Does case capacity effect velocity?

[Iamero]

I’ll make this one short. Did some velocity testing last week on the 260 and today on the 6.5-284 over the magneto and got the following results. Temperature difference of about 20 degrees is the only variance (colder today).

42.3gr H4350 in a 24” 1:8 260 Rem with 140 ELDM = 2795 FPS 5-shot average
Round count about 200.

47.3gr H4350 in a 24” 1:8 6.5-284 with 136 Scenae-L = 2760 FPS 5-shot average
Round count about 60.

Why would 5gr more powder cause a slower velocity? Same primer in both calibers, same muzzle brake.

 

[Supersubes]

Absolutely it affects velocity! Or more specifically, it affects pressure.


Looking at the data for the 6.5-284, a slower powder will be needed to take advantage of that extra capacity.

 

[lash]

Like Supersubes said.

Just for comparison, I use 41.8gr H4350 under 136 Scenars in my .260 with 26" barrel and am getting 2770 fps. That is my preferred medium velocity load. I have a 2850 load developed, but prefer the longer barrel life, accuracy and consistency I get from the medium load.

 

[CJS-6.5]

Less pressure.

 

[Iamero]

Gotcha, that’s what I was curious about but didn’t know it would make that much of a difference! Does it make sense to load more powder into there or switch powders to get the velocity? I don’t want to rip it over 2,900, just get to a consistent 2850.

 

[Supersubes]

Youre already past hodgdon book max, so you’ll need more of a slower burning powder to take advantage of all the capacity you have.

Edit: Man, hodgdon data on the 6.5-284 just sucks! They must have shot that the same day and with the same shoddy sensor calibration they shot 140gr 6.5 creed with.

Do yourself a favor and try RL26, bet you can hit 3000fps easy.

 

[bullfisher]

That 284 should be well over 2900 with those 136gn.

 

[Supersubes]

http://forum.accurateshooter.com/threads/rl-26-with-6-5-284-norma.3913676/

 

[cperazza]

6.5-284 with H1000 (Retumbo also works)
- 3020 fps / 74° / 27.5" Barrel / Krieger #18 / 8.5T
- 140 gr Hybrids/VLDs Bergers

 

[Iamero]

I saw that RL26 is another real common powder so I picked up some of that to try. If that doesn’t pan out then I’ll just still to H1000 since I use that in my 300 and 28 Nosler as well.