Velocity changes with different brass manufacturers?

[Tesoro]

This is in reference to my 6xc Tubb .160 freebore .270 loaded neck od in a .278 neck chamber

I read a 10 yr old post somewhere on here where David Tubb stated that switching from Norma brass to the then new Peterson shot 25fps faster even though the capacity was the same. Noone commented on this.

I have been shooting Peterson brass with copper DTAC's at 2985 fps. I recently converted some Lapua Palma brass and am now at 3012 fps. Same everything other than the case. I did my own water weight tests and the cases were very close in capacity ie .5% difference. SD's for both loads were in the 10 range with ES around 20.

Based on my trimming experiences the brass alloy softness to hardness ratings on these 3 brands is as expected: Norma>Peteterson>Lapua

So what is happening here? My only guess is that some how the tougher brass has more resistance to fireform and thus exerts a tad more initial pressure down the bore on combustion? If not then what is it?

 

[6ShotsOr5?]

Just brainstorming… I don’t know how neck tension influences muzzle velocity, but if you are using the same size neck bushing for all these then the stiffer brass probably exerts more neck tension on the bullet.

 

[Tesoro]

Just brainstorming… I don’t know how neck tension influences muzzle velocity, but if you are using the same size neck bushing for all these then the stiffer brass probably exerts more neck tension on the bullet.

good point. I thin my necks down to .0145 and set at .0025 expansion. My necks are brushed carbon inside and I use light imperial wax on the bullets as well. They seat on my arbor like butter. If one dosent then it goes into the fouler box or gets pulled. Cases are always annealed.
As you cant measure tension other than in the form of seating or extraction pressure then I dont know as I dont have that tool.

 

[lotech]

Weigh your brass for an average and check internal capacity as well. This can account for signficant differences in velocity and require an adjustment of powder charge. Just one example of several I've run across... I bought some new PPU 7mm Rem. Mag. brass several years ago because that was the only brass I could find. Good brass, but 14% heavier than Winchester brass. That's significant. I had to reduce the powder charge 1.5 grs. with the PPU to get the same velocity (and accuracy) I was getting with Winchester.

Neck tension is certainly of some importance. but it seems to have become a fad concern the last few years. I won't argue the point, but based solely on my experience, I haven't found it to be nearly as important as some others claim in putting together an accurate load.

 

[Tesoro]

I mentioned I had checked water weight capacity and they were very close. The lapua brass is 5gr heavier which aint much and most likely because it has more brass on the top part of extractor groove and is a different alloy. I guess one thing I could do is increase the NT on the Peterson and see if this speeds it up 30 fps to match the Lapua.
But I can tell you the Lapua brass is hard! compared to the Peterson when trimming. And it also takes longer to anneal. There is too much leverage using the neck expander mandrel to feel any difference between the two.

Pic of Lapua on left vs Peterson to see extractor groove extra beef and also 5gr of shavings. Might look like alot but it dosent take much .0005 thick shavings to make a pile.

 

[Idaho-Lefty]

It's a KNOWN Fact that, Peterson Brass in most Cart's, IS, a tiny Bit, Thicker and has, slightly LESS,. Volume than Lapua.
I use both, Pete and Lapua, in 6.5 Creed with, 1/2 to a Full grain, less Powder to AVOID higher, Pressures with, the Peterson's.
I've had, NO "Issues" with, Norma 6 XC Brass ( All, I use for,. IT ).
Making 6 XC, Brass from .308 Lapua, Palma brass,.. IDK ?? ( Different, Animal ).
Makes a NICE Case,.. BUT, way more Work than, I want to do, for another,.. 25 FPS.
Might want to "Look at" the 6 SLR for, Higher Velocities with,.. the 115 DTAC's

 

[Tesoro]

When I did my tests, will admit only on 7 cases, I found the Lapua to have slightly less capacity but not enough to note. Thus I was perplexed on the Lapua being heavier until I noticed the diff in the base. I think capacity was 3.5% to be exact without digging out my notes. All the capacities were consistent so I didnt do a large batch. I dont notice any pressure differences with same charge in each case. Maybe the 6xc and 308 are different than the Creed.

 

[RegionRat]

Were your averages and stats based on just 7 shots each?
Was the humidity of the powder the same on both kinds of cases?

When zooming in on a difference in velocity that is less than 1%, it takes more outings and more data to be sure if that theory will hold.

ETA: Maybe think about asking David what his reasons were for changing brass?

 

[Tesoro]

Were your averages and stats based on just 7 shots each?
Was the humidity of the powder the same on both kinds of cases?

When zooming in on a difference in velocity that is less than 1%, it takes more outings and more data to be sure if that theory will hold.

Over a hundred shots of each. Gamin makes things easy! Same climate and temp and same jug. I said I checked h20 weight on 7 cases.

 

[lotech]

Tesoro- I apologize. I overlooked your mention of already weighing your brass.

 

[Tesoro]

Tesoro- I apologize. I overlooked your mention of already weighing your brass.

When threads get too long this happens! I'm calling this done and racking it up to a tighter grab on the bullet.

 

[J_idaho]

I know I don’t have the experience you guys have, but I do know that trying to use different lots of brass even if there sorted you end up with different velocities accuracy goes all over the place. Also consider that you can take a sparkler and wrap black tape around it really tight and it explodes so I know that any slight amount of pressure difference is going to cause serious changes in burn rate and velocity. I would say alloy variations alone would cause velocity change.

 

[Ned Ludd]

A 0.5% difference in case volume may not seem like much, but it can be significant when we're talking about 4-digit velocity values that typically range from 2500 to over 3000 fps. Although it may be possible to explain away the increased weight of Lapua brass versus Peterson by the difference in the extractor grooves, the weight difference is certainly consistent with the velocity results you listed for the two brands of brass. Something caused the difference in velocity, and a difference in case volume is certainly a likely contender. Further, really accurate measurement of case water volume is not such a simple thing to do. If it is not a difference in case volume, then it may be going down a rabbit hole to try and figure out what exactly the difference is. For example, a difference in neck tension, flashhole diameter/geometry, carbon/patina lubrication of the case neck, etc., etc., etc. The question is how much time and effort is it worth to determine a cause when you already have the result (i.e. the cases clearly generate different velocities with the same load)?

 

[Tesoro]

I have the curiosity gene! If case volume made that much difference then should be able to seat deeper to gain speed!
The one common denominator is that as brass gets tougher the fps rises. Slightly but there is a correlation.

 

[RegionRat]

If case volume made that much difference then should be able to seat deeper to gain speed!

When doing a sweep for depth/jump... as often as anything else, you will see a decrease in speed with deeper seating, until you get to where things get really really deep.

A picture is worth a thousand words, so here you go. Below is an example of a 6 Dasher running Varget.

 

[JFrank]

I mentioned I had checked water weight capacity and they were very close. The lapua brass is 5gr heavier which aint much and most likely because it has more brass on the top part of extractor groove and is a different alloy. I guess one thing I could do is increase the NT on the Peterson and see if this speeds it up 30 fps to match the Lapua.
But I can tell you the Lapua brass is hard! compared to the Peterson when trimming. And it also takes longer to anneal. There is too much leverage using the neck expander mandrel to feel any difference between the two.
Pic of Lapua on left vs Peterson to see extractor groove extra beef and also 5gr of shavings. Might look like alot but it dosent take much .0005 thick shavings to make a pile.
View attachment 1515873

Chris,
5.0 grains seems like quite a bit, I'd have to dig out some notes from weighing brass but if I recall correctly that was more than enough to change pressure and show on paper.
This screenshot was posted awhile back and I thought it was interesting to note the differences in manufactures so I saved a copy for reference.
As far as NT changing speeds… I don’t see that on paper nor do I see seating depths changing speeds beyond white noise, it may happen on one test’ but I don’t see it consistently using my chronograph and small sample sizes.

 

[JFrank]

Not to dispute anyone else’s tests or findings, but just for something to ponder I’ll post a couple things.

One) a difference in .015 bullet jump with virtually no variation in point of impact at 300 yards.

Two ) While sorting brass by chrono it’s easy to see quite a variation in speed within the same lot.

Three) Even with such a variation you may still not see a point of impact change once in tune.

For myself the target is King..

 

[SeabeeKen]

When doing a sweep for depth/jump... as often as anything else, you will see a decrease in speed with deeper seating, until you get to where things get really really deep.

A picture is worth a thousand words, so here you go. Below is an example of a 6 Dasher running Varget.

View attachment 1516102

Correct me if I'm wrong but if running light neck tension, Primer impulse can pop the bullet into the lands prior to powder ignition thus creating a larger powder capacity. GRT has a setting for just such a situation. Is this what we are seeing here?

 

[jelenko]

sparkler and wrap black tape around it really tight and it explodes

How interesting.

 

[Tesoro]

Chris,
5.0 grains seems like quite a bit, I'd have to dig out some notes from weighing brass but if I recall correctly that was more than enough to change pressure and show on paper.
This screenshot was posted awhile back and I thought it was interesting to note the differences in manufactures so I saved a copy for reference.
As far as NT changing speeds… I don’t see that on paper nor do I see seating depths changing speeds beyond white noise, it may happen on one test’ but I don’t see it consistently using my chronograph and small sample sizes.

If you look at the pic I posted you can see quite a bit of extra meat on the Lapua base that wouldn't affect capacity.

All I can say for certain is the harder the case the higher the velocity for some reason, when capacities are equal! I'll contact Mr Tubb and see what he has to say as he noted the same thing. Will report back.