Re-chamber for 7RM Question...

[AZShooter]

This is off topic but now that is has been mentioned twice...

John and MagnumManiac,

What chamberings are you guys setting back and rechambering? Have either of you bore scoped the barrel before and after? I would think there would still be an area of heat damaged rifling in front of the new chamber. I have read the second throat will not last as long as the original. I would imagine is has to do with the fact the rifling is already heat damaged.

Have either of you setback and rechambered a barrel that was "shot out" or are you doing it for a barrel that lost its competitive edge?

I have several barrel that are "shot out". One in particular is a Bartlein in 6mm-284. I considered cutting of several inches and rechambering but have thought it is a waste of effort as the bore scope shows crazing and cracking up the bore for at least 10".

 

[Ingwe]

Just wondering why you ordered a 1:9 barrel to shoot those long bullets?

 

[Innovative]

johnlittletree . . . .

Right. As long as you are able to spend the time, money, and effort, it's better to go for the best accuracy possible. A standard production reamer has way too much neck clearance to consistently provide top accuracy.

Unlike any pre-chambered barrel production barrel, the custom barrel deserves to be chambered perfectly concentric, with bore run-out (within a tenth of a thousandth of an inch). Someone once said, "Only accurate rifles are interesting".

 

[MagnumManiac]

This is off topic but now that is has been mentioned twice...

John and MagnumManiac,

What chamberings are you guys setting back and rechambering? Have either of you bore scoped the barrel before and after? I would think there would still be an area of heat damaged rifling in front of the new chamber. I have read the second throat will not last as long as the original. I would imagine is has to do with the fact the rifling is already heat damaged.

Have either of you setback and rechambered a barrel that was "shot out" or are you doing it for a barrel that lost its competitive edge?

I have several barrel that are "shot out". One in particular is a Bartlein in 6mm-284. I considered cutting of several inches and rechambering but have thought it is a waste of effort as the bore scope shows crazing and cracking up the bore for at least 10".

I do this on my comp barrels only. I set-back 3 times on a barrel, if I think a 4th will work, I do it and see, but this is hit and miss and the barrel may only last half a season instead of a full season. I also don't clean for 200-300 rounds without accuracy issues.
These barrels are in 6.5x47, 264WM and 300WM.
The reason is simply to keep the throat fresh. It's not the haze cracking that 'shoots out' a barrel per se'. It's the throat 'ballooning' that creates the accuracy to drop, a loss in velocity is always accompanied with the accuracy loss. A lot of the time you can increase powder and gain velocity and accuracy back, not always as good as what it was, but you can re-tune.
I have a barrel that has NO RIFLING AT ALL for 4" from end of chamber, it still shoots sub MoA with the right load.
The other myth is that the rifling near the muzzle doesn't erode as some think, it is generally rounded by cleaning rods.
Setting back and re-reaming keeps the throat end square and the transition to the rifling true and at the correct angle.
The haze cracking is generally not as deep as the new throat is, so it cleans it up almost 100%.
I have not tried to set-back a burned out barrel. Don't see the point.

Hope this answers your question.

Cheers.

 

[MudRunner2005]

Just wondering why you ordered a 1:9 barrel to shoot those long bullets?

Yeah, I was wondering the same thing. Nobody has even brought that up yet. I would recommend the OP call Bartlein up and see if they can swap him for a 1:8 if it's not too late.

 

[Innovative]

I use Berger bullets as a benchmark for accuracy testing. They have a spectacular selection of bullets, and every bullet variation is designed for maximum accuracy (including hunting bullets).

Every box of Berger bullets is marked with the optimal twist rate.

I've found that if you are building an accurate rifle, begin with the exact bullet you would like to shoot . . . . then get the best barrel to make it work.

 

[Ingwe]

I use Berger bullets as a benchmark for accuracy testing. They have a spectacular selection of bullets, and every bullet variation is designed for maximum accuracy (including hunting bullets).

Every box of Berger bullets is marked with the optimal twist rate.

I've found that if you are building an accurate rifle, begin with the exact bullet you would like to shoot . . . . then get the best barrel to make it work.

What do you do then if that bullet doesn't shoot well out of your gun/barrel?

 

[Innovative]

Ingwe . . . .

Simply try another bullet of the same length and weight. There is no need to make this complicated.

The selection of 30 caliber bullets is incredible. It simplifies things when you start with what usually works best. Then once you establish a benchmark accuracy load you can experiment.

The whole point is to select the bullet size and type before selecting a barrel. You'll get better results this way.

 

[crazyhorse]

Just wondering why you ordered a 1:9 barrel to shoot those long bullets?

The bullet I'm really wanting to shoot with the new barrel is the Berger 175 Elite Hybrid. Berger lists optimal twist as 1:9 and since this likely the heaviest bullet I'll shoot (my stand by for hunting is the 160 AB) I elected to go with this particular twist rate.

 

[Innovative]

Looks like you're all set I would purchase the reamer to be sure the neck diameter has .0015" to .002" clearance. Also, make sure the leade and the throat fit your Berger 175 Elite Hybrid. This requires you to send one sample round to your reamer maker.

You select the seating depth when you build your sample round, and specify how far off the lands you want to be. This stuff makes one heck of a difference.

It is just as important to get a rifle accuracy specialist to cut your chamber "perfectly" concentric (.0001" or less).

 

[crazyhorse]

Looks like you're all set I would purchase the reamer to be sure the neck diameter has .0015" to .002" clearance. Also, make sure the leade and the throat fit your Berger 175 Elite Hybrid. This requires you to send one sample round to your reamer maker.

You select the seating depth when you build your sample round, and specify how far off the lands you want to be. This stuff makes one heck of a difference.

It is just as important to get a rifle accuracy specialist to cut your chamber "perfectly" concentric (.0001" or less).

Showing how green I am to the process but within the neck tolerances you mentioned (.0015 to .0002) would I have any concerns with lot consistency on brass? I'm currently running regular Winchester brass but don't mind upgrading if necessary.
Second, would I need to order a separate reamer for roughing in or will a finish reamer be sufficient. I understand the concept but not the actual process.

 

[AZShooter]

Crazyhorse,

While it is suggested by others Innovative it is also mentioned on Accurate Shooter website by Jerry Tierney: "Chambering can play a role as well. Jerry Tierney, a past NBRSA 1000-yard champion and .284 Win "guru", has observed that overly tight chamber neck tolerances can cause accuracy problems. Jerry says "some guys who were running minimal neck clearance .284 chambers weren't getting the accuracy they expected. If you open up the neck to allow more clearance, say .0015-.002″ per side, that seems to solve the problems."

You are right when questioning such a small release clearance. You must turn the necks to be able to work with such a small amount. Even the best brass would have some neck thickness variance to cause release issues with accuracy or even worse pressure spikes.

You could specify a bit larger dimension of .0025" to .003"clearance. You will need to measure all your Winchester brass to ensure none are out of spec. You could skim turn the ones that are too thick.

IMO there is no need for a roughing reamer. You are not into production work, just a few chambers.

 

[Innovative]

Crazyhorse . . . .

You're right that case neck thickness does vary. Case necks can also thicken as brass migrates forward at almost every firing. However, these increases are in the realm of ten thousandths of an inch. Measure the neck diameter on one of your fired cases, and you will be looking at the exact neck diameter of your chamber. Then measure the diameter of your handloads. The difference is your neck clearance. (Watch your decimal point when measuring things.)

AZShooter . . . . .

You are right that case necks can be too tight. I once had to purchase another reamer with a larger diameter neck to solve the problem. Tight necks are not a problem if the handloader wants to neck turn cases. In fact, if you neck turn your case necks for a tight neck chamber, it will always produce a more accurate rifle. Just ask any BR shooter.

However, for the handloader who absolutely doesn't want to neck turn, .002" to .003" is probably a better way to go. I wouldn't go larger than that expecting to get maximum accuracy.

 

[crazyhorse]

Thanks for all of the advice so far! I'm learning as I go here while taking the next step in customizing my rifles, all the while trying not to make an expensive mistake!

A few years ago I was able to borrow a reamer from one of the local BR guys and build a 300WSM with a "tight neck" and while it was accurate, I learned to despise turning necks. I later had it set back and re chambered to the standard measurements and didn't notice any discernible difference in accuracy.

My goal for exploring a custom reamer is A) take advantage of a large mag box and extend my seating depth, and B) an attempt to gain consistency from barrel to barrel when a change is required.
Measuring my current chamber shows fired brass to be .314 and a loaded round at .311 which seems pretty good based on what you guys have illustrated as acceptable.
Wether it's coincidence or not, ive noticed getting the bullet up and out of the case and powder seems to be conducive to accuracy. Having worked with explosives for many years (and still have all my fingers), I've noticed a lot of obvious parallels to reloading as it regards to pressure, rate of burn, etc.
What I do know is that when you have the same charge and components, simply changing how they physically relate (position) to each other can make a considerable difference. The added component in making an accurate rifle is after the round is fired it now has to enter and travel down a barrel.
I'm getting way off in the weeds from the original point but it plays into my logic. All that should be clear as mud but I thoroughly enjoy exchanging thoughts and ideas in this forum!