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?

 

[fkimble@charter.net]

Do you throw or weigh powder charges, and how much variation in you charges?

Frank

 

[MMH]

Do you throw or weigh powder charges, and how much variation in you charges?

Frank

I weigh using a Charge Master 1500. The charges are within 0.1 grain. However, I have never verified the accuracy of the scale.

 

[Ned Ludd]

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?

I routinely do all preliminary load development for my F-TR rifles (charge weight/seating depth) at 100 yd. Once the load is acceptably tuned at 100 yd, I always shoot a few practice groups at 600 yd to ensure the precision and velocity have not deviated from the initial workup. However, the majority of the "tuning" is done at the shorter distance. For most shooters, too many other variables including wind and terrain effects can impact the interpretation when trying to do load development entirely at 600 yd, especially with relatively light .224" bullets. I have yet to find a load that tuned well at 100 yd that wasn't also more than capable of winning F-TR matches at 600 yd. There are certainly disciplines such as BR that do not follow this approach. However, it should be more than adequate with your AR15 setup. My suggestion would be to make it easy on yourself and tune the load at a shorter distance such as 100-200 yd where external factors are less likely to make interpretation of the results more difficult.

 

[dnellans]

You should test the elds before investing in a lot of them. It is not meant to load to mag length and you may have give up velocity to do so, negating some of the improvement in BC.

 

[CharlieNC]

Start with a ladder test 2/charge at 0.2gr increments from 22.5 - 24.5 and you should find a node where there is little difference in vertical poi at 200yd. There are several harmonic factors at play, and this should provide a clear result.

 

[kevinbritvec]

If you have 22" barrel your velocities seem extremely high. I would expect 2700 fps to be a hot load (safe maximum) from a 22", I look for 2650 fps from a 20" service rifle as my max.

My 75/77 grain ammo runs 2750 fps from a 24" barrel. I have been accused of being conservative regarding load data, but I look at actual velocity being produced from my rifles versus published data taking barrel length into consideration when working up my loads.

IMO AR-15's shouldn't be pushed the way you can a bolt action. I use .3 grain increments in .223 and am happy with any load that groups under 1 moa for ten shots. I am shooting NRA high power and our targets are pretty generous. We don't shoot past 300 yards feeding from a magazine, at the 600 yard line ammo gets loaded one round at a time so I am allowed to shoot 80 grain bullets seated close to the rifling.

You might want to try lowering your charge weights to see if your rifle will group better with less fuel.

 

[JFrank]

PRS guys can tune in a phone booth with just a chronograph.
ES & SD are very important whereas br shooters only concern themselves with group size and shape.