05-01-2024, 02:20 PM
You'll find several methods and strong advocates/critics for each method.
I use an OBT based approach I developed for myself.
Some will use a method that works for them in larger calibers (308, 30-06, mags) and port them over to the smaller-case rounds like 6Arc... well anyway you get the picture.
I had to first of all evaluate myself as a shooter. In order for non-chrony, shoot-for-best-group on-paper styles, you have to be a consistent sub-moa shooter no matter what, type of guy. I'm not that good. So I adopted a style based more on the physics and engineering concepts rather than just eyeballing groups on paper -- more often that I would like, I may have a flyer or twitch in my shooting position that would ruin a group and I wouldn't objectively be able to tell the difference. So, use mechanical engineering and physics, then add in the shooter as the last piece.
1. For any ladder of rounds you shoot, keep the delta at 1% or less of case/powder capacity. Lots of guys will use loads 0.5 grs apart, but like I said that is really for larger case calibers, not a 6Arc or grendel. So load deltas of 0.3 or 0.2 grs over your range.
2. Keep in mind that too-small of a delta will give you readings that are not significant since MVs they yield will no doubt be within your SD of MVs for the previous load: "noise", "in the weeds". If you're a competition shooter you already have a method, so for the rest of us, where MV std deviations will be anywhere from 8-ish to 20+, load deltas of 0.1 grains will use powder but not give you anything significant.
I will do a ladder of shots first. This step is to discover how the bullet, primer, case, powder and barrel react -- use mfr or published data first. Shoot over a chrony, I use a magnetospeed but the radar types will work fine also. Never have used the older styles since I shoot on public ranges and a magnetospeed doesn't require the range to go cold for setup... plus accuracy % is much better. The radars came after I had already bought, so no real gain for me there.
But you have to have a chronometer, no question.
Shoot 1 round at the 0.3 gr interval over a range of at least 1.5-2.0 grains, starting somewhere low in the powder range -- don't start up close to max for your start. Load and shoot one at a time. Write everything down. Now take that data home and log it and graph it in Excel, vs the mfr data, to see how your setup compares to the mfr data.
I might pause at this point to say, why do this? First of all, for a given powder and caliber combo, inside of "normal" limits, the response to an increase in powder should be a linear increase in MV (in real world there will be some fuzziness but that's ok). If not, then compression, bullet jams, too much void inside the case, etc, some abnormality is happening. So with Excel I can see what that linear response "ought" to be (I use a linear curve fit, it's an excel function in the graph so you don't have to draw it yourself).
Now you know what the real-world MV response for your barrel and your combination should be. From this you can look for a range that is close to a node (as defined in "optimal barrel time" - see Chris Long).
For a particular obt node (this is based primarily on barrel length and MV, not powder or even the bullet!), pick a range surrounding it and shoot 4- or 5-shot groups at 0.2 intervals. Again chrony and look for the best SD and tightest MV average. Not worried about actual group size (remember I'm less than a perfect shooter anyway!). That's your target load/MV for that combo.
Now for that target load, shoot again 4- or 5-shot groups varying only the OAL, I use 0.010" intervals. By the way, unless it's a vld I try to have my OAL end up somewhere close to 0.060-090" off the lands. But sometimes, esp in the Arc, you don't have that capability, in that case use OALs that you can reasonably get. Example, the Hdy 6mm 105 gr BTHP usually touches my lands at ~2.270-ish so I use 2.220. Likewise the 95 Nosler ballistic touches at 2.222, I use about 2.200. With an AR especially I don't do any jams into lands.
This varying of OAL changes the "timing" of the bullet's exit and 1 group should have significantly better group at 100 yds than the others. That would be the OAL you load for, at that charge, for that bullet, primer, case, powder-lot#, barrel setup.
One final note: If you change anything, ANYTHING, in that combo, start again lower and work up. Just like grandma's biscuit recipe, any change of ingredients can mean flat, yucky biscuits!
I use an OBT based approach I developed for myself.
Some will use a method that works for them in larger calibers (308, 30-06, mags) and port them over to the smaller-case rounds like 6Arc... well anyway you get the picture.
I had to first of all evaluate myself as a shooter. In order for non-chrony, shoot-for-best-group on-paper styles, you have to be a consistent sub-moa shooter no matter what, type of guy. I'm not that good. So I adopted a style based more on the physics and engineering concepts rather than just eyeballing groups on paper -- more often that I would like, I may have a flyer or twitch in my shooting position that would ruin a group and I wouldn't objectively be able to tell the difference. So, use mechanical engineering and physics, then add in the shooter as the last piece.
1. For any ladder of rounds you shoot, keep the delta at 1% or less of case/powder capacity. Lots of guys will use loads 0.5 grs apart, but like I said that is really for larger case calibers, not a 6Arc or grendel. So load deltas of 0.3 or 0.2 grs over your range.
2. Keep in mind that too-small of a delta will give you readings that are not significant since MVs they yield will no doubt be within your SD of MVs for the previous load: "noise", "in the weeds". If you're a competition shooter you already have a method, so for the rest of us, where MV std deviations will be anywhere from 8-ish to 20+, load deltas of 0.1 grains will use powder but not give you anything significant.
I will do a ladder of shots first. This step is to discover how the bullet, primer, case, powder and barrel react -- use mfr or published data first. Shoot over a chrony, I use a magnetospeed but the radar types will work fine also. Never have used the older styles since I shoot on public ranges and a magnetospeed doesn't require the range to go cold for setup... plus accuracy % is much better. The radars came after I had already bought, so no real gain for me there.
But you have to have a chronometer, no question.
Shoot 1 round at the 0.3 gr interval over a range of at least 1.5-2.0 grains, starting somewhere low in the powder range -- don't start up close to max for your start. Load and shoot one at a time. Write everything down. Now take that data home and log it and graph it in Excel, vs the mfr data, to see how your setup compares to the mfr data.
I might pause at this point to say, why do this? First of all, for a given powder and caliber combo, inside of "normal" limits, the response to an increase in powder should be a linear increase in MV (in real world there will be some fuzziness but that's ok). If not, then compression, bullet jams, too much void inside the case, etc, some abnormality is happening. So with Excel I can see what that linear response "ought" to be (I use a linear curve fit, it's an excel function in the graph so you don't have to draw it yourself).
Now you know what the real-world MV response for your barrel and your combination should be. From this you can look for a range that is close to a node (as defined in "optimal barrel time" - see Chris Long).
For a particular obt node (this is based primarily on barrel length and MV, not powder or even the bullet!), pick a range surrounding it and shoot 4- or 5-shot groups at 0.2 intervals. Again chrony and look for the best SD and tightest MV average. Not worried about actual group size (remember I'm less than a perfect shooter anyway!). That's your target load/MV for that combo.
Now for that target load, shoot again 4- or 5-shot groups varying only the OAL, I use 0.010" intervals. By the way, unless it's a vld I try to have my OAL end up somewhere close to 0.060-090" off the lands. But sometimes, esp in the Arc, you don't have that capability, in that case use OALs that you can reasonably get. Example, the Hdy 6mm 105 gr BTHP usually touches my lands at ~2.270-ish so I use 2.220. Likewise the 95 Nosler ballistic touches at 2.222, I use about 2.200. With an AR especially I don't do any jams into lands.
This varying of OAL changes the "timing" of the bullet's exit and 1 group should have significantly better group at 100 yds than the others. That would be the OAL you load for, at that charge, for that bullet, primer, case, powder-lot#, barrel setup.
One final note: If you change anything, ANYTHING, in that combo, start again lower and work up. Just like grandma's biscuit recipe, any change of ingredients can mean flat, yucky biscuits!