Forming Die

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Re: Forming Die

Post by tcoz » Sat Feb 07, 2015 11:33 pm

This is from the FAQ section on the Lee Precision website and explains the reason why some people have problems forming 300 BLK from 5.56 cases using Lee sizing dies and other people don't...

Forming vs. Sizing
Case forming dies are usually cut to size the case slightly smaller than the minimum SAAMI (Small Arms Ammunition Manufacturers Institute) dimensions, to account for the brass case's tendency to spring back after sizing.

Our full length sizing dies are cut to size the case closer to the middle of the SAAMI dimension, because that is where most rifle's chambers are made, and to size to the minimum would shorten case life. Whether or not our full length sizing die will work as a case forming die depends upon the chamber dimensions of your rifle, and how much spring back there will be when sizing the case.


Case Forming
The full length sizing die included with the Pacesetter die set is not technically a case forming die. There is a certain amount of spring back in the brass case, and this spring back has to be taken into consideration when designing the die and its intended purpose.

Case forming dies usually need to be made to smaller internal dimensions to compensate for more spring back, because the case dimensions are being altered to a greater degree. Sometimes a full length sizing die will work as a case forming die, but it depends on a large number of variables, and is impossible to predict with any accuracy.
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Re: Forming Die

Post by eugenesan » Sun Jan 19, 2020 1:40 am

I know I am resurrecting a very old thread but previous post is probably the most valuable piece of information on the subject, so I decided to continue.

Recently, I've attempted to form a test batch of 223/5.56 (several headstamps) to 300blk and was experiencing some difficulties.
(I use Lee Pacemaker set with a full sizing die)

While forming the brass, I was referencing the datum measurement of once fired and resized factory cases.

Here is what I've found:
1. I couldn't get the datum measurements for the freshly formed cases to match the measurements of factory cases. The measurements of the formed cases were always ~3-4 thousands longer, no matter how deep the sizing die was pushed.

2. 2/3 of the cases (regardless of the maker, caliber or year) did pass the Lyman ammo checker, while the remaining 1/3 didn't. I tried to to play with depth of the sizing die and do multiple sizings, but that didn't help. The datum measurements of those cases were stuck at +7 thousands.

3. Later, after a break, I've attempted something strange and to my surprise it did work! What I did is to size each case twice with a short delay (a few seconds or immediately after). Every case, including those that wouldn't size properly even after several attempts, suddenly formed properly. Well, almost. factory cases still have datum measurement 3 thousands shorter, but they all pass the ammo checker test.

Now, after reading the previous post by @tcoz, I think I have an idea what I've experienced.
By not letting the brass fully "relax" before the next sizing I was actually "forging" it into the new dimensions.

It would be nice to hear opinions of a more experienced people about my observations.

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