Factory/Manual Discrepency

[CSI-WALLEYE]

I am developing a load for Hornady GMX 6.5 mm in Creedmoor brass. The Hornady manual has a recommended COL of 2.710 but the factory ammo has a COL of 2.695 (as measured by me. I am new to reloading. Is this unusual?

 

[Icefire]

Best to find your max COAL using the ogive mesurement and go for a 0.015” jump if mag permit it.

 

[emerson]

Coal is gun specific. Factory is short enough to work in all (mostly) production rifles. Another reason handloading could give performance gains in your gun.

 

[hometownhero]

Use the length for your gun not the gun used to test their load data in.

 

[Chas59]

I'm guessing every factory round from different Mfg, would have different COAL. As has been mentioned, measure the bullet length to the lands, I use the Hornady OAL gauge, then load to give you various jumps to the ands to see what your rifle likes best. For reloading rifle rounds, the OAL gauge is invaluable.

 

[CSI-WALLEYE]

When developing a load is it better to vary the cartridge length or powder charge first?

 

[Chas59]

Powder charge is a bigger variable than cartridge length IMHO. I would go with the the loading manual COAL, play with what powder charge is most accurate, then adjust COAL.

 

[Chilly807]

Powder charge is a bigger variable than cartridge length IMHO. I would go with the the loading manual COAL, play with what powder charge is most accurate, then adjust COAL.

I agree. Find what powder charge/primer/brass combination gives you the best results and fine tune with seating depth after that.
You may be surprised at how insensitive the rifle is. A few thousandths either way won't make much difference in accuracy in my experience. My Tikka shoots about the same between 0.010" jump all the way out to about 0.040". It's slightly better at 0.010", but not much. Beyond 0.040" jump, my groups start to open up.
Your ideal jump may vary depending on the bullet you use. IE, you may need to experiment with different jump dimensions when you change bullets.

 

[Ganderite]

Powder charge is a bigger variable than cartridge length IMHO. I would go with the the loading manual COAL, play with what powder charge is most accurate, then adjust COAL.

I start all loading work ups with the bullet about 20 thou off the rifling. I find this by seating long and chambering the round. If I see rifling marks, i seat deeper and deeper until there are no marks. Then i give the seater a quarter turn and call that a good start length for the COAL.

Then I load my survey of powder charges with that length. When I find a good powder charge I will try it with 20 thou longer and 10 thou shorter, to see if there is an improvement.

For a hunting rifle, I will load to just under magazine length and if those bullets don't hit the rifling, that is the length I load.

As mentioned, the length of factory ammo and the loading manual means nothing.