Variation in group size between charge weights

JohnnyBWildered

Private
Minuteman
Jul 10, 2019
24
1
New reloader here! I Went to the range yesterday to try a few charge weights to find my preferred load and wanted to know how much variation in group size do you see when gradually stepping up charge?
My plan was to find the best 5rd group, then load 10 rounds each of +./- .1 and check the velocity and if it’s good enough then id start fiddling with seating depth.

Shooting a 20” factory Remington 700 SPS tactical in .308.
Loaded 168 SMK in Federal GMM once fired brass with Varget. Plan was 43gr-45gr in .3 increments.

43grs did best at .67 MOA
43.3 was about 1” so much larger I didn’t bother to use a measuring app)
And it continued with everything being around 1-1.2
Until it dropped to .78 MOA at 44.8.
Everything felt good on all these aside from the occasional shooter induced flyer that I called. I allowed barrel/shooter to cool for a few minutes between groups.

How frequently do you find multiple promising charge weights that far apart?

Do you see groups open that quickly with just .3gr difference?

Also any other advice or knowledge on my process would be appreciated.
 

918v

It’s not the primer!
Full Member
Minuteman
Jul 15, 2007
5,886
3,093
Miserable CA
First you need to learn about the relationship between case weight, volume, and pressure.

You should not use data developed in lightweight brass with your heavy brass without first reducing the starting charge.

In your case you went way over pressure with 44.8grs in Federal brass. You should not go above 43.5grs in that case.

I would redo your load development starting at 41.5 grains.
 
  • Like
Reactions: camocorvette

JohnnyBWildered

Private
Minuteman
Jul 10, 2019
24
1
First you need to learn about the relationship between case weight, volume, and pressure.

You should not use data developed in lightweight brass with your heavy brass without first reducing the starting charge.

In your case you went way over pressure with 44.8grs in Federal brass. You should not go above 43.5grs in that case.

I would redo your load development starting at 41.5 grains I didn’t see any pressure signs on my brass even on the 44.8. I stayed under all the published maxes I could find, I wish they’d specified the case and pressure
I didn’t see any pressure signs on my brass but damn, that’s concerning. I stayed under all the maxes I could find published.
 

straightshooter1

Private
Full Member
Minuteman
Dec 5, 2017
1,665
951
AZ
New reloader here! I Went to the range yesterday to try a few charge weights to find my preferred load and wanted to know how much variation in group size do you see when gradually stepping up charge?
My plan was to find the best 5rd group, then load 10 rounds each of +./- .1 and check the velocity and if it’s good enough then id start fiddling with seating depth.

Shooting a 20” factory Remington 700 SPS tactical in .308.
Loaded 168 SMK in Federal GMM once fired brass with Varget. Plan was 43gr-45gr in .3 increments.

43grs did best at .67 MOA
43.3 was about 1” so much larger I didn’t bother to use a measuring app)
And it continued with everything being around 1-1.2
Until it dropped to .78 MOA at 44.8.
Everything felt good on all these aside from the occasional shooter induced flyer that I called. I allowed barrel/shooter to cool for a few minutes between groups.

How frequently do you find multiple promising charge weights that far apart?

Do you see groups open that quickly with just .3gr difference?

Also any other advice or knowledge on my process would be appreciated.
Using increments of .3 grs is good for a .308 cartridge, however . . . you started pretty high for the Federal brass, which has a case capacity of ~55.9 grs H2O (that's filled to the rim of the mouth, as it's typically measured). It sounds like you found a node at the high end with 43.0 grs and that's pretty much where I also found an accuracy node (at the high end). Now, I'd say the next step for you would be to verify that by running the test again and starting with powder weight well below that moving with the 3 gr increments till to get to 43.3 and see if you still get the best result at 43.0. If you do, THEN you can work with seating depths to tighten up the grouping.

There are a number of nodes ("promising charge weights) below the high end where you're at. How far apart they are depends on your barrel and its configuration.

With .3 grs difference in a .308 cartridge, you'll see groupings change somewhat quickly. But you can certainly see the significant change as the charge changes. What you want to look for is not just a smaller group, but a small difference is you vertical.
 
  • Like
Reactions: simonp

Dogtown

Ke = (mv^2)/2
Full Member
Minuteman
Jun 21, 2007
5,046
1,879
CONUS
Some folks look at groups as they move through charge weights. Others look at POI shift between charge weights. Generally with these incremental charge weights you'll see a sinusoidal pattern as group size / POI shift moves around in a periodic way. Groups can open up then start to shrink, then open up again, and/or the POI moves away from the POA then back. With really consistent ammo across increments, there's usually some space between shifts as things tend to happen gradually. Personally, I've noticed that short bull barrel (ie very rigid*) tend to increase the frequency of those shifts. But if you're having something really abrupt happening, then it's probably something in your reloading/shooting process. Inconsistencies can show up all over the place.

* that's a clue that all of this is directly related to vibration in the barrel
 
  • Like
Reactions: JohnnyBWildered

camocorvette

Send it
Supporter
Full Member
Minuteman
Supporter+
  • Apr 1, 2010
    1,456
    1,131
    Minnesota
    First you need to learn about the relationship between case weight, volume, and pressure.

    You should not use data developed in lightweight brass with your heavy brass without first reducing the starting charge.

    In your case you went way over pressure with 44.8grs in Federal brass. You should not go above 43.5grs in that case.

    I would redo your load development starting at 41.5 grains.
    Yep.

    I used Winchester brass load data in FGMM brass and it was bad.
     
    • Like
    Reactions: JohnnyBWildered

    JohnnyBWildered

    Private
    Minuteman
    Jul 10, 2019
    24
    1
    Using increments of .3 grs is good for a .308 cartridge, however . . . you started pretty high for the Federal brass, which has a case capacity of ~55.9 grs H2O (that's filled to the rim of the mouth, as it's typically measured). It sounds like you found a node at the high end with 43.0 grs and that's pretty much where I also found an accuracy node (at the high end). Now, I'd say the next step for you would be to verify that by running the test again and starting with powder weight well below that moving with the 3 gr increments till to get to 43.3 and see if you still get the best result at 43.0. If you do, THEN you can work with seating depths to tighten up the grouping.

    There are a number of nodes ("promising charge weights) below the high end where you're at. How far apart they are depends on your barrel and its configuration.

    With .3 grs difference in a .308 cartridge, you'll see groupings change somewhat quickly. But you can certainly see the significant change as the charge changes. What you want to look for is not just a smaller group, but a small difference is you vertical.
    I’m surprised that
    Using increments of .3 grs is good for a .308 cartridge, however . . . you started pretty high for the Federal brass, which has a case capacity of ~55.9 grs H2O (that's filled to the rim of the mouth, as it's typically measured). It sounds like you found a node at the high end with 43.0 grs and that's pretty much where I also found an accuracy node (at the high end). Now, I'd say the next step for you would be to verify that by running the test again and starting with powder weight well below that moving with the 3 gr increments till to get to 43.3 and see if you still get the best result at 43.0. If you do, THEN you can work with seating depths to tighten up the grouping.

    There are a number of nodes ("promising charge weights) below the high end where you're at. How far apart they are depends on your barrel and its configuration.

    With .3 grs difference in a .308 cartridge, you'll see groupings change somewhat quickly. But you can certainly see the significant change as the charge changes. What you want to look for is not just a smaller group, but a small difference is you vertical.
    The #2 post gave me the same advice with the federal brass an even some specific numbers. How do you calculate those based on case capacity/weight? I went off the Hodgdon data center and figured with a 20” barrel I’d end up needing to go on the higher end so I started in the middle between their starter and their max/high pressure warnings. I’ll try it again like y’all said.
     

    918v

    It’s not the primer!
    Full Member
    Minuteman
    Jul 15, 2007
    5,886
    3,093
    Miserable CA
    He prolly meant bad in terms of primer pocket expansion. Leaking primers, blown primers, etc.

    A good rule of thumb for the 308 Winchester is to reduce the load by 1.5-2grs between Winchester commercial brass and Federal brass. Winchester brass, which Hodgdon used to develop their data, averges 157+\-2grs in weight whereas Federal brass is 20 grains heavier. So when you’re looking at data, consider the brass being used and adjust accordingly.

    For example, if your brass weighs 165 grains like most Hornady and Remington then lower the charge by one grain. If your brass weighs 185 grains like some CBC, or some special Hornady cases, lower the charge 3 grains.

    Otherwise you’ll wreck your brass.
     

    JohnnyBWildered

    Private
    Minuteman
    Jul 10, 2019
    24
    1
    He prolly meant bad in terms of primer pocket expansion. Leaking primers, blown primers, etc.

    A good rule of thumb for the 308 Winchester is to reduce the load by 1.5-2grs between Winchester commercial brass and Federal brass. Winchester brass, which Hodgdon used to develop their data, averges 157+\-2grs in weight whereas Federal brass is 20 grains heavier. So when you’re looking at data, consider the brass being used and adjust accordingly.

    For example, if your brass weighs 165 grains like most Hornady and Remington then lower the charge by one grain. If your brass weighs 185 grains like some CBC, or some special Hornady cases, lower the charge 3 grains.

    Otherwise you’ll wreck your brass.
    Thanks! I just fooled around on federal reloading data on their website and their max for their 165 is 44. But it’s 46 on Hodgdon with the same weight. Luckily I didn’t go any heavier! Thanks again for the knowledge.
     

    camocorvette

    Send it
    Supporter
    Full Member
    Minuteman
    Supporter+
  • Apr 1, 2010
    1,456
    1,131
    Minnesota
    Damn, how bad? Anything I should be checking on my rifle? My brass didn’t have any pressure signs. How/where do you find data specific to heavier brass?
    20220319_175319.jpg
    20220319_204806.jpg
    20220319_195641.jpg


    I've got a thread in the reloading section called "what not to do".
     

    straightshooter1

    Private
    Full Member
    Minuteman
    Dec 5, 2017
    1,665
    951
    AZ
    The #2 post gave me the same advice with the federal brass an even some specific numbers. How do you calculate those based on case capacity/weight? I went off the Hodgdon data center and figured with a 20” barrel I’d end up needing to go on the higher end so I started in the middle between their starter and their max/high pressure warnings. I’ll try it again like y’all said.
    A common rule of thumb is to start at 10% below the max load listed for a particular powder. Then work in 2,3 or 5 gr intervals, depending of the size of case you're working with.

    As far as doing any "calculation" base on case capacity or weight, you just need to be well aware that the lighter cases (like Winchester produces) have thinner material for case walls resulting in more volume than the heavier cases (like Lake City brass or most others). For example, Winchester brass tends to have about 1.5 grs H2O more is case volume than Federal brass. That means more volume for more powder or less pressure, less velocity for the same amount of powder that's used in a case like Federal's. A significant difference in case weight can tell you whether there'll be a significant difference in volume. But, there's not a direct correlation between case weight and case volume.
     

    Dogtown

    Ke = (mv^2)/2
    Full Member
    Minuteman
    Jun 21, 2007
    5,046
    1,879
    CONUS
    I figured I’d do .3 and get a window and try .1 increments around the best group and see if there’s any improvement. When would you use .5gr?

    When your charge weights are above 60gr. For example, when I load 6.5x47, my charge range is 37-40gr and I do 0.3gr increments. But when I load .300NM the range is 84-90gr so I use 0.5gr increments. On the other end of the spectrum, loading for .300BLK where your range may be 16-19gr, increments of 0.2gr (or even 0.1) are reasonable.

    The increment should be proportional to the amount of powder filling the case.
     

    JohnnyBWildered

    Private
    Minuteman
    Jul 10, 2019
    24
    1
    When your charge weights are above 60gr. For example, when I load 6.5x47, my charge range is 37-40gr and I do 0.3gr increments. But when I load .300NM the range is 84-90gr so I use 0.5gr increments. On the other end of the spectrum, loading for .300BLK where your range may be 16-19gr, increments of 0.2gr (or even 0.1) are reasonable.

    The increment should be proportional to the amount of powder filling the case.
    That makes sense. Appreciate the help in not blowing myself up!
     

    Dogtown

    Ke = (mv^2)/2
    Full Member
    Minuteman
    Jun 21, 2007
    5,046
    1,879
    CONUS
    Since you're relatively new at this, on the subject of charge weights, the idea of working up to max is pretty straight forward. But something a lot of new reloaders don't understand is that there's also a minimum charge weight you don't want to go below. Generally, you should use a powder that can safely work with filling the case >80%. If you drop below that you can start getting into a danger zone.

    "But why?" you may ask. "As long as there's some powder in the case, won't that just be a low velocity load?" Sure and getting a slow bullet stuck in the barrel is NOT the issue here. The issue is that when your case fill percentage starts to get in the 70s% and lower, when oriented on its side the powder sits low on one side of the case instead of being packed up against the primer. With a normal fill percentage the primer ignites the powder next to the primer and it burns quickly and generates pressure. But if there's this big air gap left above the powder laying along the side of the case, the primer can ignite ALL of the powder (both powder next to the primer, but also next to the bullet). This will generate pressures much too rapidly and you'll get a detonation. This is why there are fast burning large volume powders like TrailBoss so that they will fill up a case but not generate crazy pressures.

    Anyway, something to be aware of.
     

    JohnnyBWildered

    Private
    Minuteman
    Jul 10, 2019
    24
    1
    Since you're relatively new at this, on the subject of charge weights, the idea of working up to max is pretty straight forward. But something a lot of new reloaders don't understand is that there's also a minimum charge weight you don't want to go below. Generally, you should use a powder that can safely work with filling the case >80%. If you drop below that you can start getting into a danger zone.

    "But why?" you may ask. "As long as there's some powder in the case, won't that just be a low velocity load?" Sure and getting a slow bullet stuck in the barrel is NOT the issue here. The issue is that when your case fill percentage starts to get in the 70s% and lower, when oriented on its side the powder sits low on one side of the case instead of being packed up against the primer. With a normal fill percentage the primer ignites the powder and it burns quickly and generates pressure. But if there's this big air gap left above the powder laying along the side of the case, the primer can ignite ALL of the powder (both powder next to the primer, but also next to the bullet). This will generate pressures much too rapidly and you'll get a detonation. This is why there are fast burning large volume powders like TrailBoss so that they will fill up a case but not generate crazy pressures.

    Anyway, something to be aware of.
    No, I appreciate it. I’m at the point where I know enough to be dangerous. I don’t think I’d ever be inclined to load below any of the starting weights but it’s definitely good to know in the off chance I tried to light load something for the wife to shoot.
     

    b6graham

    MMPRL
    Full Member
    Minuteman
    Supporter+
  • Jul 29, 2014
    6,976
    5,985
    Colorado
    you could reshoot the test (with reduced/appropriate loads this time) and likely have opposite results. the .7" groups would be 1.2" and the 1.2" groups would be .7"

    all likely within the capabilities of the shooter or gun, or ammo
     

    Doom

    Balding Eagle
    Full Member
    Minuteman
    Mar 24, 2013
    610
    310
    South of the Mason Dixon Line.
    Not trying to start any trouble but I have reloaded for this same rifle/same bullet for at least ten years. Your Remington 700 308 probably has a long freebore so the pressures tend to run low and despite what others said you probably did not go overpressue. That said much of the advice on working up loads here is spot on.

    Your starting load is around a known node for Varget and IMR 4064 with the 168 SMK. It should give you results very similar to Federal GMM. Going higher in Federal brass is going to reduce brass life.

    I’m going to suggest yo look into Dan Newberry’s OCW reloading system. It’s not the only one but it seems to fit what you are working with.
     

    MarkyMark007

    Sergeant of the Hide
    Full Member
    Minuteman
    Jan 31, 2018
    350
    128
    Europe
    not-on-google.shit
    New reloader here! I Went to the range yesterday to try a few charge weights to find my preferred load and wanted to know how much variation in group size do you see when gradually stepping up charge?
    My plan was to find the best 5rd group, then load 10 rounds each of +./- .1 and check the velocity and if it’s good enough then id start fiddling with seating depth.

    Shooting a 20” factory Remington 700 SPS tactical in .308.
    Loaded 168 SMK in Federal GMM once fired brass with Varget. Plan was 43gr-45gr in .3 increments.

    43grs did best at .67 MOA
    43.3 was about 1” so much larger I didn’t bother to use a measuring app)
    And it continued with everything being around 1-1.2
    Until it dropped to .78 MOA at 44.8.
    Everything felt good on all these aside from the occasional shooter induced flyer that I called. I allowed barrel/shooter to cool for a few minutes between groups.

    How frequently do you find multiple promising charge weights that far apart?

    Do you see groups open that quickly with just .3gr difference?

    Also any other advice or knowledge on my process would be appreciated.
    it is good.

    load 43gr if you don't have pressure issues, and change seating depth.
    it would be the best if you can do it in 0.005'' intervals, but if you have large range to discover, than you use 0.010'' interval.

    for now, when you are new reloader, it is all you should do.

    in next experiment you can change powder charge in 0.1gr increments around 43gr at your best seating depth. with use of chrono for best velocity spread.
     
    • Like
    Reactions: Doom

    JohnnyBWildered

    Private
    Minuteman
    Jul 10, 2019
    24
    1
    Not trying to start any trouble but I have reloaded for this same rifle/same bullet for at least ten years. Your Remington 700 308 probably has a long freebore so the pressures tend to run low and despite what others said you probably did not go overpressue. That said much of the advice on working up loads here is spot on.

    Your starting load is around a known node for Varget and IMR 4064 with the 168 SMK. It should give you results very similar to Federal GMM. Going higher in Federal brass is going to reduce brass life.

    I’m going to suggest yo look into Dan Newberry’s OCW reloading system. It’s not the only one but it seems to fit what you are working with.
    I didn’t have any pressure signs shown immediately. I’ll report back once I get them cleaned and deprimed . But I’ll still err on the side of caution, as evidenced there’s plenty I don’t know that I don’t know. 😅 I’ll look into it, thanks for the rec!