Brass lots with different case capacities (H2O)

[sandman]

Okay, so I recently got back to reloading for my 7mm-08 and I need to make a change. I had been using Federal Nickel cases (case capacity 53.5 grains) but my brass lot is near it's end. After 5 firings, I've started to see a few split necks, so it's probably time to pitch the whole lot. No complaints as I've gotten plenty of use out of these 100 cases.

I just bought 500 Hornady match 308 cases for a steal, and have full length sized 100 for the 7-08. I built a dummy round to ensure it chambers, given what Rocky, et al have recommended about the increase in neck thickness. It chambers with no problem. Good so far.

Case capacity of the new cases is 55.5 grains of h2o.

Is it safe to assume that my current petloads (under book max with no signs of pressure in my rifle) would be safe in the new brass?

Also, can someone clarify a question I have about pressure. Does the size (capacity) of a given case impact the pressure generated? I know I could fit more propellant in a larger case, but if the powder charge and projectile are kept constant would the pressure produced be equal between the two cases? Is the 3-4% increase in case capacity that significant?

Sorry this is long and maybe I'm overthinking it. I am going to back off a couple grains and work back up, but I was curious to know what the gurus think.

Also, do you guys have any reliable (preferably permanent) way to mark the brass that now sports an incorrect headstamp? Unfortunately I also load for 308. I may just keep this whole lot of Hornady brass for the 7-08 to keep it simple.

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Steve

[dovehunter]

If it were me, I'd want to work up to your pet load using the new cases just to play it safe. I remember years ago I was having trouble with flattened and/or cratered primers using loads right out of the manuals for the 8x57 Mauser with surplus military brass. I switched over to W-W brass and didn't have any more problems. I weighed the military cases and they were a lot heavier than the Winchester cases which presumably was indicative of less case capacity. I also tried the water bit and the military cases definitely did not hold as much. That was enough to confirm at least to me the importance of working up a load from scratch every time you change components. JMHO.

[GoodOlBoy]

Since most folks never check their case capacity, but do stick with a pet load once they have it up I would load a few with your pet load and see what you get.

my 2 cents.

GoodOlBoy

[buckhunter]

Pretty much had the same experience as dovehunter. Also found that normally by changing case's the pet load wasn't the pet load anymore. Sorry to say you will probably have to develope another pet load. I usually try to stay with the same manufacturer once a sweet spot is found.

[sandman]

Collecting data to answer my own question (though i do appreciate everyone's input so far). Pet load in question is Nosler 120gr BT in front of 49.0 gr of Win760, Fed 215 primer to a COL of 2.850.

Federal Nickel (old case): 53.5gr capacity, avg vel 2924, group 0.44" (4 shots)
Hornady (new case): 55.5gr capacity, avg vel 2868, group 0.51" (4 shots)

So, a 3.74% increase in case capacity results in a 1.91% decrease in velocity (essentially a 2:1 ratio). Does this relationship generally hold true for changes in case capacity?

As for the new pet load, I may just leave well enough alone. The decrease in velocity really makes little difference in trajectory, especially hunting normal distances in Pennsylvania woods. I'll probably still load a few at 49.5, 50.0, 50.5, and 51.0 to test--plenty of time on my hands now that the season is over. Plus, it gives me an excuse to shoot more.

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Steve

[Catfish]

Steve, Your test seem to be right on with what the wildcatters use as a rule of thumb. Your in crease in velocity will be 1/2 of the % of the increase in case volume. That assumes the same presure levels.

[sandman]

Catfish,
Are you saying that the same powder charges are operating at the same pressure between the two different cases? I would think the pressure is lower in the larger case, hence the decrease in velocity of the projectile. Or am I wrong thinking that there is a direct relationship between pressure and velocity?

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Steve