Trim Length vs Neck Tension

[Phil3]

I see much discussed about neck tension, but very little about any effect the trim length has on bullet grip. It seems to me that the longer the neck (all else being equal), the more bullet grip there will be. Perhaps not enough though to matter. Consider a 223 Remington case. I used case dimensions as shown on the 6mmbr home page.

223 Remington Case Neck/Bullet Contact Surface Area (sq. in.)
1.7598" Case Length = .1426
1.7500" Case Length = .1357

The longer neck has 5.08% greater contact area with the bullet than the shorter neck. Enough to cause performance differences? I have no idea, but if the purpose is to make one round identical to the next, it seems to me having consistent trim lengths are desired. ???

The effect per .001" change in trim length is less as the cartridge grows in size. The 260 Remington has a trim length min to max of .010". That changes case neck to bullet contact area by a total of 3.96%.

Given that I have found new Winchester 223 cases very nearly exceeding the maximum length of 1.760", while others were at 1.752", it seems prudent to at least check these, if not trim.

Phil

 

[searcher]

These are very valid points for sure. I shoot a lot of tiny varmint bullets that I seat WAY out in .223 and .243 cases. barely anything left to hold them in. Same thing when I shoot 62 Bergers in my 6ppc. So if I seat my bullets out with only half of my neck (or less) making contact, the contact percentages change dramatically more. A long time ago, I bought one of those K&M presses that have the crush washers and you can affix a dial indicator to the top to check for differences in seating pressure between rounds when loading. Quite interesting playing around with different length cases, using different chamferring technique, etc.

 

[Luke_NL]

Phil,

Solid point. I had the same qualms about chamfering, as chamfers that aren't uniform would also make for differing amounts of contact surface. I resorted to taking a bit off of my chamfer tool, tapping a small thread in the end so i can screw in different "bottom stops" that I made for different cases. Of course I could have just bought a K&M chamfer tool that has an integral stop but I only saw that after. (I did try the wilson chamfer tool in my trimmer but that never worked well on the brass I had on hand. Also, for longer cases I couldn't screw the stop back far enough.) Also, I wanted to say you've been posting sensible and interesting stuff - in what seems a short time. Helps my learning curve, fwiw. Is your shooting showing the results? Thanks either way. 8)

 

[jcampbellsmith]

I see much discussed about neck tension, but very little about any effect the trim length has on bullet grip....

Phil. It's a question of priorities. If you've cracked neck tension, then by all means move on to the next most significant factor. However, after 7 years of trying, consistent neck tension still has me beat. Regards JCS

 

[DogPatch06]

Nubie question about bullet grip:

If trim length and # of firings is controlled, and one starts to work up hotter loads, does the neck expand to chamber diameter faster leaving less time for the bullet to bear against the inside of the neck as the pressure increases?

 

[Phil3]

Phil,

Solid point. I had the same qualms about chamfering, as chamfers that aren't uniform would also make for differing amounts of contact surface. I resorted to taking a bit off of my chamfer tool, tapping a small thread in the end so i can screw in different "bottom stops" that I made for different cases. Of course I could have just bought a K&M chamfer tool that has an integral stop but I only saw that after. (I did try the wilson chamfer tool in my trimmer but that never worked well on the brass I had on hand. Also, for longer cases I couldn't screw the stop back far enough.) Also, I wanted to say you've been posting sensible and interesting stuff - in what seems a short time. Helps my learning curve, fwiw. Is your shooting showing the results? Thanks either way. 8)

First, thanks for the kind words on my posts. The community here has been understanding, patient, and mature, and for that, I am appreciative. I know for my guns and ranges (100 yards mostly), some of my ammo efforts may have no benefit, but hopefully some things I find and report on will be of benefit to others. And maybe to me too, once I find myself living closer to a range that has distances at least as long as a fairway at a golf course.

Your chamfer idea works but have a question. Does the "stop" spin with the chamfer tool and spin inside the case against the bottom? Having a stop is nice since one can cut too much pretty quickly. And of course you need all the cases to be trimmed to exactly the same length, if I understand your process. I was thinking of affixing the chamfer tool in a drill press and hold the shell on a predetermined spot on the press table. Set the drill press quill stop where you need it. Hold shell on table, lower quill, chamfer, go to next case. Of course, you need to chamfer both inside and outside, and have not thought through this process.

Phil

 

[ridgeway]

I see much discussed about neck tension, but very little about any effect the trim length has on bullet grip....

Phil. It's a question of priorities. If you've cracked neck tension, then by all means move on to the next most significant factor. However, after 7 years of trying, consistent neck tension still has me beat. Regards JCS

Annealing regularly helped my consistency in a big way.

 

[Luke_NL]

Your chamfer idea works but have a question. Does the "stop" spin with the chamfer tool and spin inside the case against the bottom? Having a stop is nice since one can cut too much pretty quickly. And of course you need all the cases to be trimmed to exactly the same length, if I understand your process. I was thinking of affixing the chamfer tool in a drill press and hold the shell on a predetermined spot on the press table. Set the drill press quill stop where you need it. Hold shell on table, lower quill, chamfer, go to next case. Of course, you need to chamfer both inside and outside, and have not thought through this process.

Phil

The stop looks like the length gauge for the lee trim, except it is smaller in diameter as I don't want it rubbing the sides of the case mouth. Basically it is a pin that tapers from say 5mm to 2mm at the end to allow for centering in the flashole. Easy to turn on a lathe (even for me) and it works. It doesn't 'rub the bottom in a disturbing way, I do clean up the flasholes with a very light touch of the deburring tool (not removing metal) so the height of the stop is not affected by grit or residue. I'd be hesitant to use a drill press for reasons of mass and torque: lowering the drill without proper control might deform the case mouth as it's heavy, solid and brass less so, also when using a drill press taking off to much happens in an instant. Alignment might also be more trouble than it's worth.

As to whether my shooting improves from all I glean here and (try to) implement, :-\. But I do think I know more so I have lots more variables to blame except the shooter 8). Joking of course, I actually enjoy bringing things to the level of "no-one to blame but me". Surprising amount of work that takes, though.

 

[tenring]

At extremes, trim length can affect tension. Not all my brass is consistent to .001 or .002", and shoots well. When it gets to .005" difference, I throw them away or trim all down to uniformity.

One area where trim length is important is if you turn brass. Here a thou difference in trim can change the distance the case will bottom out in the turning tool.

 

[Elkbane]

Phil,
Good point. Does it matter? I honestly don't know, but I trim my brass to within +/- 1 thou because its a cheap and easy way to mitigate controllable variability. And on some short-neck calibers (like 7MM WSM) a few thousands could be significant. If your powder charges were "off" by +/- 3% would you do anything about that? What about seating depth? Like I said, I do it because I can nearly eliminate another source of potentially meaningful variability.....

Re' depth of chamfer, I fixed that by using one of these:
http://www.kmshooting.com/catalog/case-neck-chamfer-tools/controlled-depth-tapered-reamer_large_6mm-338.html
Not only does it center by indexing off the flash hole, it has stops built into the tool to set depth of cut. On uniformed necks, the chamfer is spectacular...and it's adjustable for different calibers by sliding the tapered cutter head in or out of the tool. Couple of turns on a 45 degree tool to knock off the major chaffe, then a few turns on the K&M and they are GTG.

Elkbane