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Chambering goals

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Unread 02-18-2009, 02:37 AM
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Join Date: Dec 2006
Posts: 27
Chambering goals

It appears that few to nobody wants to commit to answering a question about what is considered a "perfect" or optimum chambering job.

So, specifically what is OK runout and what is perfect runout?

How much over the actual reamer size is considered good.

Clearly GO-noGo gauges determine chamber length but how about finish and concentricity to bore?

These are to help me know when my chambering practice has become good enough for prime time.

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Unread 02-19-2009, 10:28 PM
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Join Date: Jan 2009
Posts: 21
Re: Chambering goals

It depends on what exactly you are looking for. Benchrest smiths are alot more particular in what they accept than a smith who primarily barrel hunting rifles. There is no such thing as a "perfect" chambering job, but you have to strive to get as close to perfect as possible. I generally shoot for .0005" runout, but I have settled for more depending on what I was trying to accomplish. I have seen Remington chambers that were as much as .012" out, and still shot. It's a matter of just how well you want it to shoot.
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Unread 02-20-2009, 02:50 AM
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Join Date: Dec 2006
Posts: 27
Re: Chambering goals

I'm going for ultimate accuracy, like benchrest shooters. So, 0.0005 is helpful as a start.

Is there a measurement beside runout that describes how much over reamer size is considered benchrest quality?

Oh, and thanks for replying, it seems very difficult to get this data.

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Unread 02-20-2009, 07:00 AM
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Join Date: Jan 2009
Posts: 21
Re: Chambering goals

Well to start with, I'm not a BR smith. I'm just a welder/machinist by trade. If you really want to learn about chambering a rifle do a search over on benchrest central. Those guys are very helpful, and there are several threads on chambering if you do a search. In fact one was posted a few days ago with pictures detailing the whole process. Chambering is like the old saying there's more than one way to skin a cat. Not everyone does it the same they just go with what works for them.
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Unread 02-20-2009, 12:28 PM
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Join Date: Jan 2007
Location: South Dakota
Posts: 753
Re: Chambering goals

I have chambered a lot of guns.

I've also done a hell of a job on every one of them because its something I just like doing and I take it very seriously.

Dakota Arms and Nesika paid me a lot of money and gave me a blank check with clear instructions to figure out how to cut a chamber perfectly every time in the shortest amount of time possible.

It starts with the right machine. You use one that's designed with a spindle cartridge that uses a preloaded tapered spindle bearing. The one we picked out used a class 9 bearing and thats about at good as it gets.

Next you indicate the barrel through that spindle. This is a long winded debate on what is the best process. The one I use takes an average between the minor and major bore dimension on a barrel and the barrel gets indicated precisely where the bullet enters the lands and where it exits.

What the barrel does between these two points is beyond my control and I really don't care. Tie the thing in a knot if you wish so long as its nuts on at those two points.

Fitting the tennon to the receiver is next and there is more to threading a barrel than just getting the right number of threads per inch and getting the action to screw onto the thing. Pitch diameter, flank angles, all that stuff needs to be right. If you understand the engineering dynamics and limitations of a 60* thread form you will see why this is important. Surface finish is also critical because this has to do with friction coefficients and that affects torque.

Now comes chambering.

spin a barrel that has been indicated at both ends and you'll discover very quickly that it is not straight. It's a skip rope. This has to do with how barrels are made. Imagine this piece of material as a solid bar in a deep hole gun drill at the barrel plant. It's spun and the center runs out a bit. The drill doesn't care because it's going to follow the spindle theoretical centerline as best as it can. So, when the machine is running the hole is pretty damn straight.

Now shut the machine off and what happens? the barrel snaps back to being straight and your hole turns into a banana.

OMG that's terrible? No it's not. It's still round and it's still a consistent diameter and that's all you really care about. In fact the crooked ones sure seem to shoot better than the ones that are straight. If a guy is an accuracy minded gunsmith he'll take the time to clock this bend so that it runs on the vertical. Rifles seem to shoot better this way.

Ok, so if this hole is a skip rope we know that it's not right when the pilot of the reamer enters the barrel. It won't be right until the pilot gets to that point that we initially indicated the barrel at. BUT it's a piloted reamer and what can't a reamer do?

It can't have an origional thought and make it's own hole where it wants to. It has to follow what is already there. Neither can a drill bit (HINT HINT)

So, IMO people who use drills to hog out chambers are take a lot for granted because they assume the hole was right to begin with. IT'S ALMOST NEVER RIGHT GUYS

This is the beauty of single point tooling. It will make a hole conentric to spindle centerline and since that is what we datum everything off of, it's perfect.

The last thing you want to grab is a boring bar so just leave that put away. Get yourself a two or three flute carbine end mill and set it up with a corner on center then have at it. Get to a depth and diameter that still leaves meat on the barrel for the reamer. How much is debatable but reamers need to have a certain amount of chip load in order to mitigate chatter. Experience comes with this because a 22PPC is a bit different from a 338LM AI.

Also be mindful of the size of the reamer and what this does to surface speed on the tool.

your hole will be concentric so as long as your reamer is set up on center to spindle bore line it will feed concentric eventhough the pilot is hanging in space. (trust me)

done this way I have cut a pile of chambers that barely make a .00005" resolution indicator move.

These guns have won Olympic gold, World long range championships, and yada yada.

there's no voo doo or dead chicken blood in my shop. Just a little of my own from being a clutz.


Coolant and chip evacuation. You stick a chip and you will ruin the chamber. It'll have more rings that Saturn. I designed and built a through barrel coolant delivery system that delivers up to 1800lbs of oil pressure to the tool. I also had specialized reamers made with through coolant in the smaller varmint cartridges because of how shallow the chip gullets are due to the smaller diameter of the tool.

One thing to think about is when you push that kind of oil pressure to the tool you hydraulically stabilize the pilot. just like a crankshaft bearing does on an engine. (HINT HINT)

I'm not saying this is the perfect way or that it is the ultimate way, but its a way I came up with that overcomes certain challenges and it's produced guns that have placed in marquee events so its certainly not hurting anything and its FAST when done with some creative CNC programming.

I could do this entire process (set up to completion) in 20 minutes (including the crown) at Nesika. That's no bull. Fluted barrels take longer because I always clock the flutes to the action and that just takes eyeball experience to get right. Muzzle brakes add about another 15 minutes to the job.

You'll find that a lot of gunsmiths won't talk openly about this process because they feel they have some proprietary edge over the next guy by being tight lipped, or they are just full of chit and don't wanna get called on the rug for spewing BS.

I'll blab away cause what I'm talking about here takes a machine that costs close to six figures. Then you have to dump another 15-20 grand in tooling and specialized fixturing and coolant pumps/filter/lines and convince your boss all the while that you have not lost your mind. 99 percent of the gunsmiths out there can't afford that so this process is essentially meaningless to them. that and a lot of guys are very old school manual machinists. CNC programming is intimidating as hell when you first get into it. (trust me, I used to be one of those manual machinists guys) You get a decimal point on the wrong side of a zero or a "-" where a "+" should be and suddenly you just caused 10K worth of damage to a machine.

This sucks believe you me!

I think personally that any chamber cut to within .001" TIR of the bore will shoot well. That's assuming that the rest of the gun is built right, the ammunition is prepared properly, the glass is working, and the guy yankin on the trigger knows what he/she is doing.


Last edited by NesikaChad; 03-02-2009 at 10:17 PM.
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Unread 02-20-2009, 04:16 PM
Junior Member
Join Date: Jan 2009
Posts: 21
Re: Chambering goals

Great post Chad. It must be nice to have that caliber of equipment at your disposal. I own/operate a small welding and machine shop, and do my own rifle work. I have all older manual machines, but they are tight, and in good shape. I'd love to run a machine of that caliber, but I'll have to just do the best I can with what I got right now.
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Unread 02-20-2009, 04:31 PM
Platinum Member
Join Date: Apr 2005
Location: Alaska
Posts: 4,618
Re: Chambering goals


How will you be doing it when your return from Iraq and get back into gunsmithing work in your own shop? Sounds like if you had all the money required to purchase that quality equipment in the first place, you could think about retiring... that's a bit of an exaggeration...

Interesting information. I always enjoy catching your posts.
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