Tell me why my OAL lengthens going through the crimp dye..... ??

[awshoot]

We're talking 0.01" as having suspected yet measurable influence on POI on target? Have you measured a sample of factory rounds to determine their range of compliance with given spec?

I would say it could. When I worked up a load for the Federal Guard Dog bullets Midway was selling at 9c per thousand some time ago, I made tests at various COAL increments (there is no published data for this bullet) and noticed a difference in accuracy. For example, same powder charge, headstamp, and primer:

COAL	Group Size
1.13"	3.0"
1.12"	2.7"
1.11"	2.0"
1.10"	2.1"

EDIT: fixed mistake at 1.11" -- got crosseyed looking at a spreadsheet.

 

[awshoot]

I check OAL on a box of HST and it seems it's normal to vary +/_ .07 or more. I may be letting my OCD get the better of me on that. I read on another forum one reloader describing his willingness to allow a variance of anything in the thousandths. If he wanted 1.125, he would accept anything from 1.120-1.130. Would any of you take issue with that rule of thumb? I still cannot explain the crimp die elongating the OAL. I reset everything and it's not happening now.

I would not take issue with that. Even though I definitely let my OCD run wild when ammo testing it is normal to see a little variation like that in the ammo that comes off the press -- no two bullets or cases are strictly identical. Realistically, and it takes some practice to get the touch fairly right, the amount of pressure you exert on the calipers can change the reading a few thousandths.

Do you have the anvils that attach to your calipers? Those help with the touch issue.

EDIT: in the load I was referencing above, I settled on 1.105 as that lay between two values that gave good accuracy, and being off a few thou either way not likely to hurt.

 

[DizzyJ]

I can't fathom why the crimp station would cause your OAL to grow.

You may have mentioned it but I'm going to ask again, this is with every single cartridge? They ALL grow by approx .010" between bullet seating and crimping?

 

[Fasterthanu]

I can't fathom why the crimp station would cause your OAL to grow.

You may have mentioned it but I'm going to ask again, this is with every single cartridge? They ALL grow by approx .010" between bullet seating and crimping?

Thats what it seemed at first. In further experimenting I now think that my seating dye had creeped out of adjustment, partly contributing to my problem . After setting it back to the target OAL, I did see a 1.124 OAL grow to 1.126 several times in a row. That I cannot explain. I just got done running one batch of FC headstamp and one batch of WIN headstamp through with fairly consistent results. Without adjusting the dies between batches, FC brass loaded in a range of 1.113-1.121 (a fairy wide range I think) and WIN brass loaded in a range of 1.119-1.124. It was curious to me that the FC averaged a much shorter OAL though.

 

[DizzyJ]

Thats what it seemed at first. In further experimenting I now think that my seating dye had creeped out of adjustment, partly contributing to my problem . After setting it back to the target OAL, I did see a 1.124 OAL grow to 1.126 several times in a row. That I cannot explain. I just got done running one batch of FC headstamp and one batch of WIN headstamp through with fairly consistent results. Without adjusting the dies between batches, FC brass loaded in a range of 1.113-1.121 (a fairy wide range I think) and WIN brass loaded in a range of 1.119-1.124. It was curious to me that the FC averaged a much shorter OAL though.

Ok. Good to know!

 

[41mag]

Although I have moderately decent calipers, it is difficult to measure the same object 3 times in a row and obtain the same reading to 0.001"

 

[orygun]

Although I have moderately decent calipers, it is difficult to measure the same object 3 times in a row and obtain the same reading to 0.001"

True enough, but especially when trying to measure a rounded object!
If I was trying to be this precise I would use a micrometer and that's still challenging on non parallel surfaces.

 

[Certaindeaf]

There'll be variation. Especially if you don't use a comparator.. the bullet seating stem seats the bullets by engaging the bullet ogive but you are measuring the very point of the bullet to arrive at oal.
And also, don't crimp, per se, unless it's for a revolver, which uses a true roll crimp.
Also, it's "die", not "dye".

 

[orygun]

There'll be variation. Especially if you don't use a comparator.. the bullet seating stem seats the bullets by engaging the bullet ogive but you are measuring the very point of the bullet to arrive at oal.

Thumbs up on this.
Bullets themselves aren't always perfect and measuring OAL might not give a valid measurement. Besides getting the cartridge up the feed ramp on a self loader, the important part is where the shoulder of the bullet engages the rifling. Too tight (long) and pressure builds quickly.

 

[Fasterthanu]

There'll be variation. Especially if you don't use a comparator.. the bullet seating stem seats the bullets by engaging the bullet ogive but you are measuring the very point of the bullet to arrive at oal.

Great point! I hadn't considered this. I have already come to the conclusion that some level of variation is normal and to a greater degree with mixed brass, but what is acceptable? Once I have determined a target OAL for my barrel, do I look for variances of .005? .01? How much is generally too much?

 

[41mag]

the bullet seating stem seats the bullets by engaging the bullet ogive but you are measuring the very point of the bullet to arrive at oal.

most excellent point!!!....however, NOW you've made me go check out MY set up....

 

[Member 5605]

Lead has to go somewhere when you compress it.

Flow, flow, flow your bullet, gently up the nose..

 

[DizzyJ]

The auto feeders utilize a taper crimp to help keep the bullets from getting pushed back into the case when they hit the feedramp. It also helps to keep the bullet from moving outward when the case comes to an abrupt stop after the slide slams it into battery.

Roll crimp is more for revolvers to keep the bullet from getting "pulled" out upon recoil from the chambers. Neither should be applied excessively as to significantly distort the bullet to the point of affecting accuracy.

 

[noylj]

If you can hold COL variation to +/- 6 mils, you're doing fine.
Never noticed variation traceable to crimp or case length. Most variations are variations in where and how you measure the bullet and variations in press operation.
Measure factory rounds and your bullets. Also, measure rounds and bullets going around the bore axis.
Starting to see variations?
If crimping is changing COL, my first thought is over crimping. Pull bullets and inspect for crimp damage and any change in length or diameter.