Seeking Input: Throat Erosion

  • Last Post 11 May 2019
MarkinEllensburg posted this 06 May 2019

I'm into my third year of CBA benchrest competition. The summer I started I made a chamber cast of my rifle. .30BR on a Ruger No.1 built by my father. I looked at the casting then just to try to determine what bullet to use after reading time and time again that for accuracy with cast bullets the bullet must fit the throat.

That first season I had some success although never improving on my first match (197-7X) but not ever doing terrible. Last year my season was dismal. In spite of shooting a great 10 shot group in January .611" @ 100 yards, not in a match I shot poorly at nearly every match wondering what was going on. I think it was late in the season last year that I actually measured the chamber casting. My throat not only was gone but had been since before i inherited the rifle.

Through the fall and into the winter I wondered the best course to take. I don't own a lathe but know several people who do. I have another rifle to build when I do get either a lathe or access to one. I have a .308 barrel blank and another Number 1 action, just needs the machining done. However for this current rifle I thought quickest and possibly easiest thing might be to run a reamer in to create a .308x1.75"

Here are some pictures with measurements, the top are diameter the bottom is the throat length.

chamber casting..

Then yesterday happened. At the match at Spokane Rifle Club I shot better then ever before there. two 5 shot groups were remarkable, a third was good and the other was not bad but not noteworthy. The ten shot groups had nice clusters with some fliers that I think were my fault. On the score targets shot a 49, then next two targets dropped a few, final target was a 50. However no x hits. I have more to say about this and will be submitting an article to TFS.

My bullet is Lyman 311335 sized to .311 and poured with an alloy close to lynotype. They are seated into the case only the depth of the gascheck cut.

If this was your rifle what would you do? Ream the chamber to have a new throat? cut the barrel shorter and start over with a fresh .30BR chamber, or just shoot it and hope for more days like yesterday? I had decided but just had not followed through yet. Now I have different thoughts.If yesterday only proved one thing to me it would be that we know much less than we think we do.

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RicinYakima posted this 06 May 2019

My opinion, for what that is worth, is try a bullet with the top driving band closer to the nose, like 311284.

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45 2.1 posted this 06 May 2019

Ric's suggestion could work with a larger bullet diameter / softer alloy used. Throats usually erode in a somewhat tapered configuration. To have one at a single diameter that far would suggest a freebore was cut when it was chambered.

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Brodie posted this 06 May 2019

Freebore,  THat's the name I couldn't think of when I first saw the photo of your Chamber cast.  Were it I, I wold set the barrel back and cut a new chamber without the freebore.


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Ken Campbell Iowa posted this 06 May 2019

i agree with 45 2.1 ...the throat/freebore might have cut that way.   throat wear ( actually corrosion )  usually occurs in a nasty spiral pattern; yours looks pretty smooth and concentric .  also, a " shot out " throat is usually 2 or 4 inches of dried river bed in front of the chamber.

most match chambers are cut with separate chamber and throater cutters.   it is very easy when throating to go too deep; almost no metal is removed.   when you did a chamber cast 3 years ago did you include the throat image then ?  that is really the main reason to do a chamber image. 

if it were mine, just for educational purposes, i would first try a 0.3135? rear end bullet just to see if the only rule we have actually works.  a long 0.313 rear and short 0.308-0.3085 front end. ( loverin or LBT ) then tell us the results ( g ) .

but to take the most promising route to more accuracy, assuming that is a match grade barrel, i would use the same barrel, set back and cut a fresh match chamber ... that barrel is probably a really good one starting at the 0.308 dia.  barrels actually get better with shooting, not worse...   the reason mj benchresters just screw in a new barrel is because the profile of those barrels don't allow more than one or two small setbacks ....  and many use " name " gunsmiths that charge $600 for their knowledge and prestige, added cost of a new barrel is only a little more.

just some thoughts.  ken

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Paul Pollard posted this 06 May 2019

You might want to try a few more groups without messing with it. My own rifle with worn throat continues to shoot well enough that it teases me into keeping the same barrel.

mtngun posted this 23 May 2015


Lee Wiggins wrote: The Ardito throat is 1/2 degree per side (1degree total angle).  Thanks for the clarification, Lee. . The Ardito article in #207 says “I used a 1 degree tapered reamer,” but he didn't specify whether it was “per side” or “included angle.”   Apparently he meant included angle.

John also states “since then I have played with different degree tapers but it did not make much difference.”   Hmmm.

I found it interesting that John used an unpiloted chucking reamer for his freebore experiment:  'a standard .308” chucking reamer.'   Note that the cutting edges of a chucking reamer are about 45 degrees per side. 

John said “I did this in one inch increments up to six inches.  It still did not make much difference in accuracy.”   Though unfortunately he seemed to be recalling an experiment he did some time ago and did not list any specific loads, velocities, or groups.     Not that I doubt John's veracity or his proven track record, still, I find the results hard to swallow.

In conclusion, if the leade angle doesn't make much difference, and if the freebore or lack thereof doesn't make much difference, and using a piloted reamer doesn't make much difference,  then what does make a difference?  Why do some guns shoot better than others if none of those things make much difference?

I also found it interesting that John built a rifle with an action at both ends of the barrel -- why didn't I think of that!  :cool:

John concludes by saying “I know I am a nut for doing these things but this is how I learn what works and doesn't work with lead bullets.  I love to try things other people haven't done.”   Amen.  ude:

It would not be that difficult for me to repeat John's freebore experiment.  I have an '06 takeoff barrel that would be a good candidate.    If ever I can find time ......




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Lee Wiggins posted this 06 May 2019

The Ardito Taper of 1 degree is 1 degree total ( 1/2 degree per side ) ..  In your chamber cast you have a measurement of .315 . Check that and see if it is a taper from case mouth to where you can see lands. If it is a taper, you just need to make the same taper on the bullet  , A long bullet , I suggest a Lyman 311284 . I use a chucking reamer with the end ground to .200 diam. and the base of a 223 cart. case press fit and lathe turned to .300 ( now I have a brass pilot)  next the reamer is reground with the 1/2 degree taper starting behind the pilot cutting about .295 deep and tapering up to the full reamer diam. The throat is cut with this reamer.

   This reamer is also used to cut the cavity of a bumping die . After the bullet ,that has cylinder sided driving bands is run into the bumping die it comes out with the driving bands having the exact taper of the throat . With little more than the gas check in the case neck , on bolt closing the tapers come together and the bullet is pushed just a little but further into the case. This is a "perfect  bullet  fit "

                                                                     Lee Wiggins 

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Ross Smith posted this 06 May 2019

As the owner of an Ardito rifle, I second what Lee said. Mine only has a .5" throat and tapered like Lee said. Interesting enough is that the best bullet so far has been an Eagan bullet bore ride style with a short driving band section at the base of the bullet which is exactly the same diameter as the rear section of the throat. That .315 is a little scary to me.

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MarkinEllensburg posted this 07 May 2019

Thanks for the input gents.


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M3 Mitch posted this 08 May 2019

I will just say that I think your idea to make it into a 308X1.75 would be my choice if I decided that the throat erosion was severe enough that re-cutting the chamber was justified.  I have never done a job like that, but is is possible to do this with the barrel still in the action?  Maybe the answer to that depends on what kind of gunsmithing equipment you have on hand. 

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Ken Campbell Iowa posted this 09 May 2019

 a complication of re-cutting a new chamber in a barrel where the throat has been wallowed out... is that if your cutter depends on a guide-button to center the cutter ....  the oversize old throat is bigger than your normal guide button.  those buttons are normally supplied only ~3/4 inch long.

so we like to get the cutter and the bore lined up the best we can before cutting.   the good news is that the old chamber will tend to center a new chamber cutter ....  ( path of least resistance ) ...  so a 1-piece chamber and throater kinda works easier ... but then they never come ground to exactly what you decide at the last minute. ( g )  .

if your new chamber is not cut to " fresh bore " depth, your throating cutter might not be centered.  so we like to cut the chamber to reach the " fresh bore" .    which usually means setting back the whole barrel enough to do that. usually at least a half-inch.  then a lathe is necessary. ( a lathe does not solve the long wallowed out throat guidance though. ) .


so, tentatively, yes you can do a fresh chamber and throat with the barrel still in the action, ... sometimes .... but you need to do a few measurements and thinking sessions beforehand .

1)  will you clean up the entire old chamber ?   yes is good.

2)  is your new cutter going to leave some of the old bad throat ?  no is good.

just some thoughts, hope this helps.



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RicinYakima posted this 09 May 2019

Take the barrel out. It takes 15 minutes and you will have a better job.

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lotech posted this 10 May 2019

Maybe a little off topic, but regarding #311335...I bought a mould stamped #311335 about thirty or more years ago not long after Lyman introduced (or re-introduced) the mould. In an older Lyman manual, there is a photo of a bullet cast from a #311335 mould. A spitzer design is depicted; weight is listed at 206 grains. My mould casts a roundnose design that weighs about 200 grains or so with gas check affixed. From the Lyman book again, I think this design is really the 197 grain Lyman #311333. 

I've read something about this mixup before. Anyone know more about this? I'm not sure which design the original poster is referring to here. The mould I have makes a bullet of good dimensions, including the nose, but I've never been able to get the accuracy with this bullet that it appears to be capable of and I've tried many loads in a variety of rifles.   

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MarkinEllensburg posted this 11 May 2019

 Both of my molds are of the roundnose version.

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