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Join Date: Jun 2009
Location: Northern Berks County, PA
The effect is related to internal capacity. The thickness is usually related, but it's not the cause. It's the capacity.
Generally speaking, military brass tends to be thicker-walled, and since external dimensions should be standard, then thicker brass means less internal capacity.
Less internal capacity means higher pressures for the same load specs. So if you're shooting a 30-06 load of a 150gr Hornady SP bullet with 51.5gr of IMR4064 (a load I used a lot in bolt guns over the years) in RP, Hornady, Federal, etc brass and you go and stuff it into some LC or HXP brass, you're likely going to experience higher pressures, even though all the load parameters were kept constant.
Does this effect anything? Generally, yes.
1- if you're near max pressure and change brass, whether to military make or not, you should back off the load and work it back up to be sure you're ok.
2- even if there's "slack" and you have capacity for for increased pressure to remain safe in your load/rifle, the change in pressure will map to changes in velocity, and the overall results on paper are almost certain to change to some degree, as this will change the barrel harmonics.
How do you know if this goes on?
Easiest way is a chronograph. Higher speed = higher pressure. Really no way around that bit of physics.
How do you know if there will be problems before they occur?
You measure the internal volume of the cases. How? You take 5 cases from each make. These should be fired in the rifle you are using them in, but NOT RESIZED. Weigh them. Make sure you can ID each case (I use sharpie and number them 1 through 5). Record the weight of each case.
Now use an eye dropper, and carefully (not making air bubbles), draw up water in the eye dropper, and then transfer it (carefully again) to a case. Fill it till the water bulges over the neck but doesn't drip (you'll see it when you get to that point). Weigh it. Record the weight for that case. Do this for the rest of the batch.
Now calculate the difference in the empty and filled weights. Average it for the 5 cases (you should average this, as there will be some variation from case to case).
If you do this for two different makes of brass, you'll quickly see the difference in internal capacity between the brands/makes.
Higher average case volume = reduced pressure for a given load
Lower average case volume = increased pressure for a given load
Either way, your groups and POI can change simply from a change in brand of brass. The safety concern is if you're up there in pressure on your load and then change brands of brass.