How is this Possible???

[INTJ]

A couple weeks ago I chambered a barrel for my 6BRA LRBR light gun. It was a .237 Krieger, 4-Groove, 1.35" straight; cut to 28". My plan was to use it for NBRSA matches with their new 22.5lb LG weight limit.

Well, my 1.35" BAT B just didn't hold that 11 lb barrel very well. Add to that I am running out of time, so I went ahead and chambered a .236, 4-Groove, HV contour Brux, also cut to 28".

I have about a .001" longer chamber in the Brux. I used the same reamer and HS gauge. I did have the lathe much better aligned when I cut the Brux.

I just finished checking the throat length for the Brux. It's throat is .010" longer than the Krieger. I used both the Hornady tools and the Wheeler method on both barrels. Fireforming bullet is a Hornady 108 ELDM, and that is the bullet I used to measure the throat.

I went back and rechecked head space on the Brux. It's exactly like it was on Sunday, about .002". (I didn't get any more crush from when I had the barrel hand tight on the lathe vs torqued in the barrel vise. I think my realigned lathe cuts a little more precisely.)

So how does .001" more headspace give me .010" more throat?

Is it a Brux barrel thing?

 

[TylerFromMS]

Is there a chance the freebore is cutting farther down the barrel due to the tighter bore on the Brux? I dunno- just spitballing.

 

[INTJ]

Is there a chance the freebore is cutting farther down the barrel due to the tighter bore on the Brux? I dunno- just spitballing.

I am thinking along those lines too.

 

[INTJ]

well if it was me i would know i had the HS on both those barrels way closer than .010 just like you know that. Id be looking at the measuring system. i dont even use the hornady tool anymore as i could never get reliable repeat results with it. i do use wheeler but its not always absolute with my touch anyway. JMO

I very sure of my measurements. They have repeated several times. Also, both the Hornady tool and the Wheeler method are giving me the same measurements.

 

[Dusty Stevens]

Is that brux one of those .236 bores they love so much?

 

[INTJ]

Is that brux one of those .236 bores they love so much?

Yes. It's my first. Though I'd give one a try.....

 

[boltfluter]

If you did the math, by trigging it out, that would be about right with such a shallow angle. Food for thought. Hmmmm! Lol. Same theory as my Vortex fluting, where a .022 depth of cut gives a flat flute about .350 wide.
Paul

 

[INTJ]

If you did the math, by trigging it out, that would be about right with such a shallow angle. Food for thought. Hmmmm! Lol. Same theory as my Vortex fluting, where a .022 depth of cut gives a flat flute about .350 wide.
Paul

So the Brux's tighter bore is the cause?

 

[jackieschmidt]

No, the throat is cut away with the reamer. The size of the original material makes no difference.
look at it this way. If you bore a hole to 1.000 starting with a 3/4 hole and then take another piece of material and bore it to 1.000 but started with a piece of material with a 7/8 hole, both are still 1.000.

As Boltfluter alluded to in post #7, The culprit in your situation is the .001 added to the headspace. Since you are dealing with a very shallow angle in the throat, it will result in the bullet moving in a certain percentage more before the ogive actually touches the lands If the measurement is taken from the bolt face.

 

[GenePoole]

I'm envisioning this in my head and the trig doesn't work out. For the same bullet, a 0.001 difference in headspace equates to a 0.001 difference in ogive contact no matter the angle of the throat. I'll have to get out a paper and pen and check the math, but intuitively, it doesn't make sense.

 

[Alex Wheeler]

I'm envisioning this in my head and the trig doesn't work out. For the same bullet, a 0.001 difference in headspace equates to a 0.001 difference in ogive contact no matter the angle of the throat. I'll have to get out a paper and pen and check the math, but intuitively, it doesn't make sense.

Agreed, the angle doesnt change, it all moved forward .001". Now, if the reamer got a little loose in the front and cut a little fat, the headspace wouldnt get affected near as much as the bullet touch point.

 

[jackieschmidt]

Agreed, the angle doesnt change, it all moved forward .001". Now, if the reamer got a little loose in the front and cut a little fat, the headspace wouldnt get affected near as much as the bullet touch point.

This is kinda of a brain teaser. If I had an optical comparator, I could plot both chambers and then see the actual point of contact.
I am thinking you might be right.

 

[GenePoole]

So, it looks like an OOR situation of less than 3 tenths can throw the ogive contact point off by 0.010 (assuming 1.5° throat angle).

 

[Walt Krafft]

What size bushings did you use for the different size bores?

 

[Edd]

So how does .001" more headspace give me .010" more throat?

It won't, but a .0005" larger freebore diameter will.

Post #11 and 14 may describe how that happened.

 

[INTJ]

What size bushings did you use for the different size bores?

What size bushings did you use for the different size bores?

Don't remember the exact numbers, but one was .237ish and the other .236ish

 

[TylerFromMS]

Throwing 1 more thing out there… did you use the same bullet and piece of brass?

 

[INTJ]

Throwing 1 more thing out there… did you use the same bullet and piece of brass?

No, because the same bullet gets ogive impressions that can change the reading. However, I did check it with more than one bullet and they all read the same.

 

[INTJ]

The guy at Brux told me it was the smaller groove diameter that made the throat longer. I think he is saying that the 1.5 degree angle would be longer in a .236 vs a .237, and since it's an angle it could allow the bullet to move farther forward.

 

[INTJ]

Agreed, the angle doesnt change, it all moved forward .001". Now, if the reamer got a little loose in the front and cut a little fat, the headspace wouldnt get affected near as much as the bullet touch point.

Is there anything I could see visually with the Hawkeye as evidence the reamer got loose in the front?