New brass vs fired brass math

[912173]

I've started to come to the conclusion that a lot of the common high/low node charge weights might have more to do with fired vs new brass than most people consider and that it definitely needs to be mentioned in the load data context.
For instance, 77 smk with 8208 xbr, most guys will say about 23.6-23.7 or they will say 23.1-23.3. I use 23.7 with new brass for 2700 fps in my gun and in fired brass it seems hot. I back it down to 23.2-23.3 to get back to 2700 and it goes right back to normal. 20 rd avg change from new (2705) to fired (2741) was 36 fps. I found the same change with my .308 175 smk /4064 loads. Almost the same velocity change and percentage of change. It changes from [email protected] (new) to [email protected] (fired) which are also both both popular loads for a 175 smk. I've found if I multiply my new brass charge by .98, it gives me an almost perfect fired brass load. If I want to find a load for new brass and I developed the load on fired, I can multiply it by .019, add it to the charge and get a theoretical new brass charge within the accuracy of my chargemaster. These numbers are a pretty good ballpark and I'm sure they could be refined with more data. It works with my 5.56, .308 and it's pretty close with my 300 Norma, although I don't have too many numbers for that one yet. I just figured I'd throw this out there because it's pretty interesting and I've never heard anyone mention a mathematical solution.

 

[eschafer]

it certainly requires a little more energy to propel the bullet down the barrel and fire-form the case at the same time... just never did the testing you have... your numbers make good sense, thanks for sharing......

 

[Dthomas3523]

......it’s literally mentioned all the time when loading is discussed.

There are generally two schools of thought:

1: shooters have a generic safe load they run in their virgin brass during practice or club matches and then load develop with fired brass

2: those that don’t worry too much with nodes and just run the speed they want. They may or may not change charge weights based on virgin vs fired (basically if they don’t change the charge weight they just chrono the batch of loaded ammo and adjust)

 

[912173]

Yes, fired vs new is constantly mentioned, but usually people leave it at "It changes". What I haven't seen is a constant or predictable change identified. I also thought it was really coincidental that I kept landing on other popular charge weights when adjusting loads (41.8gr 4064 with a 175smk works great in .308 at low 2600s in new brass or at 2700 in fired brass). I just figured it could save some precious time and components doing load tests. It would be cool if other people could refine it with more data.

New brass load x.98 = fired brass load

Fired brass load x .019 + fired brass load= new brass load

 

[mjphawk]

I’m not refuting the idea that new and fired brass result in differences in MV I can’t conceptualize how/why that would occur.

Let’s say new brass is 3% smaller by volume in your chamber because of extra headspace. When the primer/powder ignite, the case instantaneously expands to the chamber and “grips” the chamber of the barrel. There is no vent in the chamber so whatever volume of gas the powder is going to produce is fixed regardless of brass sizing. If by the time the bullet leaves the barrel the brass will have been resized to the chamber, I don’t understand where the difference in pressure could come from.

Not refuting what’s being said but that’s what I had heard before and that made sense to me (certainly took some of the stress out of my brass prep steps).

 

[Alan Warner]

I’m not refuting the idea that new and fired brass result in differences in MV I can’t conceptualize how/why that would occur.

Let’s say new brass is 3% smaller by volume in your chamber because of extra headspace. When the primer/powder ignite, the case instantaneously expands to the chamber and “grips” the chamber of the barrel. There is no vent in the chamber so whatever volume of gas the powder is going to produce is fixed regardless of brass sizing. If by the time the bullet leaves the barrel the brass will have been resized to the chamber, I don’t understand where the difference in pressure could come from.

Not refuting what’s being said but that’s what I had heard before and that made sense to me (certainly took some of the stress out of my brass prep steps).

The energy required to fire form new brass softens, --- changes the pressure curve.

 

[mjphawk]

The energy required to fire form new brass softens, --- changes the pressure curve.

Thanks. That makes sense.