Procedure to Tune load at 600 yds.

[MMH]

I have an AR15 that I use for PRS and want to tune my load at 600 yards. The load that I have been using for quite some time is a Hornady 75 gr. BTHP with 23.5 grain of XBR 8208 under it. I am not seeing any pressure signs but think that 23.5 is close to a maximum load. The base to ogive distance is 1.821" and the bullet jump is 0.022". The average velocity is 2862 fps (shot out of a 22", 1-7 twist, 4 groove, Bartlein barrel). The Std.Dev. on velocity is 11 fps (based on labradar data but not very high round counts (four sets of 10 shots)).

At shorter ranges the rifle shoots 0.75 moa, but I have never grouped at longer ranges. I know that accuracy really starts falling off at 900 yards but expect that I am beginning to go transonic at 800 or so. Switching to 75 ELD-M, should get me to 1000 yards but I want to burn up my BTHP bullets first.

Anyway, I was going to load 5 rounds starting at 23.0 grains and going up to 24.0 grains in 0.1 grain increments and shoot at 600 yards. I am going to look at group size as well vertical dispersion. The vertical dispersion will be compared to the muzzle velocity (which should correlate). Anything else that I ought to consider?

 

[Newbie2020]

This came up in a different thread and I think the general consensus was that switching between 75 gr bullets wouldn’t change load data enough to matter but I’m curious to see more feedback.

 

[nn8734]

At shorter ranges the rifle shoots 0.75 moa, but I have never grouped at longer ranges.

Are you saying you’ve done grouping drills at 600y and weren’t happy with the results or you have never done grouping drills at 600?

At distance, group size itself isn’t as indicative of a load’s consistency as vertical dispersion. Lateral dispersion is almost always driven by the shooter (fundamentals or wind pattern compensation). If your rifle is consistently shooting .75 MOA at 100 with a 40 round SD of 11, I don’t see any reason why those numbers shouldn’t hold up at 600, assuming the shooter does his part, unless there are some inconsistencies in the projectiles themselves. But even then, it may not show up until 800 or so.

 

[smoothy8500]

Anything else that I ought to consider?

Consider that you don't need to over think your plan. It's all good.

 

[MMH]

Are you saying you’ve done grouping drills at 600y and weren’t happy with the results or you have never done grouping drills at 600?

At distance, group size itself isn’t as indicative of a load’s consistency as vertical dispersion. Lateral dispersion is almost always driven by the shooter (fundamentals or wind pattern compensation). If your rifle is consistently shooting .75 MOA at 100 with a 40 round SD of 11, I don’t see any reason why those numbers shouldn’t hold up at 600, assuming the shooter does his part, unless there are some inconsistencies in the projectiles themselves. But even then, it may not show up until 800 or so.

I have not done any grouping drills at 600 and have not really done enough to tune my load.

When grouping I would analyze the vertical dispersion and correlate against velocity. I will look for flat spots in velocity & vertical dispersion as the charge is varied.

No doubt, in my case at 600 yards group size will be dominated by wind, shooter error, etc. Nonetheless, it should be an interesting exercise, good practice and a chance to see how I group at 600.

 

[nn8734]

I have not done any grouping drills at 600 and have not really done enough to tune my load.

When grouping I would analyze the vertical dispersion and correlate against velocity. I will look for flat spots in velocity & vertical dispersion as the charge is varied.

No doubt, in my case at 600 yards group size will be dominated by wind, shooter error, etc. Nonetheless, it should be an interesting exercise, good practice and a chance to see how I group at 600.

I also do grouping drills at 600m with my rifles and agree it’s great practice for a variety of things.

Id do a grouping drills at 600m with your current load to establish a performance baseline before spending time/resources on additional test samples of different charge weights. You may not have to do anything but if you do, at least you have a test target for basis of comparison.

I set three 2/3 ipsc targets at 600m then do 5 shot groups on each, letting the barrel cool between each group. Then I’ll look at the vertical dispersion (I also eliminate any called flyers from the measurement).

 

[Newbie2020]

I will look for flat spots in velocity

Why look for flat spots in velocity? Flat spots have been thoroughly debunked in several threads recently. Chasing phantom flat spots is a waste of time and components.

If you’re looking for pressure signs near maximum charge weights that’s a valid practice; flat spots are a statistical fragment caused by normal variation around the mean.

 

[nn8734]

Agree that flat spots in context of ladder testing is a myth. Do three ladders, same load recipe and you will get three different flat spot segments in your charge weight curve.

At 600m all you’re really looking for are signs of unusually high vertical dispersion, the cause likely be either sourced to the shooter or to the bullets themselves.

a load thst groups well at 100-300 will perform at distance, all other things equal. For performance at the max effective range of the system/cartridge, getting powder charge and neck tension as consistent as possible is critical to keep that hit probability up in the last ~10% of the round’s supersonic flight envelope.

Start with your current load and let the target tell you if the load needs tweaking.

As an aside, over pressure condition can be identified comparing OD of the case web before and after firing. It will show up there before physical signs on the brass are seen.

 

[Dthomas3523]

I have not done any grouping drills at 600 and have not really done enough to tune my load.

When grouping I would analyze the vertical dispersion and correlate against velocity. I will look for flat spots in velocity & vertical dispersion as the charge is varied.

No doubt, in my case at 600 yards group size will be dominated by wind, shooter error, etc. Nonetheless, it should be an interesting exercise, good practice and a chance to see how I group at 600.

Vertical “flat spots” on paper don’t happen because of actual velocity flat spots.

More powder = more speed