Variance in Base to Datum....

[PinesAndProjectiles]

I'm loading up some 308 brass that I bought off the PX. It was once fired and obviously not fired in my rifle.

I'm measuring the brass from base to datum and I'm basically seeing two measurements.

The first one is 1.626+/- .001
The second is 1.630+/- .001

I'm sizing them using the RCBS Comp FL die with expander ball as a good portion of the necks have dents in them.

After sizing, I'm getting erratic measurements and they range from 1.624 to 1.628. I'm thinking this is because I've got two groups of brass, and they size differently.

I have some once fired Hornady brass from my rifle, and the base to datum is 1.6265. So, I figured 1.624-1.625 is good for pushing shoulders back about .0015-.002.

I think if I measured all the 1x fired brass and split it into two groups, 1.626 and 1.630, I could then bump shoulders on the smaller group, adjust the die, and then bump the shoulders on the larger group.

Or, am I overthinking this? Will the .002-.005" variance between the brass actually make that much difference?

I could size them all the same, load them up, and shoot them and on the 2nd sizing, I could adjust the die to reflect a .0015-.002" shoulder bump.

Thanks.

 

[PinesAndProjectiles]

I was doing some reading and stumbled across this article, which basically says if your die is too long internally, you can size the body of the brass, without actually bumping your shoulders back, and in turn, lengthens your brass.

I wonder if this has something to do with what I'm seeing.

This is a link to the Accurate Shooter page.

You are being redirected...

 

[kingaling]

I was doing some reading and stumbled across this article, which basically says if your die is too long internally, you can size the body of the brass, without actually bumping your shoulders back, and in turn, lengthens your brass.

I wonder if this has something to do with what I'm seeing.

This is a link to the Accurate Shooter page.

You are being redirected...

That article doesn't sound like your issue.
Varying brands will have varying hardness. I'd sort the brass by brand and anneal them all. Then start with one group, load and fire, and see if you get consistent resizing results.

The article you mention is describing a situation where you get no shoulder bump at all because the shoulder doesn't touch the top of the die inside.

 

[PinesAndProjectiles]

All the brass is LC 10 LR, so all the same brand.

I believe it to be all once fired, but may have been fired from two different rifles, judging by bast to datum length.

Or, different lots of the same head stamp and it reacts differently when FL sized.

 

[Threadcutter308]

All the brass is LC 10 LR, so all the same brand.

I believe it to be all once fired, but may have been fired from two different rifles, judging by bast to datum length.

Or, different lots of the same head stamp and it reacts differently when FL sized.

Yeah, provided you can chamber all of the brass (in one gun), load it, shoot it/fireform it. Then, you'll have a known consistent baseline to work with.

 

[Steel head]

1-Brass isn’t perfectly identical even in lots of premium brass.
2-Iube/inconsistent lube can make Big differences in sizing.
3-annealing really helps to get consistent sizing.
4-dwell time in die can make a big difference in sizing.
5-if brass is fired in multiple chambers variations in sizes are common.

 

[Reload10]

All good points. One point that hasn't been made is your press ram runout. Consistent headspace sizing of brass is dependent of your press ram having zero runout. Look at the ram pivot pin, and the handle bracket pivot pin. Do you think the ram TDC (top dead center) is exactly the same with every stroke? It is not. You can set up a dial indicator and check. The pivot pins can rotate in a lot of presses and they are far from perfectly cylindrical.

I had the same problem as you. I started using the redding shell holder set to set my sizing die. This reduced the variation in sizing down to maybe a thou. Then I started using wilson inline seater dies with an arborpress and seating depth has been perfect.

 

[PinesAndProjectiles]

All good points. One point that hasn't been made is your press ram runout. Consistent headspace sizing of brass is dependent of your press ram having zero runout. Look at the ram pivot pin, and the handle bracket pivot pin. Do you think the ram TDC (top dead center) is exactly the same with every stroke? It is not. You can set up a dial indicator and check. The pivot pins can rotate in a lot of presses and they are far from perfectly cylindrical.

I had the same problem as you. I started using the redding shell holder set to set my sizing die. This reduced the variation in sizing down to maybe a thou. Then I started using wilson inline seater dies with an arborpress and seating depth has been perfect.

I'm using a Forster Coax but I've never thought to measure these things. Thank you.

 

[bax]

I'm loading up some 308 brass that I bought off the PX. It was once fired and obviously not fired in my rifle.

Or, am I overthinking this? Will the .002-.005" variance between the brass actually make that much difference?

I could size them all the same, load them up, and shoot them and ...

Yes, you are overthinking it. For whatever reason, you have two sets of brass. Resize them all so that they fit in your chamber, load and fireform them. Now they should all be the same. Develop loads and go forth and sin no more.

I would anneal all of it. Then size, load, shoot, tumble, anneal and then size as you like. These days of powder and bullet scarcity, I might just anneal and then size and load.

For me, the point of annealing is two things - I want the neck tension to be the same and I want the brass to be the same size. Annealing reduces mechanical characteristic variation for things like ductility. With reduced characteristics variation, the force required to jam a bullet into the neck and the "grip" that the neck imparts on the bullet makes neck tension more uniform. Second, making the mechanical characteristics more uniform means that if I apply sufficient lube and let every case dwell in the die for the same amount of time, I should get the same final shape. Same shape means same case capacity. If the powder charges are similar ... and all the rest, it should shoot better - assuming that the nut behind the wheel is not asleep at the switch.

I shoot a ton of LC LR - 05, 06, 10, and 11 and a few other years. In my weapons and with my skills, it shoots sub 0.6 MOA about 90% of the time - maybe a bit more. About half the time it shoots less than 0.5 MOA. Maybe 5% of the time, less than 0.4 MOA. I ask no more than that. Flyers are on the driver.

A clean barrel starts at 1.5 MOA. After 30 to 50 rounds it achieves the patterns above. I used to clean after every trip to the range and swear that not cleaning was the devils work. While it was true for benchrest guns PRS-types are different and targets don't lie.

 

[zeroz]

The spread you are talking about is less than the thickness of copy paper. I have learned to live with sub copy paper difference in my headspace. If I was shooting F class, maybe not but 1-2 MOA targets don't seem to care. I am loading on a Dillon so there is more slop than a co-ax for sure but I'd say load em up and go shoot them. If you really wanted to see if there is a difference on target you could sharpie the headspace measurement on the case and see. Love to hear back if you do find a consistent measurable difference.