Cut case for measuring COAL/CBTO

[Wolfdog91]

Quick question. For you guys using the deal where you take a pice of brass cut the neck long ways so you can squeeze it to hold you bullet but still let it move so you can measure you CBTO/COAL , how are you doing your brass before cutting ? Like do you just fireform and that's it ? Or to you for from them resize it the same way you would if you where reloading it for that gun ?
Thanks !

 

[jds holler]

You must be the same kind of dinosaur I am!

I just full size it. My neck-cut is only done to the bottom of the neck. I also drill out the primer pocket to make it easy to push the bullet out.

I cut both sides, just down to the shoulder. I like to feel decent tension on the bullet.

Now you're about to be asked how come you don't just buy the Hornady or Stoney Point tool. jd

 

[rfwind]

What tool is best to cut the slot in the case neck?
Thanks

 

[GLShooter]

What tool is best to cut the slot in the case neck?
Thanks

A Dremel will work. I bought a jewelers saw 40 years ago that works well. BTW I use a fired case that I size the body on not moving the shoulders when I do this.

I got the Sinclair tool about 7 years ago after using the Hornady set up. They are pretty cheap and I feel give me much more repeatable numbers. I keep exemplar bullets so I always have the same bullet in play for measurement. I have maybe 675 bullets labeled with all their specs done up in a spreadsheet.

 

[Webster]

Quick question. For you guys using the deal where you take a pice of brass cut the neck long ways so you can squeeze it to hold you bullet but still let it move so you can measure you CBTO/COAL , how are you doing your brass before cutting ? Like do you just fireform and that's it ? Or to you for from them resize it the same way you would if you where reloading it for that gun ?
Thanks !
View attachment 1339404

I make a cut with a dremel tool, much less width than your cut. Done many touch determinations with a slit neck. The neck tension seems to be critical. If the tension is just right I can determine touch for 4 different bullets very quickly. If the tension is wrong you get a huge scatter in results. After pushing a bullet into the case about 5 times the neck loses tension and I resize it. Sometimes the case just doesn't work for touch after several good measurements.

A guy on this website made fun of my method but it is accurate and indicates if you did a good job. One guy called it averaging. It's a statistical best fit to the data. I know from experience that if you just put the bullet in the chamber a couple of times and accept the extracted length as touch it's possible to be way off. The R squared indicates the accuracy. Ideally it should be over 98%. If the data doesn't cross the zero value which is touch, you divide the smaller y value into the larger value.


Top graph poor quality results. Bottom graph much better results.



I tried it last week with my 6BRX and gave up because of the scatter I was getting.

Added later: Bottom chart comments. The out column shows the variation I got after extracting the bullet. Each number in the out column should be touch. If the determination was perfect, all the numbers in the out column would be the same. I think the variation is caused by barrel erosion and I don't remove the ejector. The ejector pushes the bullet hard against the receiver when extracting. I push on the case body when extracting to try to prevent the bullet from rubbing against the receiver. The touch value is just a starting point for reference when seating bullets. I would think any machinist would tell you any method to determine touch is an approximation. If you used the square rifling mark method you would get a slightly different result. Each bullet is a slightly different length from ogive to tip and we are usually measuring overall length with a digital caliper.

If touch is 2.169" as was determined you will notice that 5 of the 6 out numbers are bigger than 2.169" indicating that the bullets stuck a little and got pulled out 3 to 8 thousands. None are smaller than 2.169".

 

[Tesoro]

I prefer the stoney point, extra bucks yes, but I can feel the bullet kissing the lands instead of jamming them in with the bolt. In the past, I have used fire formed cases and used locktite with a lightly snug bullet to get a permanent measurement into the lands. I found that when using split case sometimes the bullet would grab on the lands and get pulled out a hair on extraction.

 

[Dave M.]

I just don’t understand why you can’t make a dummy round and polish the bullet tip with 0000 steel wool and put a tiny coat of imperial sizing wax on the bullet and seat progressively deeper until it leaves no mark on the bullet under a magnifying glass. It’s simple and takes me less than 5 minutes to get consistent results. Use very high neck tension to assure the bullet doesn’t pull out.
Dave

 

[Webster]

I just don’t understand why you can’t make a dummy round and polish the bullet tip with 0000 steel wool and put a tiny coat of imperial sizing wax on the bullet and seat progressively deeper until it leaves no mark on the bullet under a magnifying glass. It’s simple and takes me less than 5 minutes to get consistent results. Use very high neck tension to assure the bullet doesn’t pull out.
Dave

I tried this method. Maybe I had bad luck because of erosion of the rifling?

 

[Dave M.]

I tried this method. Maybe I had bad luck because of erosion of the rifling?

I think it’s really easy. Firing pin removed and close bolt fully and open. Either you will see no marks at all or tiny rectangles/squares. Keep seating the bullet deeper in .001” increments when the marks get really faint, because you are getting very close. I’ve found this method to be repeatable to +/-.001”

 

[JEFFPPC]

Will the insanity ever end?

 

[Beiruty]

Someone did the whole research for you. Seat at 40/1K" and call it a day

More Bullet Jump Research!

This is the 4th article in a series on some new research on bullet jumps. Here is a recap of the pr...

precisionrifleblog.com

 

[338 dude]

I just don’t understand why you can’t make a dummy round and polish the bullet tip with 0000 steel wool and put a tiny coat of imperial sizing wax on the bullet and seat progressively deeper until it leaves no mark on the bullet under a magnifying glass. It’s simple and takes me less than 5 minutes to get consistent results. Use very high neck tension to assure the bullet doesn’t pull out.
Dave

Basically the same system ,I just paint the bullet black with a sharpie and seat deeper and deeper until there are no marks on the bullet

 

[winston]

Basically the same system ,I just paint the bullet black with a sharpie and seat deeper and deeper until there are no marks on the bullet

I cut one slit in the neck, drill the flash hole out with a 1/8 drill bit and tap it with an 8-32 tap so I can screw the case on a cleaning rod section to insert it into the chamber to check the max coal for that bullet.