Pre bore the chamber results?

[Walt Krafft]

So let me wander a bit more. I've always wondered on the Brownells Chamber sheet about the .708 c'bore depth. Line 4 says A minus C which normally will run about .150" and then add .010" more for a depth of .160". You already have accounted for the .010" in line 2 so why do it again. I'm drawing it up in CAD and now there is .020" gap. I'm programing the .708" c'bore and pre drill and bore the taper a little under size with the same tool.
So do you guys use the .150" difference from the front of the bolt to the lug face knowing the tenon is already short or do you do like Line 4 and go another .010" deep for a .160" c'bore depth?

On the late model RR and RAR actions I have been doing, I cut the cbore 0.704-0.710 dia 0.155 deep. I then ream the chamber so the go gage is 0-0.005 below the face. I then cut the face so the go gage is only 0-0.001 deep. Then face the shoulder to dimension. Usually 0.697 plus the recoil lug thickness if one is used. This is how I do prefit barrels for about 100 actions being used for ammunition testing.

 

[Alex Wheeler]

Agreed.

As it is with nearly everything I read on the internets that is given so much importance concerning this topic of barrels and "what makes them shoot" .

I dont think you will get much argument from anyone that they have to be straight to shoot. Thats something the customer has no way of measuring anyhow. You may just need to switch up your methods to still get a throat thats centered in the bore. Most will agree that matters, and the customer can see that with a bore scope. It would be nice if they were all perfectly straight, you could dial anywhere and it would not matter. It would be nice if all the lands and grooves were all even and the correct radius as well. Would make it fool proof.

 

[DSMWerks]

Yes, this is on the lathe today. It has an intregal lug. It is a Defiance AntiX. They are as good as it gets for being to the print +-.001. This one is well with in .0005 total. B minus shank length gives .0103" go gage protrusion. A minus C equals .1497 + .010" like line 4 says is .1597". That gives .020" between C and the .708 c'bore face when you draw it up in CAD. I think the minus .010" in line 2 already accounts for the gap needed. A minus C is exactly .150" so why add another .010"? There is already a .010" gap.

 

[LVLAaron]

I dont think you will get much argument from anyone that they have to be straight to shoot. Thats something the customer has no way of measuring anyhow. You may just need to switch up your methods to still get a throat thats centered in the bore. Most will agree that matters, and the customer can see that with a bore scope. It would be nice if they were all perfectly straight, you could dial anywhere and it would not matter. It would be nice if all the lands and grooves were all even and the correct radius as well. Would make it fool proof.

But then any fool could do it.

 

[jackieschmidt]

Agreed.

As it is with nearly everything I read on the internets that is given so much importance concerning this topic of barrels and "what makes them shoot" .

^^^^^^^
years ago, I thought with my capabilities I could analyze various Premium Barrels to ascertain which ones were worth the effort to mess with. In short, looking for that ever evasive Hummer.

I could not. When it came the barrels produced by the top manufactures such as Krieger, you simply establish the very best chamber, threads, shoulder, crown, etc, take it to the range, and shoot it.

If one seemed to perform better than others, I had no way of knowing why. It had to be some intangible that was beyond our control.

I have had a few barrels that were absolutely phenomenal. One is a 30 cal 1-18 Krieger that is now on it’s third set back. It’s down to 19 inches. It still shoots Really well.

Why? I wish I knew.

 

[SKWERLZ]

^^^^^^^
years ago, I thought with my capabilities I could analyze various Premium Barrels to ascertain which ones were worth the effort to mess with. In short, looking for that ever evasive Hummer.

I could not. When it came the barrels produced by the top manufactures such as Krieger, you simply establish the very best chamber, threads, shoulder, crown, etc, take it to the range, and shoot it.

If one seemed to perform better than others, I had no way of knowing why. It had to be some intangible that was beyond our control.

I have had a few barrels that were absolutely phenomenal. One is a 30 cal 1-18 Krieger that is now on it’s third set back. It’s down to 19 inches. It still shoots Really well.

Why? I wish I knew.

Barrel voodoo!

 

[butchlambert]

To me this is exceedingly simple. Rule #1 is to make sure the throat is dialed in concentric to the lathe spindle. Preferably with a high quality direct reading indicator. After that, there are a couple of options.

1. Drill and prebore. This makes the breech area, both the tenon and chamber, concentric to the dialed in in throat so the chamber is straight.

2. Dial in the breech as well as the throat and use a finish reamer without preboring. If you make that 3" of barrel straight, then the tenon and chamber will be straight.

I use method 2 right now. I personally find it is faster and there are less things to mess up. And yes, fired brass from these chambers comes out at or under reamer spec.

I have chambered a several dozen hunting rifle barrels over the last few months, many of which are those stupid, button rifled, carbon wrapped POS barrels with more than .050" of muzzle runout when the throat and breech are dialed in, AND have uneven rifling. They all shoot sub 1/2 MOA at 650 yds when we do the load development.

My personal LRBR barrels shoot very competitively as well. While I don't pretend to do as good of a job as @Alex Wheeler or anyone else, it's isn't my chamber jobs that open my groups. My competition group size is determined by how much effort I put into tuning and how well I adjust to the conditions.

BTW, because he was mentioned, Bruce Thom absolutely knows what he is doing. If there is disagreement with what someone thinks they heard him say, well, go to the source.

I watched him set up a big heavy lathe to cut chambers with a rigid reamer setup. That method in that kind of lathe absolutely requires drill and prebore. The chamber jobs from that lathe come out perfect with no tool marks, even when a new lathe operator cuts the chamber.

OK

 

[urbanrifleman]

My brain exploded.

 

[butchlambert]

Well that's the point, isn't it? Like you have said, we all need to find a way to cut chambers that make use feel warm and fuzzy.

I would add to that to keep looking for a better and trying new stuff. We are either going forwards or backwards.....

Did I disagree with you?

 

[Edd]

You guys ain't got that barrel chambered yet?

 

[LVLAaron]

Every way but my way is the wrong way!

 

[WSnyder]

Every way but my way is the wrong way!
View attachment 1432925

Reminds me of this kids book

 

[butchlambert]

No, and I didn't disagree with you either. Should we find something to disagree about?

I hope that is not what you want. I'm old and get confused easily.

 

[DSMWerks]

Well I've got to say I'm a fan of drill and prebore. Saved time and very happy with the results. Just finished a 6.5 PRC that was the first chamber I used my new flush system. Only got a face full once! Need to work on the plastic soda bottle splash guard a little more. What really felt good was putting the taper in the pre bore. Having CAD and CNC ability I can program the body profile .010" undersize. If you put the reamer in by hand it fits like a glove, no wiggle. Used my new JGS holder and the neck runout was Zero and the lands were .0002" TIR. Finish was great with the flush system and Astro Swiss oil. Better than the heavy sulfer based oil I have been using. Reading threads and input from you guys I think really stepped up my chambering.
thanks

 

[LVLAaron]

Nice! I found if you put a splash of ATF into the mix, you can run lower pressure, the chips evac a lot easier... so you wont get a face full of industrial moisturizer : )

 

[jackieschmidt]

Well that's the point, isn't it? Like you have said, we all need to find a way to cut chambers that make use feel warm and fuzzy.

I would add to that to keep looking for a better and trying new stuff. We are either going forwards or backwards.....

I would add that often a craftsman is faced with figuring out how to do a certain job using the equipment on hand, which often accounts for the various different methods used by different persons to arrive at the same destination.

 

[Road_Clam]

So after reading 4 pages of method arguing, i'll play the anti-christ . For those that feel its mandatory to hold a few tenths of chamber to bore concentricity , how do you know for fact your bullet seated inside a stamped brass casing that typically shows a .0006" deviation in wall thickness is chambered and locating perfectly concentric with the chamber bore rifling ? Unless you jam into the lands your cartridge (bullet nose) "could" be off center of bore axis. If I have this wrong, then correctly educate me. In theory we all would like to assume we are seating our bullets perfectly concentric but in reality they are probably not . To my knowledge theres no real definitive method to accurately measure seated bullet concentricity.

 

[LVLAaron]

You're right... the way I look at it....

There are a bunch of methods...
Do your own experiments and trials...

Some barrels will shoot with a 5 side, crooked, ugly chamber. Some wont shoot with a perfect chamber.

You've gotta find the method that produces the the least amount of bummers and the most amount of hummers.

 

[swd]

So after reading 4 pages of method arguing, i'll play the anti-christ . For those that feel its mandatory to hold a few tenths of chamber to bore concentricity , how do you know for fact your bullet seated inside a stamped brass casing that typically shows a .0006" deviation in wall thickness is chambered and locating perfectly concentric with the chamber bore rifling ? Unless you jam into the lands your cartridge (bullet nose) "could" be off center of bore axis. If I have this wrong, then correctly educate me. In theory we all would like to assume we are seating our bullets perfectly concentric but in reality they are probably not . To my knowledge theres no real definitive method to accurately measure seated bullet concentricity.

Neck clearance is typically.003”-.005”. That leaves the free bore and throat to straighten the bullet to the extent it can. As far as holding our set up tolerances to a few tenths or less, well it beats the hell out the other option of not dialing a set up as close as we can. If I told you I only dial to .001” would you you be happy or would you have more confidence in my chambering if I dialed to .0001”?

Also as far as holding set up to a tenth or less it just depends on what your doing. You can build half in rifles with .001” or more runout but your repeatability goes way down and come back rate goes up. It’s about reducing variables. If your a professional doing it for a living it’s part of the job if your building accuracy rifles. A lot of guys here myself included bank our reputations on guns that can win at a championship level.

 

[STS]

The old saying "You don't know what you don't know" is often applicable to rifles. Sometimes they won't do what they are supposed to do, and other times they will do what they shouldn't be able to do. I have a 6 BR barrel that I had chambered about 25 years ago before I got my lathe. It is a Shilen Select Match take off benchrest barrel. The fellow that chambered it clearly missed the mark with regards to the set up. You borescope that barrel and 2 of the lands extend back to the end of the neck, and the 2 on the opposite side probably have .050 freebore. It shouldn't have shot worth a crap, but it shot lights out. You just never know.