What to Look For in a Custom Rifle – Chambers and Throats

In his next article on what to look for in a custom rifle, John Stewart of Kiote Corp explains the right way and the wrong ways to cut a chamber and throat for a rifle…

The number one problem with most rifles both factory rifle and also some custom rifles as well, is the manner in which the chamber is cut.

Factory guns are made with reamers that get used on hundreds and hundreds of chambers. Over time, the reamers become dull, chipped, and dimensionally smaller due to wear. Scoring, gouges, too loose, too tight are all problems commonly seen in factory and less quality custom guns.

Just recently, I was asked to correct problems with chambers on two rifles that you’d expect to be near flawless: one Kimber 84L and one GA Precision HRT rifle. Both exhibited excessive scoring caused by chips not being removed during the reaming process. Both exhibited longitudinal gouging caused by the reamer being forced into the chamber. These are irreconcilable errors on guns that the owners paid in excess of $3000 for.

When I cut a chamber, I never use a reamer that has been used on more than 4 other rifles. Once that reamer completes its fourth rifle, it either gets sent back for re-conditioning or it gets tossed in the recycling bin. This ensures that my tooling is sharp and dimensionally sound before it ever cuts steel.

I also never cut more than 0.010” in depth in one pass. This ensures that I’m not overheating my tooling Even with oil or coolant applied, heat is generated which creates problems. More importantly, this ensures that I don’t have excessive chip build-up which could cause scoring or gouging inside the chamber while it’s being cut.

300 Norma Mag Chamber Cut Before Oil Removal and Honing (image courtesy of Kiote Corp)

My lathe does not have integrated coolant nozzles. Even if it did, it wouldn’t really work for chambering because I don’t want to push/flush chips into the chamber. I’ve found that using ISO 32 machine oil on my tooling allows for both proper cutting and ease of chip removal during the cut, allowing the chips to slide down the flutes of the reamer and out of the chamber. I also use a floating reamer holder from Manson Precision that ensures that my reamer stays inline and parallel with the bore. Even if my barrel were to be a half-thousandths out, the chamber would still be cut properly.

Once I’ve achieved proper headspace, I use a honing brush from Flex-Hone to ensure a near-mirror finish on my chamber walls. I don’t necessarily believe that a true mirror finish is necessary. In fact, I prefer not to have one because the walls should provide a slight resistance to the brass during expansion so that everything stays in line with the bore during the burn process of firing.

Properly cut and throated chamber after 1,000 rounds. (Image courtesy of Kiote Corp)

Another problem is that all factory rifles are chambered with a long throat. This is primarily done to ensure that no matter what brand of ammo the consumer is using, it will fit in their rifle. While it’s convenient, it’s also detrimental to both accuracy and life-expectancy.

Factory cut chamber after 1,000 rounds. (photo courtesy of Kiote Corp)

The longer the jump is between the ogive of the projectile to the start of the bore, commonly known as “free bore”, the bigger the chance is for that projectile to get off-center, commonly referred to as “sideways”, before entering the barrel. This is exaggerated by out of square, off-center chamber cuts commonly seen in most rifles.
The long throat also allows for quicker throat erosion. With the projectile essentially banging against the throat of the bore, it erodes microscopic pieces of the metal with forces in excess of 40K PSI found in most center fire rifles used for hunting or competition.

Several well-known custom have led the consumer to believe that they are required to have a longer throated chamber now so that they can set the projectile out farther to achieve greater velocity. Time and time again, history has proven that most calibers are more accurate and have longer life expectancy by setting the projectile a specified length off of the lands, typically 10-20 thousandths of an inch.

comments

  1. avatar Ryan says:

    With the longer-throated chambers, you’re still setting your projectile’s ogive 0.010-0.020″ from the lands, but with more volume in the boiler room of the case. This also allows you to run the longer, heavier, higher BC bullets without taking up case volume.

    None of this necessarily improves pure accuracy, but allows for more case volume and thus more velocity. In a Palma rifle, for example, you need all the case capacity you can get to cram more powder behind a .308 projectile. It needs all the help it can get to make it to 1000 yards without getting blown out of the 10 ring by a 1 mph wind gust or switch.

    It’s all a compromise…. magazine length restrictions, case capacity, velocity, projectile selection.

  2. avatar Joe R. says:

    “Irreconcilable” as in no-can-fix those screw-ups (without a new barrel)?

    I know mr. Coyote is not really responding here, if anyone knows the answer i’ll pay in “TTAG-coin”, my newly minted crypto-currency of the Realm.

    /sarc [wink, meet me on the dark web]

    P.S.
    I bet those cut-in-half chambers are loud as hell (for a few milli-seconds).

    1. avatar Joe says:

      P.P.S.

      Don’tcha just hate when somebody comes along and makes you think you need to upgrade something?

    2. avatar Dyspeptic Gunsmith says:

      Yes, it would take a new barrel. There are only two ways to fix the sort of problems Mr. Stewart is referencing there:

      1. Put on a new barrel, and then profile the barrel to match the inletting on the stock.
      2. You could “set the barrel back by a turn,” ie, you move the barrel shoulder forward by one turn, decrease the length of the tenon by that one turn, extend the threads forward by one turn, and then deepen the chamber to a point where you can close a stripped bolt on a “go” gage. The problem with doing this is that the barrel’s taper will no longer match the inletting, and you basically need a new stock.

      Now you understand why so many match barrels, where shooters do set their barrels back by a turn, don’t have a taper on the barrel; when you set them back, nothing about the inletting changes.

      1. avatar J says:

        Ummm…..no…?

        Unless your barrel is reverse-tapered, and/or full-length bedded (a fashion that thankfully died with the ’60s), setting your barrel back will only add a little clearance to the float, or change where the forend’s pressure point contacts the barrel – not a problem, in either case. Setting a x/16 thread barrel back 1/16″ of an inch, the taper shift will only change the diameter of the barrel at any point in the inlet by a few 0.001″. Float is float, whether you have 0.010″, or no forend at all.

        Also, this has nothing to do with why “match” barrels are often not tapered. If a match barrel is not tapered, it’s because the match rules allow it, and a non-tapered barrel generally performs better, where allowed.

        I know it’s fun to say things, but you really should think about them before you do.

        1. avatar Dyspeptic Gunsmith says:

          I take it you’ve set back barrels in a high-dollar stock then?

          When I say “high dollar,” I mean stocks where the walnut cost about a $K, never mind the cost of the finished stock, and the owner paid for the barrel inletting to have barely any clearance…

        2. avatar J says:

          Yes. I have.

          By the time a client had put enough rounds through their custom rifle that it was ready for a set-back, they were willing to accept a float going from 0.010″ to 0.015 or 0.020″. If the rifle was set up with a pressure point, I’d sometimes have to build that back up with glass, but not always.

          But, of course, the topic being discussed, chiefly, is precision rifles, which are usually stocked in something other than wood, and, as of late, painted some sandbox-ninja color, so it’s a non-issue.

          I notice your reply had nothing to do with your bunk comment about “why match barrels have no taper”, though.

  3. avatar MiserableBastard says:

    I watched the first 2 or 3 minutes of a bond arms show (turned out to be a 30 minute commercial). In the first minute or so, there was a montage of notable people holding a Bond Arms derringer. One of the pictures was of TTAG Resident war hero Jon Wayne Taylor. Oh, Former Texas Governor Rick Perry was also in the picture 🙂

  4. avatar Dyspeptic Gunsmith says:

    I agree with most everything here, especially Mr. Stewart’s practice of using a reamer for only four chamberings before sending it out for re-sharpening, or putting it aside to be used as a roughing reamer or something else (ie, recycling the HSS for some other tool). For higher-use chambering, I might spring for a carbide reamer; they hold their sharpness longer. Most all of my reamers are live-pilot reamers, which I prefer over the older-fashioned stub pilot reamers.

    In my shop, I like using high-sulphur cutting oil when I’m cutting chambers or other similar operations. I too cut only 0.010 to 0.015″ depth at a pass. Mr. Stewart didn’t say how he rough-cuts the chambers (to remove the vast majority of the material before using the finish reamer). I tend to not use roughing reamers (which are ground/sized to cut an “undersized” chamber, allowing perhaps no more than about 0.015″ that the finish reamer needs to cut. I use a twist drill, judiciously applied, and then a boring bar to clean up the hole to be round, concentric with the bore axis and have the majority of the case taper cut.

    One of these days, I’ll sink some money into Greg Tannel’s through-the-bore chamber flushing setup, which uses a hydraulic pump and a special hydraulic fitting on what would be the muzzle of the barrel, to push cutting lube up the bore and the chips out of the chamber, up towards the tailstock of the lathe. It’s a bit of a bother to build up and to set up, but it saves a bunch of time during the actual chambering, because you don’t need to cut in 0.010 to 0.015″ depth steps – you just make sure your reamer is clean and sharp, put it into the bore, start the high pressure lube pump, and then start cutting. The chips come out of the chamber over the end of the reamer. You might need to use live pilot reamers that have little notches cut into the pilot bushing to get the lube to flow over the pilot effectively, tho.

    The free-floating reamer holder. There is probably more debate over this issue in chambering rifles than most anything else. Some people swear you need them, some swear you don’t, some people use them and swear at them. I know of the type of floating reamer holder Mr. Stewart is using, and it is a good one. I’m in the camp of “I like that type of reamer holder, until I get a chamber that starts to chatter… and then I don’t like them.” There’s an alternative reamer holder called the “Bald Eagle” type of reamer holder, where the reamer isn’t held as it is in Dave Manson’s reamer holder. With the Bald Eagle type of reamer holder, if the reamer “grabs up,” I just let go of the handle and the reamer spins with the barrel, and the chatter stops immediately, faster than it would if I hit the brake on the lathe. I then stop the lathe, and then I start cleaning things up, re-lubing, etc, looking for the cause of the chatter. Sometimes, it’s just the barrel steel, and there’s not much I can do, other than put a shop towel over the reamer before I send it in, or I change spindle speeds, etc.

    Here’s a pic of Dave Manson’s reamer holder:

    https://i0.wp.com/rifleshooter.com/wp-content/uploads/2015/02/12-bolt-nose-recess-set-up.jpg?ssl=1

    Here’s a pic of the Bald Eagle reamer holder:

    http://pacifictoolandgauge.com/reamer-holders-arbors/112-bald-eagle-floating-reamer-holder.html

    As I said, they both work. To me, the nice thing about the Bald Eagle style reamer holder is that when things start to go wrong, I can literally feel it through that handle w/ the knob. If I’m not holding onto that knobbed handle, the reamer won’t cut in the barrel – it will just spin on the pilot mandrel that’s mounted in the tailstock. As soon as I feel something start to go awry, I let go and then stop the lathe.

    Only a couple of other things on the subject of throats, etc. The high-precision guys are increasingly preferring chambers with under-sized necks, which allow them to “neck turn” their brass in a little widget that looks like this:

    https://www.brownells.com/userdocs/products/p_749006477_1.jpg

    The idea is that by turning the neck brass with a pilot inside the neck of the bullet’s diameter, you get the brass thickness of the neck truly uniform, all the way around. The object of this entire exercise, starting with shortening up the throat, under-sizing the neck of the chamber, turning the case necks to get them truly concentric, etc – is to get the bullet launched out of the case and into the bore perfectly on the axis of the bore, so that the bullet does get “bobbled” or deformed from one side to another, and a high quality, non-deformed bullet (eg, start with a Berger/Barnes/etc all-copper, match VLD pill) should be turning perfectly on its long axis, with as little runout and precession as possible, thereby reducing group size downrange.

    PTG can supply “undersize neck” reamers in some of the chambers more commonly used in precision shooting:

    http://pacifictoolandgauge.com/6mm-635mm-nopix-showcart-/11059-6mm-br-order-any-oal-freebore-neck.html

    6mm BR is one of the cartridges used by the benchrest community for decades, and the basis for much experimentation in rifle accuracy/precision over the years.

    Enough of my prattle here. This is a subject where, if we collected a half-dozen gunsmiths together over a bottle of whiskey, we’d arrive at 14 different opinions on the matter in one evening.

  5. avatar Slow Joe Crow says:

    The guy at rifleshooter.com has a lot of articles on barrels, chambers and receiver tuning since he builds custom rifles. Well worth a look to see how it’s done.

  6. avatar James Ivy says:

    My Ruger American ranch in 5.56 has an extremely long jump I feel, if I load a 75gr A-max at 2.580 that’s .002″ off the lands that’s a long jump for normal ammunition, but it doesn’t shoot any better than my 60gr A-max seated to the recommended length so go figure

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