How a “Cut” Rifling Machine Works

This came across my desk this morning and I thought it was really cool. Ever wonder how a barrel is rifled? What we have here is computer animation of a “cut” rifling machine, showing all the parts and how it works. “Cut” rifling does the lands and grooves one pass at a time as opposed to “broach” rifling that cuts all of them at the same time. Then there’s “button” rifling which simply presses the rifling into the barrel. Another option is “hammer forging” where the barrel is placed over a mandrel with the negative image of the lands and grooves and then pounded to fit the mandrel, transferring the image to the inside of the barrel.

[h/t Reddit]


About Nick Leghorn

Nick Leghorn is a gun nerd living and working in San Antonio, Texas. In his free time, he's a competition shooter (USPSA, 3-gun and NRA High Power), aspiring pilot, and enjoys mixing statistics and science with firearms. Now on sale: Getting Started with Firearms by yours truly!

22 Responses to How a “Cut” Rifling Machine Works

  1. avatarmatt says:

    I want to know what can cause a dimple on the inside of a hammer forged barrel. Me (and a few other people on the forums) have one in the same spot on our FNP45s.

    • avatarPlumbump says:

      A female dimple in the barrel would have to be caused by the male equivalent on the mandrel, but knowing how much even the real small carbide dies I work on cost, I’d find it hard to believe something visible to the naked eye would get past inspection. Assuming they use thermal expansion to remove the barrel from the mandrel, it could also cause difficulties even removing the barrel. The same mandrel probably formed thousands of barrels, so both of yours are prolly from the same mandrel. However, it could also be something from after that process, perhaps some type of method to hold the barrel in place, or index it while lapping or polishing it after forming.

      • avatarmatt says:

        Its a female dimple on one of the grooves. About 75% the diameter of the groove. I was rather disappointed to notice that flaw on a $1,300 MSRP pistol. Especially when most of the other parts are plastic or metal injection moldings.

  2. avatarMark N. says:

    OK, I got the hammer forged and this wonderful little simulation of cut rifling, but I don’t understand how button rifling is “pressed” into the barrel. Could you explain?

    • avatarPlumbump says:

      For button rifling its kind of the same as hammer forging, i’d imagine, except the force that is forming the metal comes from the mandrel or tool itself forcing itself through the barrel, not the barrel being pressed or hammered down onto the mandrel.

  3. avatarBLAMMO says:

    Very cool. I’ve always been curious about the process, researched it and seen it described in texts. But I’ve never seen it illustrated. Thank you.

    Now go find a video on button rifling. :-D

  4. avatarSanchanim says:

    Ok where do I get one and how much do they cost! My uncle was a machinest sooooo this is keeping up in his spirit. Now all I need is a good source of barrel tubes, of the forging system for the AR lowers, and a decent CNC mill. hhmmm the possibilities are endless!

    • avatarjwm says:

      a guy in hayward ca had a whole shop set up in one of those strip warehouse places. on a weekend when he wasn’t there a water pipe broke. someone called it in to the local fd and when they checked it out found he had a machine gun shop working. he was arrested with 160 complete weapons and something like 100 k rounds of ammo. no telling how many weapons of his were put in circulation before he was shut down. another example of gun control working, except when it doesn’t, which is always.

    • avatarDyspeptic Gunsmith says:

      For custom barrel makers, the typical machine is a Pratt & Whitney rifling machine.

      The most versatile P&W machines are the “sine bar” machines made from before WWI to a bit after WWI. The higher-throughput rifling machines P&W made going into WWII were hydraulic tracers. The computer simulation above is an example of a “sine bar” machine – but it doesn’t look that much like a P&W sine bar machines. I’ll look in my collection of machine pictures to see if I can find an example of a P&W sine bar machine.

      How much do they cost? Depends on condition and location, but $50K for one in good shape isn’t unknown.

  5. avatarJason says:

    Are the barrels bored with just a really long bit on a lathe or something? It seems like it would be difficult to bore a straight hole 20 some inches through a rifle barrel.

    • avatarBLAMMO says:

      Long, thin boring tools and drill bits tend to “wander”. One way to prevent it is to back off after a distance, clear the material, continue and repeat. It’s probably done in steps with several different length tools. Maybe a final reamer for a precise bore.

    • avatarI_Like_Pie says:

      Rather than drilling though a block of steel….most barrels were manufactured as tubes with the holes already there.

    • avatarmatt says:

      I’m not sure what sources the other commentators are basing their replies from, but yes, they are drilled on a special machine.

      This place sells machines and bits:

    • avatarDyspeptic Gunsmith says:

      There’s a special type of drill used for “deep hole drilling” (where machinists consider a “deep hole” to be more than 3X the diameter of the hole) that has one cutting flute and has oil/coolant coming out of the relief behind the cutting edge. There’s a V-groove running the length of the bit and driveshaft to enable the cutting oil to push chips up and out of the hole.

      Drilling a deep hole straight is a difficult job indeed. The bit for a barrel bore can (and does) wander a bit as it’s drilling the hole. This is why barrel bores are drilled undersize, and then reamed with a straight reamer to size before the rifling is put in.

      What allows a gun drill to drill such a deep hole relatively straight is that the cutting oil or coolant is pushed down the drill shaft at pretty high pressures to keep the cutting zone clear of chips, and the rate of feed of the tool into the material is relatively slow compared to a common twist drill. When I say “slow feed,” we’re talking in tenths of a thousandth of an inch per revolution.

  6. avatarTommy says:
    Another good article explaining different barrel making techniques.

  7. avatarplumbump says:

    explosive welding !!1 :)


    yet another idea i should have patented :( along with my fiber optic christmas tree, sigh…

  8. avatarPhilthegardner says:

    Cool! I always wanted to know about this… Time to google hammer forging…

  9. avatarbontai Joe says:

    I had the pleasure and good fortune a long time ago, to take the tour of the gunsmith shop in Williamsburg, VA where they showed the methods used in the 1700′s. They were cutting the grooves in a long rifle, one at a time with a similar machine that was hand powered. If anyone has a chance to tour an historic village/park near them like Williamsburg, I strongly suggest visiting the gunsmith’s shop as I did. Amazing stuff you can learn and all done with basic and somewhat crude hand tools (and a LOT of skill!)

  10. avatarTorrey J says:

    There is an excellent, new technical book on barrel rifling available. Look for “Rifling Machines and Methods” by Clifford F. LaBounty available at

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