Before I ream my throat by .107", please check my math

[Nomad47]

I want to have the boattail/bearing surface junction of the bullet above the neck/shoulder junction of the case. I have checked and re-checked and 107 thousandths seems like a lot so I hope someone can double check my math.

6-284 Shilen barrel
105 Berger Hybrid
My ogive measurements are with my Stoney Point bullet comparator which measures .234" I.D.
My current CBTO (Case base to ogive) to lands is 2.328", and I am seating for a .020" jump, so my loaded CBTO is 2.308".
Berger reloading manual and my measurements on my cases have the case base to neck/shoulder junction at 1.917".
My bullet comparator has the bullet base to ogive at .648".
Berger says the boattail length is .180". So I have the boattail/bearing surface junction to ogive at .468".
So I think I need a CBTO of 2.385" for the bullet to be seated at the neck/shoulder jct AND the bullet touching the lands.
I am then ADDING .050" - up to a .030" jump and .020" in front of the neck/shoulder jct in case of the 'dreaded donut'. Total = 2.435"
New CBTO 2.435" minus old CBTO 2.328" equals .107".
And is .020" enough room for a potential donut?

 

[Nomad47]

I would like to add that I don't know why Shilen would chamber an 8 twist 6-284 with such a short freebore. I would expect anybody chambering a 6-284 in an 8 twist to be shooting really long bullets.

 

[watercam]

When in doubt, use the Hornady OAL tool to put a bullet in the case set to the adjusted length, hold another bullet next to the case at the same height as the bullet tip and you will see how much bullet remains within the case.

 

[WayneShaw]

With a barrel burner like that, I would leave a lot of growing room.

 

[Nomad47]

Watercam, yes, I did that. Even locked my micrometer to 2.435" and adjusted the OAL gauge to that length with bullet in the case. I also seated a bullet in a split case to see what it looked like. Trying to cover all the bases before making the cut. It's easy to cut more throat - hard to put it back.

WayneShaw, very good point. After posting and doing more thinking, I'm leaning toward not making room for the donut. The idea that if and when I have donuts in the brass, the throat will have grown enough to be out beyond the donut. Again, I can always take more off - hard to put it back.

 

[Nomad47]

In keeping in mind that the 6-284 is a hotrod and a barrel burner, therefore prone to throat erosion, I stopped my throat ream at a CBTO to lands of 2.388". So I have lengthened my throat by .050" and seated rounds look a lot better now. We'll see how they perform in two or three days.

 

[dieselten]

How did you ream the throat? Did you use one of those hand throating reamers?

 

[Nomad47]

Yes, a hand throating reamer. I got lazy and didn't remove the action from the barrel and I also didn't remove the stock.

The reamer has a flat spot on the shaft. I was able to use a 1/4" socket with a metal shim to lock the reamer and socket together. Then a socket extension and a nut driver got me to the end of the action.

I shot a 5 shot group yesterday with 51.6 grains of RL-22 and 215M primers, 105 hBN coated Hybrids seated .020 off the lands. The group showed a lot of promise but I was shocked at the velocity - 3317 fps. It was 90 degrees here in AZ but I still wasn't expecting that kind of velocity. Either my chrony is way off or my old version of QuickLoad needs replaced.

Actually I want to replace my old version of QL which does not work on Windows 7, but I can't find my old disc, so it looks like I will have to pay full price. And I want to buy the new Magnetospeed chrony. But I'm getting some static from the wife as we just spent $2500 on motorhome repairs and have a $2000 insurance bill due next month.

I wonder if anyone with a newer version of QL could tell me what their QL says velocity and pressure are for the above load from a 26" barrel. 3.099" COAL, 66.0 grains case capacity.

 

[Erik Cortina]

Bill, use non magnum primers (210M or BR-2) you'll be happier.

You don't need new version of QL, you can calibrate QL to match your setup.

 

[Nomad47]

Thanks, Erik, always appreciate your comments.

Before reaming the throat I tried BR-2 primers - the 215Ms looked better. And my cheap chrony must be way off - I had to tweak QL a whole bunch to calibrate to that velocity.

BTW, will you be in Phoenix on 11/17 & 18?

 

[Erik Cortina]

Thanks, Erik, always appreciate your comments.

Before reaming the throat I tried BR-2 primers - the 215Ms looked better. And my cheap chrony must be way off - I had to tweak QL a whole bunch to calibrate to that velocity.

BTW, will you be in Phoenix on 11/17 & 18?

No, I wanted to go but it will not work out. I was going to meet Steve Blair and shoot together but you know, work before play!

 

[Nomad47]

I remember those 'work before play' days all too well. Too bad. I was looking forward to seeing you again and meeting Steve.

 

[Erik Cortina]

Steve is a great guy and super smart, you'll like him.

 

[Nomad47]

Thanks, Erik. I think there is some sort of long range shoot at Ben Avery early next year. Be sure and let me know if you will be at that one. We'll be in AZ until some time in March.

 

[Erik Cortina]

I'm definetly planning to be at the Berger SW Nationals. That's will be in February 5-10, 2013. Hope to see you then.

 

[Nomad47]

I will plan on being there at least one of the days.

BTW, Walt Berger tells me he has never shot steel at long range. His schedule is very busy until the end of the year, so I plan on taking him out on BLM desert land sometime in January and setting up my portable steel. But maybe he can talk Ben Avery into letting us set up there.

 

[Rustystud]

Just a note:

You are moving a tapered cylinder inside a cylinder, not an easily measured fixed measurement.
I make the following suggestions.
1. use a piece of sized virgin brass with the neck turned .002 smaller than chamber as a test to insure you have no friction other than the bullet.

2. Use a nylon brush and clean out the throat and chamber before measuring.

3. Use a controlled method of cutting with a throating reamer. ie: reamer stop, dial indicator

A throating reamer cuts rifling like a knife in hot butter. Don't expect to do this by feel. If you do you will be sorry.
Believe me I have been there.
Nat Lambeth

 

[Nomad47]

Thanks for the advice, Nat. But it was a tad too late. I pretty much followed your first two suggestions. But I did ream by hand - VERY CAREFULLY - cutting a little, then checking. Did this three times to get to an increase in free bore of .060".

Shot two more 5 shot groups today and I now believe my chrony and QL. After adjusting burning rate to 12% over default and a couple other small tweaks, QL tracks my two group velocities which were 2.2 grains apart.

 

[lurcher]

Say you use a wilson bushing (which does not size the entire length of the neck) in a redding FL die. Or size by bumping with a redding body die and then a lee collet die with a washer placed over the case again to avoid sizing the base of the neck.

Would this avoid the issue of the donut with out the use of the reamer even though the bullet will still be seated deep?

 

[Nomad47]

I don't have any personal experience along that line, but it is my understanding that it could work.
Another benefit for me besides more case capacity is my loads are more concentric not being seated as deep in the case. Also easier to make concentric when they aren't.