Strange Chamber issue

B

bob3700

Member
Gents,

I recently chambered a 6mm BRX bbl with a new Kiff reamer. The reamer was designed with a .272 no-turn neck.

While forming cases, it was discovered that the neck portion of the reamer was too short and needed to be lengthened.

When the bbl was originally chambered, headspace was set with the go gauge. I had a new Lapua case with a bullet seated so that I could check feeding of a loaded rounds. It worked just fine.

Approx. 25 rds were fired thru the bbl with the original short neck chamber and I got the bbl back in order to cut the neck longer.

Wanting to check the headspace dimension, I put the go gauge in the chamber and closed the bolt. No problem.

Put the same FL resized dummy cartridge (that I used originally) in and the bolt won't come close to closing?

How did a chamber get smaller? Could a piece of brass get larger just sitting on my bench?

If you have any ideas on this, I would appreciate them as it certainly has baffled me.

Thanks

Bob
 
6mmBRX growing

When you fire form a 6mmBRX the case neck should remain the same length only the shoulder should fireform forward .100. You may need to trim the first time after fire forming, but I usually get about 12-16 firings before I anneal the necks, full length size and trim. If you don't anneal the case necks you will develope flyers in my opinion. I can usually buy 100 pieces of Lapua 6mmBR brass and shoot it the life of the barrel +/- 3000 rounds. Then I rebarrel and buy new brass at the same time. I have chambered both .268 neck and .272 neck and see little to no significant difference in accuracy. Especially for F-Class.

Rustystud
 
Sounds to me as if you've got some dirt in the chamber. Chips, or specks of brass, whatever. If the Go goes, then the case should go too, when clean.

About the only way that it wouldn't is if you had a case neck that was really long. Then it would stop the bolt. But, if this went in before you added neck length to the chamber, it certainly should go now.
 
I'm going to assume that the reamer was altered or more likely Kiff provided a new reamer. Then you ran the reamer in to lengthen the neck. If this is the case I would look at the reamer again. Sounds like the new reamer's neck is undersize.
I'm confused now. If the neck was to short before, did you shorten the cases to fit the first chamber? If the case length was correct then you would have to set the barrel back to lengthen the neck. If you set the barrel back and the headspace is correct I would still look at the neck diameter of the reamer.

Dave
 
Dave and Rusty,

Two issues.

First - when you fireform using the false shoulder method, you neck the case up, then neck it down to form the forward shoulder to capture the case against the bolt face. This makes the neck of the case become longer than a new piece of Lapua brass! You have to have the extra room to chamber the case.

The original chamber had a neck length of .197"and an OAL of 1.550".

New Lapua brass runs from 1.556 to 1.565 in length and the BRX has a neck length of .210. So the actual neck dimension was too short. New Lapua brass would not chamber.

In order to fire form a few pieces of brass, we cut the necks down until it would chamber and fireformed about 20 pieces of brass. This was done just to break the bbl in.

Once fireformed, the cases OAL was measured at 1.543, way too short.

Second - When the initial chamber was cut, I had a Lapua BRX case that had been neck turned to .268", full length resized, and seated a 107 SMK. Used this round to check feeding. No problem.

Now after bbl break-in, that same cummy round doesn't fit into the chamber. I cleaned the chamber with a copper bore brush and bore mop. Checked the chamber with my strong light and it is as clean as a whistle.

Tried to chamber the above dummy round and no dice. Bolt is 1/8 inch from closing. It appears that the brass is touching at the mouth of the chamber.

A new piece of Lapus brass will fit the chamber but the bolt won't close (neck dimension is too short) as I have not reamed it with the new reamer.

Puzzling as to why the dummy round won't chamber now.

Bob
 
What you are saying does not seem possible.
If you make a dummy round that fits it should fit wether you run in a barrel or not.
Nothing in the chamber is going to change or the dummy load that is enough to make it not rechamber.
Only dirt , rubbish or maybe powder granuales are jaming the bolt lugs at times or the projectile .
I have made a thousand dummy loads over the years and have never seen this happen , the way you explain it. Most confusing .
Are you sure you are using the correct dummy load and not another one?
Is your barrel tight ?
 
Check the reamer neck specks..

Dave Kiff does great work but I am aware of one 6mmBRX reamer that the neck was cut incorrectly. The reamer owner had me and another couple of smiths cut him chambers and it was not until he loaned the reamer to another (4th)smith that it was noticed that the reamer was out of spec. It akes very little deviation. Measure the reamer and compare it to the print. Make a cast of your chamber and measure the cast.

That is what I would recomend.
Nat
 
J. Valentine said:
What you are saying does not seem possible.
If you make a dummy round that fits it should fit wether you run in a barrel or not.
Nothing in the chamber is going to change or the dummy load that is enough to make it not rechamber.
Only dirt , rubbish or maybe powder granuales are jaming the bolt lugs at times or the projectile .
I have made a thousand dummy loads over the years and have never seen this happen , the way you explain it. Most confusing .
Are you sure you are using the correct dummy load and not another one?
Is your barrel tight ?
Click to expand...

I concour with everything you are saying. That is why I am so puzzled. It is the exact same dummy round that I used to check the feeding cause I color coded it with a sharpie.

Oh well, It will be a moot point as I am going to recut the chamber this weekend and that will be it.

Thanks for all who responded.

Bob
 
Yes a round grows longer while setting on the bench. I'm not saying that this is your issue but rounds definitely grow, sometimes ending up over a half-thou bigger in two days. Different rounds grow at different rates, this is due to differences in the initial fireforming and subsequent resizing methods and in some small part is dependent on shoulder angle.

Now, you say your bolt is "1/8 inch from closing"....... what does this mean? Are you saying your lugs are machined or lapped at a taper? That the bolt handle stops before it hits the bottom of its throw? Or that you're an eighth inch from the bolt handle beginning to drop?

al
 
Rusty,

Here are the latest drawings folks are asking for using Lapua brass. The short necked BRX reamer had been ground to an old Clymer print Bob sent me. The case length on that print was 1.530 long. I have quit grinding that reamer and we threw out all the old prints two weeks ago. It seems the best length and what people are asking for is the print here showing 1.570 long, although most customers vary on neck diameter. This print shows a "No Turn" but we sell quite a few .268 and .269 necks. Also, here is the most popular dasher print for a comparison. It also has the lapua length with no turn neck. I much prefer the dasher, the dasher will perform as good as you can read the wind. The other wildcats that will be dominating the long distance competition in the future are the 6 x 47 Long Dasher and 6 XC with the new throat configuration we are working on. We are grinding the new eliptical throat to match the bullet ogive. This new throat cups the bullet and fits like a glove with complete contact. Well, I've got to go, the phone are going crazy and I've got reamers to ship out.

Thanks,

Dave Kiff
Pacific Tool & Gauge

View attachment 6 x 47 Lapua Ack Imp aka 6mm Long dasher.PDF

View attachment 6mm BRX (No Turn).PDF

View attachment 6mm Dasher (World Record Holder).PDF
 
You can't get better service..

You have heard from the reamer guru himself. Check your reamer and measure the neck. If it is short then send it back to PT&G for regrind. Remember it is easier to take metal off than put it back.

I have two 6mmBRX reamers one with a .272 neck with .120 lead.

The second 6mmBRX reamer is a .268 neck with a .090 lead.

I personally think the BRX and Dasher are tit for tat. The advandage of the BRX is using 6mmBR dies. The Dasher requires special dies. Some say they think the Dasher is more inherently accurace because it is stiffer in the front and the powder burn turbulence is better directed.

Rustystud
 
Smoke the case and see where it is marking the strongest then alter the case and see if that corrects it...

This is just plain weird and makes no sense...
 
If the barrel was broken-in using "short" brass, you may have a very hard carbon deposit in the neck area, ahead of where the short brass ended in the chamber. I had this condition in a Dasher and it was quite apparent when I looked at the area with my bore scope.
It can be a real problem getting the carbon out of this area. A 6mm bronze brush is not large enough to reach the neck diameter.
Just a thought.
 
I ended up putting dykem on a case to see where the interferience was. It was at the base.

Resized the case and it fit the chamber. The original reamer was as Dave mentioned too short in the neck which also made the chamber too short OAL. He reground the reamer and now the neck length and OAL are OK.

The only down side of the whole deal is eating the expense of shipping the bbls and actions around to have the chamber corrected. That isn't cheap!

Bob
 
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