Case Capacity question

[QuickNDirty]

I developed a load in (.308 win) Winchester brass of which I have about 50. I have several hundred CBC brass cases which are more like military brass (crimped primer, heavier, etc).

The Winchester brass I water-weighed to an average of 56 grains.
The CBC brass water-weighed an average of 54 grains.

The load I like is 40.4 grains of N140 which is 1.6 grains below the maximum load (lyman 49th) for the bullets I want to shoot (180 grain SGK/SMK).

I haven't shot any reloads with the CBC brass, yet, but I think I found a crude way to gauge the performance, but it could be total BS.

Here it is: The 40.4 grains of powder in the Winchester brass sounds like 38.0 grains in the CBC brass when shaken next to the ear.
I have no idea if a comparison like that will be precise to any degree, but in other related fields, the shake comparison has worked well (fireworks, anyone? Fuller sound -> bigger bang, same sound; same bang)


All that aside, how much of a difference does 2 grains of capacity make? In other words, if 38 grains in one case does something, 38 grains in a smaller capacity case will do more based on what I understand about powder burn rates under compression, but approximately how much of a difference are we talking, all else being equal? I'd be happy if my 40.4 grain load only took 37.6 grains in a different type of case I have an abundance of =)

 

[BobinNC]

The difference between a 56 grain case and one of 54 grains is 2 grains or about 3.5% less capacity. Reducing your 40.4 load by 3.5% would yield a reduction of 1.4 grains. I would load 5 @ 39.0 grains in the CBC brass and do a test.

Fire one and look for high pressure signs, if OK fire the rest. Chrono them, check for accuracy, then proceed, up or down with the charge as necessary.

 

[QuickNDirty]

The difference between a 56 grain case and one of 54 grains is 2 grains or about 3.5% less capacity. Reducing your 40.4 load by 3.5% would yield a reduction of 1.4 grains. I would load 5 @ 39.0 grains in the CBC brass and do a test.

Fire one and look for high pressure signs, if OK fire the rest. Chrono them, check for accuracy, then proceed, up or down with the charge as necessary.

Huh.. I hope it's that easy. I loaded 5 up @ 39.0 grains and will see what they do on Wednesday.

Thanks!

 

[QuickNDirty]

39.0 grains had one case that had a hard bolt lift, and it shot poorly as well.

38.0 grains printed half MOA at 100 yards and no pressure signs. No idea how fast they're going, but they sure do shoot good!