Process for chambering large magnum chambers

S

skeetlee

Active member
I am sitting here at my desk thinking about a project I have coming up for my hunting partner. We are building him a 338Lapua. I have debated with myself whats going to be the best way to chamber that big long magnum chamber. If I drill bore and ream very much of this chamber, I am afraid my pilot wont engage the bore soon enough while pushing with my floating holder. Now maybe that isn't an issue, im not sure? If I bore most of the chamber out, should I just push the reamer with a dead center, would that be better?? I plan on doing a practice run on the left over I cut off the blank, but I would like to go into this with a solid game plan. I haven't ever cut a chamber that large, so I really don't know what to expect. Looking for any ideas or tricks you all might already know. thanks Lee
 
Skeet,
Just drill and single point bore, The reamer is going to follow the bored chamber. Don't worry about the pilot being in the bore. Boe
 
Boe said:
Skeet,
Just drill and single point bore, The reamer is going to follow the bored chamber. Don't worry about the pilot being in the bore. Boe
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I really can't say i agree with this at all.. Whats the point of using a piloted reamer if the pilot doesn't engage in the bore till the chamber is 95% cut?
Of all the chambers I've cut, the only ones that have chattered were ones where the pilot was not engaging in the bore.
What i do now for long chambers is to drill and bore to a depth that allows the pilot to engage in the bore just before the reamer makes contact with the breach face.
I then ream as usual, pushing with a dead center until i have about .75" of the reamer in the hole, then change over to my floating reamer holder..
I ream until i hit the un-drilled solid part of the chamber. I then remove the reamer and drill again either to around 0.060" short of finial depth or to a depth that allows the pilot to engage before the reamer starts cutting.. I don't bother to try and bore the second drilled hole as i have found that it usually runs with in a few tenths and when coming back in with the reamer, the pilot is in the bore and the reamer body is in deep enough in its own true hole to stay perfectly in line.

Cheers
Lee
 
Lee
That is very interesting, and I can see your thinking behind your method.
Little story. I broke a flute on a 30BR reamer when I first started chambering barrels. I really don't know why I broke the reamer, but ever since then I have been extra cautious when chambering anything 30cal and bigger. About the only thing I can think of as to why that reamer broke, was maybe I bored to much of the chamber out? I don't fully remember, but I don't think my pilot was engaged in the bore when that flute snapped off? Chatter is another concern I have with this 338L, and not having my pilot in the bore at all times.
I strive for such high perfection on each and ever chamber I cut, I thought maybe I should ask about this before I chamber that 338. I appreciate the info. Lee
 
skeetlee said:
I am sitting here at my desk thinking about a project I have coming up for my hunting partner. We are building him a 338Lapua. I have debated with myself whats going to be the best way to chamber that big long magnum chamber. If I drill bore and ream very much of this chamber, I am afraid my pilot wont engage the bore soon enough while pushing with my floating holder. Now maybe that isn't an issue, im not sure? If I bore most of the chamber out, should I just push the reamer with a dead center, would that be better?? I plan on doing a practice run on the left over I cut off the blank, but I would like to go into this with a solid game plan. I haven't ever cut a chamber that large, so I really don't know what to expect. Looking for any ideas or tricks you all might already know. thanks Lee
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Good timing. The 338 is based on the .404 Jeffrey, and I will be chambering one (404) this coming weekend. I generally use the reamer without drilling/boring. I'll let you know how my chamber goes.
 
Lee

When predrill and bore i pick a size that is around 0.040" under the shoulder dia of the chamber. When i bore i don't try and mimic the case taper, i just bore the hole parallel. I like to have my reamers actually "cutting" a decent amount of material as the load is a bit higher and again it helps to reduce the chances of chatter.
I can't really say why your reamer broke, except to state the obvious, in that it was likely under too much load. I hold my reamers by hand when cutting and so can feel exactly how it is cutting and when i need to remove it to clear the chips.

Cheers
Lee
 
Leeroy,
I can not agree with your method at all either. It seems your primary focus is to keep the pilot bushing in a crooked bore. I think your are missing a very important point. How do you know what the run out in throat is going to be? If all barrels were perfectly straight this would not be an issue. With your method you are not be able to indicate the throat before you start. How do you know that your barrel doesn't have one of those nasty little bends it before you get there? As we all can agree we want the run out in the throat as near zero as possible.

I want to KNOW what the run out in the throat before I get there. Everything is based off the indicated throat. I drill the chamber to get the indicator in to be able to indicate the throat first. After the throat is dialed in then I single point bore. The reamer is going to follow the bored hole and into the bore when it gets there. Unsupported .

I just came in from my shop and just happened to cut chamber using this method today. When I indicated the chamber the run out in the throat it was 0.0001 neck 0.0001 rear of chamber 0.0001 Doesn't get much better than that.

This is just another method of chambering. We all have our own methods. What works for you may not work for me. That's the great thing about this forum is being able to share ideas. And keeping an open mind that there might be a better way of doing something.

Richard Hilts
Hilts Custom Rifles
 
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G'day Richard.

I do certainly agree with you that the throat needs to be indicated in prior to any work.. Skeet wasn't asking about dialing in so i negated to detail that part of how i do it.. :D
To dial in i use a long stylus indicator. I can reach in around 5" my my setup so i dial in at the throat and then again as far in as i can reach, with cursory checks in between these two points. Like you i have to pre drill a little way to get my indicator in but Once i'm happy that it's running true i'll finish with the pre drilling, then turning the tenon complete, then pre boring and finial reaming.

Cheers
Lee
 
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It's probably been explained before, but why are some of you taking the time to drill and bore as opposed to just using the reamer to cut the whole chamber. Is it to save wear on the reamer? Drill and bore then ream would take longer than just reaming the chamber, at least for me. Do you get a better chamber and if so why? Wear on the reamer would not offset the time/cost of drilling and boring at least for me. Resharpening or replacing a worn reamer would be cheaper than the time to drill/bore it seems to me over the long run.
Where am I wrong?
Dave
 
There is a lot of taper to the 338 Lapua. Measure the shoulder diameter, and measure about halfway back on the reamer. Step bore the hole so the reamer enters far enough to engage the pilot in the bore before cutting. Not too big of deal. You then have two point of contact that WILL align the reamer for the initial cut. Usually you can bore maybe 1/3rd of the way in a few thou larger than the shoulder diameter and you will have no problems with clean-up.
 
A fellow who used to gunsmith in this area (moved out of state) really liked the .280 AI but lacked an indicator that would reach into where the chamber throat would be. To get around this, he would drill so that he had more room to reach in with his test best style indicator, and then re-indicate the barrel using the chamber throat area. He did this before cutting the tenon. Next he would bore, then chamber a bit deep using a pusher, then he would re-indicate the barrel off of the chamber, cut and thread the tenon so that the head and gap spaces were correct. Do you think that some version of this would work for your project?
 
A trick I learned with Ack Imp chambers. Run the reamer in until you have about .100" of the body cut. This area supports the reamer later and prevents chatter. Then you drill out the midsection and head back in with the finish reamer. With the Lapua case it has a pretty good taper on it. Drill it out and bore a short section a few thousandths over shoulder diameter and go back with the reamer. I use a caliber specific core drill these days.

Dave
 
burtona said:
It's probably been explained before, but why are some of you taking the time to drill and bore as opposed to just using the reamer to cut the whole chamber. Is it to save wear on the reamer? Drill and bore then ream would take longer than just reaming the chamber, at least for me. Do you get a better chamber and if so why? Wear on the reamer would not offset the time/cost of drilling and boring at least for me. Resharpening or replacing a worn reamer would be cheaper than the time to drill/bore it seems to me over the long run.
Where am I wrong?
Dave
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We do this to keep the chamber as straight as possible. If the barrel is set up by indicating in the area near the throat, the bore will usually wander a little between that point and the breech. Sometimes more than a little. If we just ream the entire chamber, the reamer will follow the hole and any wandering will lead to the rear of the chamber being out of concentricity and possibly oversize and out of round. Sometimes this is so minor that it would have a negligible effect on a hunting rifle, but for Bench Rest, shooters want chambers as straight, round, and on size as humanly possible.
 
SGS said:
We do this to keep the chamber as straight as possible. If the barrel is set up by indicating in the area near the throat, the bore will usually wander a little between that point and the breech. Sometimes more than a little. If we just ream the entire chamber, the reamer will follow the hole and any wandering will lead to the rear of the chamber being out of concentricity and possibly oversize and out of round. Sometimes this is so minor that it would have a negligible effect on a hunting rifle, but for Bench Rest, shooters want chambers as straight, round, and on size as humanly possible.
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OK, Thanks. that makes sense.
 
SGS said:
We do this to keep the chamber as straight as possible. If the barrel is set up by indicating in the area near the throat, the bore will usually wander a little between that point and the breech. Sometimes more than a little. If we just ream the entire chamber, the reamer will follow the hole and any wandering will lead to the rear of the chamber being out of concentricity and possibly oversize and out of round. Sometimes this is so minor that it would have a negligible effect on a hunting rifle, but for Bench Rest, shooters want chambers as straight, round, and on size as humanly possible.
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Let me offer some perspective on the other side of the coin. I do not build bench rest rifles. 90% of my work is tactical, the rest is hunting. I use Krieger barrels 95% of the time because I have found them to be of extremely high quality.

I do not pre-drill or bore, and I indicate to 0.001 - 0.0015. With this degree of accuracy I can consistently build 1/4 MOA rifles, and have received no complaints with regard to accuracy. As long as customers understand that ultimately in our world it is both "the indian" and "the arrow" that define quality, realistic expectations can be had as long as we know precisely what our purpose is, and what we are willing to pay to get there.

Bench rest rifles routinely require around an order of magnitude greater accuracy (than I provide) just to be marginally competitive. While I can't say my methods are definitive of anything other than the level of consistency I have achieved, it is easy to understand why so much more painstaking work is put into a BR barrel.

Considering that a BR gunsmith holds tolerances at least 10 times tighter than I do to achieve what they do, It is little wonder that certain methods have evolved over the course of time that define the standards that must be achieved in the BR world.

For those that doubt the efficacy of these techniques I would suggest trying both methods on the same barrel (if this is practical) to see what the delta might be in accuracy.

While there are too many factors other than the barrel chamber itself that define 'accuracy' to any degree, I suspect that similar methods have evolved over the years simply to control or eliminate the variables that perplex us all in our quest for a single hole shooter.
 
I think if you got the truth out of a gunsmith on the winning list of a match report youd faint on how they do things and what they dont do to produce that winning barrel.
 
Most of my builds are RUM or Lapua based cases built for ELR precision shooting and hunting. It's really just more time compared to smaller cases I think, I dial in the the throat with a gordy rod, dial in the breach with a gordy then a .0001 indicator till no more can be dialed out. Fit the action checking bore a few times then hook up the flush system, put the reamer in the newer PTG floating reamer holder and adjust then lock it down, set my feed to .0027 per rev and speed to 250 rpm. Feed .500 then clear for a little over half then go to .250 and clear. I let the reamer full float after about half way and stop about .050 shy of head space to get a measurement and I cut to .010 of that and measure with feeler gauge then cut to that and make a final measurement for .001 head space. Two passes of 320 to roughen up the chamber and two passes of 600 to mellow out the 320, bevel the chamber and polish face with 600. Just finished another 338 Lapua Improved, chambering took 40 minutes from hooking up the flush system to torquing the barrel.
I've wanted to try pre boring but I've never seen a need for it in the final measurements of my chambers nor on paper at stupid long ranges so I've been reluctant to try it.
 
AMMASHOOTA said:
Good timing. The 338 is based on the .404 Jeffrey, and I will be chambering one (404) this coming weekend. I generally use the reamer without drilling/boring. I'll let you know how my chamber goes.
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I finished my .404 this weekend. Chambering was no more difficult than any other one I have cut. I had a wee bit of chatter as the larger reamer entered the bore, maybe for the first 1/8 inch or so, then it was knife through butter. The right oil and feed make the job straightforward.

I would say as barrels go, the Lothar-Walther steel (CM) felt a bit softer than my Krieger barrels. I don't think it took more than about 20 minutes to cut the entire chamber.
 
Big chambers

Dave Tooley said:
A trick I learned with Ack Imp chambers. Run the reamer in until you have about .100" of the body cut. This area supports the reamer later and prevents chatter. Then you drill out the midsection and head back in with the finish reamer. With the Lapua case it has a pretty good taper on it. Drill it out and bore a short section a few thousandths over shoulder diameter and go back with the reamer. I use a caliber specific core drill these days.

Dave
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This is good advice..... I DO IT ON ALL BIG CHAMBERS ESP. ONES WITH SMALL BORES......IT WORKS..............
 
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