How can brass impact COAL?

[JEBruns]

I was getting some COAL variance in my 6.5CM loads using my older RCBS progressive loader. Figured it was because of the shell plate tilt factor on progressives. So I bought a "semi progressive", Dillon 550. It still has a shell plate, but the base of the shell sits on the ram surface, not in the shell plate. I use an RCBS Matchmaster micrometer seating die and Lee Factory crimp die with a light crimp.

All smiles with my normal Hornady brass. Held spot on consistent through 30 rounds. I have some Aquila brass also. It is once fired factory ammo. Those were all over the place for COAL, up to a hundredth off. All of the brass, both brands, were prepped the same way using the same equipment. Annealed, sized/deprimed, trimmed, wet clean with pins, hand primed. Load data/components (Hornady ELD-M bullets) were exactly the same, all loaded in one session.

How can be that happening? I thought I'd removed COAL variance, so this kind of threw me for a loop. I can toss the Aquila brass, wouldn't bother me a bit. But I just can't make sense of it.

 

[Blue68f100]

Different neck tension can cause variations but I would go by the o-give to base length to eliminate the points. I've seen quite a bit variation in OAL when trying to measure to the tip of any bullet, as much as 0.010" for some. Once you get way from the tip your length will be more consistent. You can use your seating stem or any thing that will contact the bullet other than the tip will give you a reference. Any of the comparators should be able to give you a more consistent measurement.

I would not crimp at all. This can change your OAL and neck tension depending on neck wall thickness. With the LFCD it should not change the OAL but will damage the bullet and change neck tension. Beside unless the bullet has a cannelure to roll into your damaging the bullet and hurting accuracy. The only rifle calibers that need to be crimped is magazine fed guns, 30-30 and mag revolvers.

 

[FROGO207]

The above post is spot on! You will get differences with brass necks being harder, different tensions ( unless you neck turn them to get constant thickness around the entire neck), and the internal volume will vary with the brand due to wall thickness and the height of the base inside the brass (webb). All All this will change your results if you are trying for max accuracy. Uniforming the flash holes helps as does inside the neck chamfering. Just how far you want to go chasing accuracy is an individual decision. Down the rabbit hole you go. Good luck OP.

 

[NMexJim]

Much depends on how much you shoot and what your goals are. Too, much depends on the rifle itself - how well it's made.

Personally, I'd stick with one brand of brass. Lapua or Hornady. It just takes that one element out of the equation. You mentioned annealing. Hopefully, you're set up for that. If not, and you have some extra money on hand, make it as easy on yourself as possible.

Buy the best precision measuring equipment that you can afford.

After that, have a lot of fun!

 

[JEBruns]

I have an Ugly annealer. Works great. The rifle I'm using is a Tikka Tac T3x A1.

Thanks for the input. I guess it may be a neck tension issue.