280 Ackley Improved Emperical Headspace Test

Top is SAAMI or Nosler type headspace gauge for 280 AI. Bottom is the “Traditional” headspace gauge for the 280 Ackley Improved, note the difference between the shoulder angles.

These Chamber gauges were made using the headspace gauges as marked. The purpose being that we can visually see the difference and measure any difference when the gauges are swapped, or ammunition is checked.

Here the gauges have been swapped to the incorrect chambers. NOTE: no difference in headspace, both gauges are flush with the base of the chamber gauge.

According to the argument, The case in the “Traditional” chamber should be .014″ too deep in the chamber. As with the gauges headspace is zero.

The discussion of headspace and the 280 Ackley Improved has been going on far too long.  Recently I decided to perform a test that would demonstrate the differences or similarities between the SAAMI (Nosler) gauge and the “Traditional” Ackley gauge.

First you can see that the shoulder angle is not the same.  The SAAMI gauge is 40 degrees like the factory brass and ammunition.  The Traditional gauge is 17 degree 30 minute just like the original 280 Remington gauge and factory 280 Remington ammunition or brass.

The Chamber gauges pictured below were both made with the exact same 280 AI reamer.  The only difference between the SAAMI and the Traditional chambers are the headspace gauges, all other measurements are identical.

Each chamber gauge was stamped with the name of the gauge used to headspace it.  The headspace was set to zero, or flush with the flat base of the gauge.

Once I had both chambers cut I swapped the gauges expecting to see some obvious disparity as the reamer makers have been emphatic that there is a difference.  However, there was no difference between the two chambers.

How could that be?

Well, when you check the drawings from SAAMI and the Traditional drawings for the Ackley Improved you find that indeed there is a numerical difference between the two drawings of .014″.  The problem stems from reamer makers applying a datum line to the Traditional Ackley drawing at the .375″ diameter on the shoulder, this is not where the datum line is on a traditional Ackley.

The drawing calls for a 40 degree shoulder, but the datum measurement is based on the traditional 17 degree gauge.  In other words the systems of measuring are mixed.  Ackley used the junction of the neck and shoulder to headspace his Improved chamber, not the datum line along the shoulder.

Hence we end up with a headspace length of 2.1542″ on the Traditional drawing.  I measured this length on my cutaway chamber and guess what, that is the length to the junction of the neck and shoulder.  Thus headspace matches the drawings correctly.

The 40 degree SAAMI gauges are made to the industry standard, datum line method.  So the drawings are correct, the datum line is at the .375″ diameter along the shoulder.  The length called out for this gauge is 2.140″, which appears to be .014″ shorter than the Traditional design.

What we have is two different methods of measurement.  However they achieve the exact same result.  Seeing is believing.

With all that said, for liability reasons I would still use the gauge the client asks for.  It’s easier than explaining all this over and over again.

The obvious question will come up, “But I am still having trouble with headspace, so the Nosler brass must be wrong?”  The answer is; More than likely a lazy or inexperienced gunsmith did not use a gauge to set headspace, but rather used brass.  The Nosler brass should work in a “Traditioinal” chamber if the headspace was set between a go and no-go as prescribed by P.O. Ackley and every reamer maker that sells these tools.

Here are some additional source of information on this subject:



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Filed under accuracy, ammo, brass, Firearms, Gunsmithing, How To, wildcat

6 responses to “280 Ackley Improved Emperical Headspace Test

  1. Here is a video to further explain:

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  4. Adam

    Isn’t this test overlooking reamer differences? IE doesn’t the SAMMI reamer have a larger radius on the neck to shoulder junction than the traditional reamer? I would expect the results you got using a traditional and SAMMI headspace gauge with the same SAMMI reamer. But if a traditional reamer has less radius it would hit the 17deg shoulder on the traditional gauge in a different spot thus resulting in a longer case body, which correlates to the fact the traditional dies are also longer. If the SAMMI reamer had more radius (to make manufacturing brass easier, etc.) then to still work with 280rem (as Ackley chambers are supposed to be designed to do) then the case wall would have to be shortened vs the traditional since the contact would be farther down on the shoulder.

    That wouldn’t be a lazy or inexperience gun smith that would be reamer differences causing a distinct difference in guns cut with a reamer from the pre-SAMMI era (which certainly would have happened in the past and would continue with any smith that hasn’t replaced their reamer with the SAMMI version). Thus someone with a traditional chamber would be wise to stick to fire forming 280rem or at minimum comparing a piece of fire formed brass from their gun against Nosler brass before loading them to review how much headspace variation there is. Also if they are using new SAMMI dies they would want to set them up to only bump the shoulder as they could push the shoulder back too far.

    • reamerrentals

      There is no difference between the reamers. SAAMI and Traditional reamers are identical in all dimensions. Only the gauges differ.

      • Adam

        There is a statement from Dave Manson that the traditional reamers were made with minimal radius at the neck to shoulder junction and the SAMMI spec one is .060″ +.025″.

        As demonstrated here its not the go gauges, since either can be used with the SAMMI reamer to cut the same chamber. So that leaves a reamer difference in the pre-SAMMI chambers using the traditional gauge that results in a .014″ longer case body which matches the traditional dies. There is no question the traditional dies are longer so there is obviously some difference (again not a lazy or inexperienced smith issue). Which is fine, folks just need to be aware of how to work with what they have.

        If I’m off base on something I’d love it clarified, just trying to add some clarification based on what I’ve come to understand. Both chambers will form from 280rem fine but they contact the shoulder in different areas and thus result in a different case body length which may cause problems when using 280AI nosler brass in a traditional chamber (depending on brass and chamber tolerances) thus the recommendation to compare some fire formed brass against the nosler brass before blindly loading/firing the nosler brass.

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