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JustC...can I get a 5th reply????

n/an/a Member Posts: 168,427
[;)][:D]...and thanks to all who have provided input! To the best of my measuring abilities with my calipers, my old fired .300 Win Mag cases measure 2.155 from the front of the belt to the line where the neck transitions to the shoulder. The fire formed AI brass measures 2.130....a difference of .025. (The length from the head of the case to where the shoulder meets the neck was 2.370 on the Win and 2.351 on the AI...difference of .019). Does this sound about right??? Isn't the datum line half way down the shoulder? How do I measure that?

Now to my next question. The fired .300 Win case is actually slightly longer than the fireformed .300 Win AI. The Win measured 2.620 and the AI measured 2.618...a difference of .002 shorter. Should this be the case? Both cases were trimmed to the same length before firing either of them. The fire formed case that the gunsmith provided with the rifle was actually quite a bit shorter (.039), but that was probably b/c it was a factory load and the brass was shorter to begin with.

Last but not least, if I understand you all correctly, I can just pick a medium load for the .300 Win and be safe using that to fireform my cases.?.?

Thanks a lot for all of your input!!![8D]

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All American Arms Company

Veteran Owned and Operated


  • sandwarriorsandwarrior Member Posts: 5,453 ✭✭✭
    edited November -1

    I'm sure you will get a better in-depth answer coming up, but the answer to your second question is that the way it should be. The AI case is .002" shorter than the .300 Win mag. That is how you can take full power standard rounds and fireform them to Ackley.

    We have the second amendment so that all the rest are secure....UNK>
  • HighballHighball Member Posts: 15,755
    edited November -1
    The datum line is difficult to locate. Ring gauges are used...honed to precise size.

    The dimensions you are giving do not correspond with the JGS prints I have...
    But the great thing about Ackley Improved is that one can fire factory rounds in the chamber to fireform new brass...assuming that the 'smith has done his job correctly.
  • JustCJustC Member Posts: 16,049 ✭✭✭
    edited November -1
    Eric, first long winded reply is on the GD post. Next,..I can say that the .002" decrease you saw was the brass flow REARWARD to make the extra brass necessary for increasing the internal volume of the mych larger body area. The cases should shrink slightly, and moreso in the heavily tapered cases like the 257rbts. I do also beleive that nononsense stated in a prior discussion we had that the base of the shoulder is slightly changed as well (but my college years have affected my memory, so keep that in mind). As for the datum line, it is ~mid shoulder. The Stoney point headspace set covers most every caliber and will attach to your caliper blade so you can keep track of such things. It especially helps when you are making very small adjustments to a FL sizing die to PFL size a case which will actually pull the shoulder foreward by a few .001's to give a good compressed fit without fully working the brass.

    Your fireformed load results sound very typical to me. Keep in mind I have not hotrodded any belted magnums, but my 6.5x55AI, 6.5-06AI and 280AI all exhibit similar results after being formed. Also keep in mind that if your original chamber was a factory chamber and your new chamber was cut with a min spec reamer, that will also create 90% of the shoulder starting point difference you are seeing. I would bet you can measure the body size of a fired case from the original chamber and one from this chamber and find it is tighter in that dimension as well.

    I just pick a charge where I want to start working my load ladder from anyway, and run them up at .3gr increments. The only change (which in the future may NOT be a change at all) is the seating of the bullet well into the lands. You want a good, sharp shoulder form on the very first firing. By using the bullet to force the case firmly against the boltface, you are assuring a good forming with only 1 firing. That cream of wheat or candle wax method often takes more than 1 firing to get good sharp shoulders due to low pressures. I find tighter chambers will often not reach book max, so don't load your first ladder up past max until you have a chance to evaluate pressure indications as you go up the ladder.

    why chase the game when the bullet can get em from here?....
    Got Balistics?
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