incremental powder change or seating depth


November 2, 2009, 04:50 PM
Out of curiosity, during load development how do you proceed? Assume you have decided upon a particular powder, case, bullet, and primer and have previously worked up from starting load. Now itís back to the bench to refine. How do you proceed? Do you use a mid velocity powder charge and vary seating depth or hold seating depth constant and vary powder charge? This is a conundrum I typically face when working up a load for a new rifle and typically results in 3 trips to the range before I begin to refine that last .5 MOA off the load. Curious to see what others feel makes the larger difference, incremental powder change or seating depth?

Thanks all,

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November 2, 2009, 05:09 PM
In the todo list to beat that accuracy I would also add consistency in shell length, neck tension (i.e. brass thickness @neck), concentricity of the neck and of bullet seating. Only doing the best for the above, I would play with the jump and barrel harmonics that you as asking about. I do not know what of the last two affects the accuracy more than the other. Probably, the answer depends on rifle and is not universal.

November 2, 2009, 05:20 PM
2 issues for me.

issue #1
Of the rifles I load for seating depth is a non issue. The lands are too far to get close and have enough bullet in the case or fit a mag. I stick the the bullet in so that I have a full diameter in the case. Then I go OCW on the powder charge. I've been very lucky with this method. The brass prep is nothing special except I am very consistent in doing the same thing every time. I trim every time. I have picked powders that are regarded as accurate for the caliber as well as 1 or 2 that are not as popular.

issue #2
My shooting has been constantly improving. At first the weak link was me not the ammo. I picked a charge weight by using OCW and didn't change anything about the load. My groups kept shrinking as I figured out how to shot more accuratley and consistently. The premise was that my ammo was not a variable and I could not develop the load any better until I shot better.

Speculation on my part:
My idea is tune charge weight first and then seating depth. I have heard that some bullets are more tolerant of jump to lands than others.

November 2, 2009, 06:28 PM
Seat the base of the bullet at the neck shoulder junction. Then up the powder charge.

November 2, 2009, 07:56 PM
I'll load a handfull of cartridges in 3 groups of minimum, medium, and max powder charge. I'll use .020" jump to start. Whatever powder charge was best I'll stick with, then starting closing the jump. Reducing jump increases pressure so if the max load was my best shooter I will reduce the powder charge slightly as I reduce jump. I never go under .005" jump even on my competition guns. Some guys jamb the lands but they are braver than I.

November 3, 2009, 04:44 AM
~z, this is a good question and I have my own theories about this tied into the OCW method. This is my thought process and procedure.

1. If the chamber/magazine combination allows, I start by seating all bullets 0.020" off the lands. If the magazine limits OAL then I seat the bullet for reliable feeding.

2. I want to find the CHARGE that produces the best results in terms of barrel harmonics as quickly as possible. In other words, a charge such that the average group center of three loads, OCW load - 0.3gr, OCW load and OCW load + 0.3gr is in the same place on the target (5-shot groups at 100 yards).

3. I generally want to maximize velocity so if the first OCW load is too slow, I push on to the next OCW load assuming that I'm not grossly exceeding published loads. I ALWAYS find the best results near the upper end of the load data so I don't waste time or money starting at the "starting" loads. I usually work up six or seven loads starting at 0.6gr over a published max and work my way backwards shooting the lightest load first and checking for signs of overpressure.

4. Once I find the OCW load with the required velocity, I realize that seating the bullet in or out won't change the OCW load ... it may affect group size a little and result in the groups moving up/down slightly due to slight changes in velocity, but whichever seating depth works best, that load +/- 0.3gr will group on the same place on the target which makes it the OCW load

5. I make up six sets of OCW loads (5 rounds each) with the bullet seated 0.005", 0.010" 0.015", 0.020", 0.025" and 0.030" off the lands.

After shooting these loads I know all I need to know. Using this method, five and six shot groups will be around 0.3 MOA at 100 yards, 0.5 MOA at 200 and 300 yards, 0.7 MOA at 400 and 500 yards and 1.0 MOA at 600 yards. Those numbers are due to my skill level and the wind and not the load. With no wind, I feel confident that 0.5 MOA is possible at 600 yards and maybe better than that. Just over a week ago I shot a 1.15 MOA 23-shot group at 600 yards with a variable 3 - 8mph crosswind. With my load, a 2mph difference in the wind means a 6" POA/POI difference at 600 yards ... almost 1 MOA so I know that a number of shots were blown off center that day.

I can usually find the BEST load (for a given powder/bullet combination) for any of my rifles in less than 100 rounds and usually a lot sooner than that (70 usually).


November 3, 2009, 08:11 AM
Yes, it is a good question. Like 1858, I'm not normally looking for an accurate low-power load, so I quickly get up to the velocity range I want to be in, and then go looking for an accurate load. My approach is to establish a OAL based on either magazine constraint or distance off the leade. I then vary powder charge weights in .3 grain increments until I find a load that performs well and remains within the velocity/pressure range I have selected.


November 3, 2009, 09:04 AM
OAL- Bullet jump and bullet jammed into the lands are the only 2 things that make a difference. Bullet jump is almost the same pressure wise. It does not matter if its .050" or .010" off the lands,the pressure on firing remains about the same because the case volume is not greatly changed. If the bullet is jammed into the lands, there will be higher pressure, maybe as much as 10,000 PSI. The key factor in accuracy on a bottle neck round is getting the round centered in the middle of the chamber. Controling shoulder bump, bushing dies, only sizing 1/2 of the neck, and bullet jam control this. Look at the pressure curve here on firing. The OAL could make a bullet exit the muzzle at a different time cycle, catching the barrel at a certain point as it flexes.

November 3, 2009, 09:55 AM
My method is simple.

Set the OAL to the longest that will feed from the magazine.

Start at the start charge and load 4rds per .5gr increments, note accuracy.

Next run 5 shots each charge between the increments that gave best groups.

Finally shoot 10 shots the most accurate charge of that last batch. That's my load.

Let's say the spread in data is 40-43gr.

4 rds each at:

Let's say the 42 and 42.5gr were the most accurate.

Then load 5rds each at:

Next we find that 42.3gr was the most accurate so we load 10 rounds of that load to confirm.

twice barrel
November 3, 2009, 10:14 AM
First, I don't preselect all of those things!

Using cases once-fired in the gun I'm loading for:

Clean brass, neck size, true up necks, flashholes, and primer pockets. Weigh cases and select a batch all within 5gr of one another. Depending upon action and magazine select longest length I can seat the bullet and still feed reliably and not jam the bullet into the rifling. This can vary a lot between guns and is not advisable for loads to be shared between guns.

Start with load somewhere between minimum and half-way between minimum and maximum. Shoot 5 shot groups in half grain increments from a solid, comfortable sandbag setup at 100 yds. Measure each group from center to center at widest point. Note the best group. Repeat with load half-grain below, same, and half-grain above to confirm. Satisfied? Yes=stop. No = depends on how close you are to being satisfied. If close, experiment with seating depth. If this gets it and the cartridge cycles thru action're done. If not, select a different bullet of same type and weight and repeat. Don't settle on final length until you've settled on the bullet and then fine tune the length.

(Watch for signs of pressure and never exceed maximum charge.)

When this fails, I make note of the bullet that achieved the best results and change powders and start the whole process over. Its my believe there is no one right way to go about it other than to be systematic and eliminate one variable at a time. Sometimes it works and sometimes not.


November 3, 2009, 10:39 AM
All, I am acutely aware of the importance of brass prep. For this discussion, let’s assume the only variables we are concerned about in this scenario are seating depth and powder charge. Also assume ANY LENGTH will fit the magazine (as I will be working up a short action cartridge in a long action rifle for this reason).

1858 I use a very similar method, just curious to see if it can be done consistently in under 100 rounds.

I typically find the powder charge that provides the best “general accuracy”, then refine seating depth with this charge weight, then fine tune the powder volume, and typically refine the seating depth again. And yes, this usually takes about 100 rounds to find.


November 3, 2009, 12:26 PM
...this usually takes about 100 rounds to find.

Yikes, that's alot of barrel life spent on load development.:eek:


November 3, 2009, 12:38 PM
Sometimes less, but you can see why I ask the question..

November 3, 2009, 01:13 PM
Yikes, that's alot of barrel life spent on load development.

Don, I'm going to respectfully disagree. I feel it's a waste of barrel life not to put the time and effort in and the necessary amount of rounds. For a .308 Win barrel, 100 rounds is just 2% of the typical 5,000 round barrel/throat life. Now include the trigger time and general practice that comes with load development and it's not something to shy away from. Typically, all that is needed after 5,000 rounds is to have a new throat cut which isn't expensive compared to the cost of the ammunition. In addition, the "best" load will still be the best load with the new throat since the mass of the barrel hasn't changed (not significantly anyway). If you shoot another 5,000 rounds then load development only accounted for 1% of the usable life.

1858 I use a very similar method, just curious to see if it can be done consistently in under 100 rounds.

~z, I don't think it can but it's not all bad. 100 rounds is a small price to pay for that information which can be applied to other loads i.e. different powder or bullet for the same rifle. Also, that load won't change for that rifle if the same barrel is used i.e. new chamber/throat, same barrel.


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