and or also Flitz loaded patches throygh the bore a few dozzen times.
    I have a couple of VERY VERY used bore snakes that I was going to use first to try and remove whatver if any lead build up.
    thank you for your post or anyone else that might make a post to this .

    JerryL




  3. Leave your barrel alone. Airgun match shooters NEVER clean their barrels!

    For additional info on barrel cleaning, search the past posts.

    B.B.




  4. very interesting post – I never new how much work went into rifling a barrel

    just a couple of points to add though – abrasives remove material. Lapping may not remove much but in order to remove tooling marks etc it has to remove some metal.

    There appears to be a new alternative method to rifling. Apparently Smith & Wesson are using an electrochemical process to rifle some their newer barrels.

    thanks again

    Phil


  5. Phil,

    Thanks for your comments. I am aware of an EDM-based rifling process, and perhaps it’s the same thing that S&W has. I don’t know enough about how it’s done to comment at this point, but it is very interesting.

    B.B.


  6. B.B. i am in serous need of help or advice. while shooting this afternoon i found that after cocking the gamo 1250 i could not close the barrel! i inspected the area around the breach and discovered that the metal piece used to lock the barrel into place was stuck to far out. this is the same aparatus used on all the gamo guns i have fired so if you have one i think you’ll know what im talking about. it just happens that on the 1250 model there is a small rectangular hole on the underside of the barrel very close to the breach. wedged against this hole i can see a spring that appears to be decompressed. im pretty sure this spring is pushing the locking aparatus out to far, causing it to hits the crossbar (that locks the barrel) at the wrong spot. i’ve tried pushing the spring with a flat headed screwdriver all to no avail. to say that im frustrated would be a severe understatment expecially having just cleaned, polished and sighted in my gun yesterday. how do you recomend that i fix this problem?
    S.O.S,
    a very weary & frustrated scopestop


  7. If B.B. can’t get back to you soon enough you may want to post your question on one of the forums, someone might have had the exact problem you did. Of course there is a lot of garbage floating around on the forums but for specific problems like this they are often very helpful.

    There may even be a forum just for Gamo, I would do a search.


  8. What about broaching? The Marlin “ballard type” barrels seem to be broached as all the cut marks travel in the direction of the rifling.

    Also, I read in American Rifleman that some slug gun maker was using electrochemical machining to cut their grooves, sometime in ’94 or ’96.


  9. Hello BB,

    Off topic! I purchased a Airforce Condor a year or two ago from Pyramyd Air. I have not had time to shoot it until today… Well today when I tried to shoot the rifle it was shooting way, way slow.I only shot five or six rounds and stopped before I threw the rifle in disgust! I was shooting 24.2 grain Exterminator XP2 hollow points on the highest power setting. They are made of pure lead by Dynamic (the company that makes Logun Penetrators). I have no chronograph but could easily tell the velocity was low. I would say under 500fps. The pellets were barely denting particle board from ten feet away! My stock Benji 393PA is shooting harder. I have several other PCP’s so I know my charging setup is okay. I know I could ship the rifle for warranty service but, I would rather not. I live in Hawaii so shipping is expensive. I have a hard time paying to ship a rifle to be fixed when it is in the same condition that it left the factory in! If you have any ideas that could help me I would be very greatful. If you want to email me instead of clogging the blog my address is
    jlaine@hawaii.rr.com. PLease help!

    Thanks,
    Jason Laine



  10. Jason it sounds like your gun is filled to high. My condor likes the 2200 psi range. Try there. If that don’t help you can always post a question on talonownersgroup.com I’m sure they will be able to figure it out for you.



  11. a very weary & frustrated scopestop,

    The chisel detent you describe is supposed to be held in position by a crosspin. It sounds like that pin has either broken or is out of its hole. The rifle will not close with that pin out, as you have noticed.

    I think this is a job for your dealer or Gamo. If the rifle is still under waranty, send it to them. Call first for return instructions.

    To replace the pin, it may be necessary to remove the barrel from the action. It all depends on access to the chisel to compress it when the pin is reinserted.

    B.B.


  12. Jason,

    I agree with baldtrucker, and there is no problem. Just shoot the gun by dry-firing untill the sound comes up to a sharp crack. Then try it!

    An early Condor is one of the most powerful smallbore air rifles in the world, but it is also EXTREMELY sensitive to over-filling. When the rifle starts cracking, attach it to your fill device and start filling. The pressure at which the tank valve opens (noted by an audible click and a jump in the gauge needle) is the MAXIMUM fill pressure for your rifle.

    You will still get the same number of powerful shots from the lower fill pressure.

    From what you said, I would guess your rifle’s serial number is lower than BC00300.

    B.B.







  13. Arcane here. Have you ever indexed them in any way? Can you direct me to one of your articles which gives a summary of what’s out there?


  14. Arcane,

    On Sep 30, 2005 I listed all posts prior to that time. However, the search function makes it much easier to find articles. Go to the current post and type your terms into the search box on the right.

    B.B.


  15. Hi BB,

    Thanks for your suggestion. I would also like to thank Baldtrucker.

    The serial number on my Condor is BC00574,if that makes any difference.

    Once again thank you guys! I will let you guys know how I fare.

    Jason




  16. I though something was wrong, the fact that it was labeled as smoothbore might put off people wanting to buy it. Now that i know that the barrel is rifled how accurate is the 760 at around 20 yards? (with pellets of course)
    Thanks for the help


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