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New vs fireformed brass

Againstthegrains · 2026-03-23T02:32:39Z

So this is the scenario. I reload for a few friends. I normally advise them to buy a box of Lapua brass, and enough primers and powder to last a while, so that we don't have to re-do the load development. Also advise to get a bullet that's easy to find. So we start off and develop a load on new brass, normally just charge, but sometimes seating as well. Once we find something that does sub MOA, we load up the rest of the box.So far so good. They now return with spent cases, and I neck size them with a collet and reload the same load. What is going to change between new and fire formed brass?1. Velocity decreases - the fireformed brass increases the size of the space in which the pressure builds up, resulting in lower presser and the need to add 0.1-0.3gr of powder to get it back on node.2. Nothing - the pressure (and indirectly barrel time and velocity) is based on the size of the chamber, and since the brass thickness and chamber doesn't change size, there is no need to change anything.3. Velocity increases - this seems counterintuitive, but the case expansion of new brass to fit the chamber absorbs energy. On fireformed brass, the pressure build up is more direct, because the brass does not need to expand in to the chamber, so all the energy is transferred to the bullet, and pressure is higher, therefore one needs to decrease the load by 0.1-0.3gr to stay on the node.ChatGPT got a bit baffled by this, I think because a lot of guys with custom build competition rifles with tight chambers seem to get different results than commercially chambered rifles with high tolerances. Does anyone have some real world experience of this?

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Treeman

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sorry, just had to ...............................

I am who I am - I am not who you want me to be.
Therefore I am me.

Posted : 31/03/2026 11:52 am

Newton

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aaaaand, yet my Oupa shot springbuck in the head at 300 yards from off a running horse using a old .303 and what ever ammo he had.
? ? / how did he do that with no special chambers then ?

With .. some difficulty 🙂 ;D

Posted : 31/03/2026 10:24 pm

Newton

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The only competitors that I know that use standard SAAMI chambers are hunters with off-the-shelf rifles and UK Bisley shooters. As store-bought ammo must be to SAAMI standard. UK bisley shooters use issued ammunition in competition that are loaded to CIP standards. They do not reload for competition.

As far as I understand World Class competitors in F-Class Open DO reload their own ammo

Posted : 31/03/2026 10:25 pm

janfred

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aaaaand, yet my Oupa shot springbuck in the head at 300 yards from off a running horse using a old .303 and what ever ammo he had.
? ? / how did he do that with no special chambers then ?

I'm not quite sure what you mean.

Posted : 01/04/2026 5:07 am

414gates

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I'm not quite sure what you mean.

I think treeman made a typo, he meant half a mile, not 300 yards.

Posted : 01/04/2026 6:58 am

Ds J

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I'm not quite sure what you mean.

I think treeman made a typo, he meant half a mile, not 300 yards.

My great grandfather witnessed it 😉

Posted : 02/04/2026 3:24 am

oafpatroll

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Posts: 1107

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I'm not quite sure what you mean.

I think treeman made a typo, he meant half a mile, not 300 yards.

My great grandfather witnessed it 😉

That seals it form me.

Posted : 02/04/2026 4:04 am

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New vs fireformed brass

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