Pre bore the chamber results?

[SKWERLZ]

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.

I had a 260 Shilen prefit like that, lands clearly showing chamber was far from concentric to bore, it was the best of the 1/2 dozen prefits I had from them.

 

[DSMWerks]

I think it comes down to process of elimination. Start with the things you have control over and get them as good as you can so you know what your dealing with then move on and try to improve the next thing down the line.

 

[Dusty Stevens]

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.

Why not get your necks turned a bit better than that? Eliminate as many variables as possible

 

[Road_Clam]

Why not get your necks turned a bit better than that? Eliminate as many variables as possible

So typically which datum locates the round cambering , the neck , or the body ?

 

[Dusty Stevens]

So typically which datum locates the round cambering , the neck , or the body ?

Shoulder

 

[Spotshooter]

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.

Road_Clam -

With Bench Rest and F Class shooters -

They know the bullet is as coaxial as possible to the bore because -

1 - When they neck size they don’t resize the neck all the way down so when it’s chambered it sits on the old neck which is already almost perfectly concentric with the bore axis the chamber was cut too

2 - When they shoulder bump they do so to a depth that the reloaded cartridge acts like a tightly fitted go gauge and the bolt closes with a very light resistance… which aligns the cartridge even more.

3- Their chambers are cut with match reamers which means they were designed with smaller brass to chamber tolerances

4- they use custom made re-sizing dies that perfectly match the chamber reamer cut so the is no front to back variation in the brass at the chamber.

All these when added together result in a near perfect alignment between the chambered round and the bore…
Number 3 &4 - are a big part of why the new Creedmoor & PRC rounds are “more accurate”…. Well the industry learned that removing slop between the brass max, and chamber min. Specification results in a better shooting rifle.. Basically they took what a bunch of good gunsmiths already knew and applied it to the new spec’s… older smiths did it in their reamer spec’s years ago..
In the end the new chambering specs reduced mis-aligned cartridge to chamber & bore standards… people noticed…


OK - my turn

I’ll play the “Engineer” to your Anti-Christ… (just in case those things are actually different)…

We know all slop is simply greater variations in tolerances of how 2 parts that fit together, and the more parts the more slop - Hence tolerances are stackable…

Meaning the more variation you have the worse it becomes…
conversely - the more you eliminate the better the system performs…
Relatively - The tighter your tolerance - the more “prefect” the chambering job will help a rifle shoot better but other slop factors can hide it… again already proven..

Now I get most people can’t shoot as well as a factory gun chambering Job… this is obvious…
But I have yet to see a guy be disappointed when his rifle shoot better than it ever did before…

Knowing on average that these improve things so they help a rifle run better, but other factors may overcome them… - who are we to say they do not ?

Remember bench shooters (a control group) proved they do improve how the rifle shoots.. otherwise the PRC and Creedmoors wouldn’t on average shoot better (they have better tolerances therefore alignment)…

I look forward to reading your letters (tongue in cheek, and often stated by late night TV hosts) for those looking to be offended..

So little time.. too much fun..

 

[SKWERLZ]

Ok, I’m now a fan of preboring, reaming was fast and effortless.

 

[Larry Koch]

I just wasted 15 mins, that I'll never get back.

 

[boltfluter]

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

Congrats on what sounds to be a great chamber you cut! Being able to program the body taper minus .010 is a great advantage. That reamer literally has no choice but to follow the hole you bored, with or without a bushing up front. Sounds like your on the right path to build some world class rifles! Well done. If I were younger, and interested in chambering barrels for a living, I would be doing it with a CNC lathe very similar to how Dave Tooley does it versus using an engine lathe. No offense intended to anyone, just how I would approach the job. Very interesting thread. Carry on men.
Paul

 

[Will Henry]

Congrats on what sounds to be a great chamber you cut! Being able to program the body taper minus .010 is a great advantage. That reamer literally has no choice but to follow the hole you bored, with or without a bushing up front. Sounds like your on the right path to build some world class rifles! Well done. If I were younger, and interested in chambering barrels for a living, I would be doing it with a CNC lathe very similar to how Dave Tooley does it versus using an engine lathe. No offense intended to anyone, just how I would approach the job. Very interesting thread. Carry on men.
Paul

If I were younger, I might well do the same. I'm not younger though (74), so I'll just struggle along as I have! WH

 

[Greg Y]

Here's a fun one, I picked this rifle up as an unfinished "gunsmith" project. I was told it was 6.5x284. I fired a round and thought it may be an AI.....then I picked up the brass and realized that someone sunk the drill too deep before boring.

 

[DaveTooley]

Here's a fun one, I picked this rifle up as an unfinished "gunsmith" project. I was told it was 6.5x284. I fired a round and thought it may be an AI.....then I picked up the brass and realized that someone sunk the drill too deep before boring.

Can't say I've never done it but I always caught it before it left the shop.

 

[LVLAaron]

I've gone too far, once. Now I just keep a couple of drills set with stop collars on them so they're always ready to go for the usual suspects I chamber.

 

[Dusty Stevens]

Theres why i dont do it anymore. Ill buy more reamers if i have to. When proof barrels were close to $1000 i did that and delayed the customer another 6mo getting a new barrel. But hey i got a 300 ultra for myself to remember it by

 

[SKWERLZ]

Here's a fun one, I picked this rifle up as an unfinished "gunsmith" project. I was told it was 6.5x284. I fired a round and thought it may be an AI.....then I picked up the brass and realized that someone sunk the drill too deep before boring.

Super duper ackley ultra improved.

 

[Tucker65]

Can't say I've never done it but I always caught it before it left the shop.

Same here, luckily there was enough shank to set it back for me.

I guess that's where the old saying "measure twice cut once" comes in.

 

[butchlambert]

If I were younger, I might well do the same. I'm not younger though (74), so I'll just struggle along as I have! WH

You're still a kid Bill.

 

[Edd]

Theres why i dont do it anymore. .....

I heard about a kid that mashed his finger closing a car door. He never opened the door again. He always got in and out of the car through the window,

 

[Will Henry]

Here's a fun one, I picked this rifle up as an unfinished "gunsmith" project. I was told it was 6.5x284. I fired a round and thought it may be an AI.....then I picked up the brass and realized that someone sunk the drill too deep before boring.

Even without the drill work, that's not a pretty chamber. I did something similar once but caught it when I tested the rifle. I bought a new barrel on that one.
Butch,
Some days, I feel like a kid; until I try to do something that requires strength, balance, or reflexes! WH

 

[jackieschmidt]

I've gone too far, once. Now I just keep a couple of drills set with stop collars on them so they're always ready to go for the usual suspects I chamber.

This is why I always establish the chamber first, and then work everything else off of it.