As the title says I worked up a load for a 338 win mag with new brass that I preped. Life was good, once I had fired all the new brass I loaded the same load again in the once fired and the group's stayed the same but velocity dropped 30 fps. I did clean the barrel but I put 10 rounds down range before chronographing the load. Why could this have happened? Primers and powder are the same lot as the first go around. The load was rem brass that was prepped with cci 250's and 70.5grs of IMR 4350 with a hornady interlock .010" off the lands. Virgin brass velocity was 2820 FPS with a sd of 11. Once fired was 2790FPS with a sd of 8.
Velocity dropped on once fired brass.
- Thread starter Mtstalker
- Start date
Dusty Stevens
Shiner
what was the temp difference of the days?
Ned Ludd
Silver $$ Contributor
With even a low to moderate load, the volume of the pressure cell (i.e. fire-formed brass internal case volume) effectively doesn't change, regardless of whether the brass started out virgin, or was already fire-formed. So the volume of the pressure cell between the two types of brass is effectively about the same. What does change is the amount of energy required to expand virgin brass out to fit the chamber, versus the energy required to expand fire-formed and re-sized brass, which starts out much closer to the fired pressure cell dimensions. This is because we typically only push the shoulder back a thousandth or two, whereas it might grow 4 to 5 thousandths (or even more) on the first firing.
In my hands, velocity in small to medium cases such as .223 Rem and .308 Win almost always increases by 10-15 fps or so in fire-formed versus virgin brass. I've always attributed this slight increase in velocity to the fact that the small amount of the total energy generated by powder combustion that goes toward expanding the brass to fit the chamber with a virgin case is free to contribute to projectile movement in fired brass, which is already closer to fire-formed case dimensions (pressure cell). I'd guess the cause of your velocity change has to do with something other than a change in pressure due to the change in effective case volume between virgin and fire-formed brass. If the starting size of the brass actually represented a bona fide pressure cell volume, it wouldn't expand to fit the chamber as readily as it does. Brass can certainly expand further with repeated firings, but the amount it grows after the first firing is only a small fraction of the amount it will generally grow on the first firing with a reasonable load.
There are many things that could explain at least part of the velocity difference you observed, such as a change in temperature or other atmospheric property. I'll acknowledge that 30 fps seems a bit large for IMR 4350, which has reasonable resistance to thermal variation. How many rounds did you fire in your two velocity determinations? Statistical variance could also be a part of it if the group size was small and/or with only a single replicate group for each brass prep. You could spend a lot of time and effort trying to specifically identify the responsible variable(s). However, it's probably simpler to carefully note and record the change, then move on and determine whether the "new" velocity in fire-formed brass (2790 fps) remains constant, and whether you need to tweak the charge weight to re-optimize the load for fire-formed brass.
In my hands, velocity in small to medium cases such as .223 Rem and .308 Win almost always increases by 10-15 fps or so in fire-formed versus virgin brass. I've always attributed this slight increase in velocity to the fact that the small amount of the total energy generated by powder combustion that goes toward expanding the brass to fit the chamber with a virgin case is free to contribute to projectile movement in fired brass, which is already closer to fire-formed case dimensions (pressure cell). I'd guess the cause of your velocity change has to do with something other than a change in pressure due to the change in effective case volume between virgin and fire-formed brass. If the starting size of the brass actually represented a bona fide pressure cell volume, it wouldn't expand to fit the chamber as readily as it does. Brass can certainly expand further with repeated firings, but the amount it grows after the first firing is only a small fraction of the amount it will generally grow on the first firing with a reasonable load.
There are many things that could explain at least part of the velocity difference you observed, such as a change in temperature or other atmospheric property. I'll acknowledge that 30 fps seems a bit large for IMR 4350, which has reasonable resistance to thermal variation. How many rounds did you fire in your two velocity determinations? Statistical variance could also be a part of it if the group size was small and/or with only a single replicate group for each brass prep. You could spend a lot of time and effort trying to specifically identify the responsible variable(s). However, it's probably simpler to carefully note and record the change, then move on and determine whether the "new" velocity in fire-formed brass (2790 fps) remains constant, and whether you need to tweak the charge weight to re-optimize the load for fire-formed brass.
Gary in MD
Gold $$ Contributor
KMart
Gold $$ Contributor
How much did the case grow, from base to shoulder, after firing? In my limited experience with belted magnums, the shoulder has a tendency to move forward a LOT on new to once fired cases
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It moved .021".
dellet
Gold $$ Contributor
Another factor that will differ in case prep is cleaning. Unless you cleaned the new brass, same as the once fired, even if dimensionally they are the same, the contact surfaces will be different.
Neck tension, the amount of interference fit, will be the same if you used the same die or mandrel to size the necks both new and one once fired. That does not mean that the force needed to release the bullet was the same.
The amount of grip or hold (yes, I went there
) the neck has on a bullet, is largely determined by surface quality of the contact points, when the interference fit is small.
Extreme examples would be a clean neck and a lubed bullet vs a fired case, only sized and wiped with a dry bullet.
It’s very possible that the neck of the once fired brass, had less “grip” on the bullet.
Neck tension, the amount of interference fit, will be the same if you used the same die or mandrel to size the necks both new and one once fired. That does not mean that the force needed to release the bullet was the same.
The amount of grip or hold (yes, I went there
) the neck has on a bullet, is largely determined by surface quality of the contact points, when the interference fit is small.Extreme examples would be a clean neck and a lubed bullet vs a fired case, only sized and wiped with a dry bullet.
It’s very possible that the neck of the once fired brass, had less “grip” on the bullet.
Temp was within 5 degrees both days. Primers and powder were from the same lot. The velocity does not bother me at all it's just that I had chronographed this load twice and now it was different. The only variable I could think of is that the brass was now fireformed.
This is what makes reloading interesting. Making blanket assumptions on whether the load should have sped up, remained the same, or dropped velocities gets people in a bind. When you said your brass grew .021" I would have assumed a fair amount of energy was expended to push your brass out and now the load should actually be warmer even though you gained case capacity. I've had cases, 6XC comes to mind where I always end up pulling powder on once fired brass. Then other cases that hold more powder upfront that need some added for the rifle to remain in the zone.
The reason may baffle you somewhat.
I run several belted magnums and own a pressure trace.
On the first firing of NEW brass the pressure doesn’t change much over once fired even though the case length from head to shoulder grows on average .016”-.026” in my rifles. However, what does change between once fired and new brass is the pressure curve and, therefore the velocity. The curve changes enough that velocity drops, not everytime though. My 375 Weatherby GAINS velocity in fully formed brass over COW formed brass, some energy is being used to blow out the brass further.
The larger capacity due to the case lengthening is what changes the curve, the same is seen with AI chambers if you hydro form and don’t get a fully formed shoulder, but to a lesser degree.
I also believe this happens only with powders that may be on the fast side for the cartridge, as in my own 338WM I see no difference between unfired and once fired cases using RE19 and velocity, I get 2890fps average over 30 rounds with 225gr Accubonds, no matter the ambient temp.
Cheers.
I run several belted magnums and own a pressure trace.
On the first firing of NEW brass the pressure doesn’t change much over once fired even though the case length from head to shoulder grows on average .016”-.026” in my rifles. However, what does change between once fired and new brass is the pressure curve and, therefore the velocity. The curve changes enough that velocity drops, not everytime though. My 375 Weatherby GAINS velocity in fully formed brass over COW formed brass, some energy is being used to blow out the brass further.
The larger capacity due to the case lengthening is what changes the curve, the same is seen with AI chambers if you hydro form and don’t get a fully formed shoulder, but to a lesser degree.
I also believe this happens only with powders that may be on the fast side for the cartridge, as in my own 338WM I see no difference between unfired and once fired cases using RE19 and velocity, I get 2890fps average over 30 rounds with 225gr Accubonds, no matter the ambient temp.
Cheers.
blah blah blah
ever buy a case holder from wilson ? what is the big question they ask ?
NEW or FIRED cases
why ? because there is a noticeable difference in size between factory new brass and fired/resized
brass...esp with a mag case.( not length, dia,, volume)
not one person that has posted an opinion has measured before and after VOLUME.
I DO. which is why i made my statement
ever buy a case holder from wilson ? what is the big question they ask ?
NEW or FIRED cases
why ? because there is a noticeable difference in size between factory new brass and fired/resized
brass...esp with a mag case.( not length, dia,, volume)
not one person that has posted an opinion has measured before and after VOLUME.
I DO. which is why i made my statement
Ned Ludd
Silver $$ Contributor
You're not the only person that measures cases before and after firing. You're also not as knowledgeable as you seem to think. There are plenty of other people here that know far more than you about reloading. I think you just post your same condescending, "I know more than you" over and over and over just to hear yourself talk, or maybe it's simply because you're a bitter old man whose best shooting days are long behind him. Neither one of those is a good reason to show your fellow shooters the consistent lack of consideration and courtesy that you do. Why don't you just give it a rest? Thank God for the "Ignore" button.
you really do not like it when the errors of your statement are shown to the public.you made a statement with no knowledge of the brass in question's volume. new virgin brass volume, 1x neck sized case volume, and unknown full length sized case volume. you make more poor assumptions about low to moderate loads, and then talk about small case volume. the case in question is not small, not medium. the volume difference is significant enough that two different case holders ( od not length) are required.
you write long "stories", but few facts. i'll pass on your opinion,
some how you are allowed to be a "know it all" and that is suppose to be ok.
ignore works both ways.
you write long "stories", but few facts. i'll pass on your opinion,
some how you are allowed to be a "know it all" and that is suppose to be ok.
ignore works both ways.
It's OK that you go both ways, but shouldn't things like that be best kept private???
but it is true...bolt and semi auto!
It's OK that you go both ways, but shouldn't things like that be best kept private???
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