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:


App for Gunsmithing/Headspace

Book has recently been published, “P.O. Ackley, America’s Gunsmith”

Web resource for P.O. Ackley:   https://ackleyimproved.com


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

25 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.

      • reamerrentals

        In speaking with various reamer makers it appears that Manson is the only one that has made this choice. However, this points out why I always recommend the use of the gauge the client asks for. Obviously using a traditional gauge with a Manson reamer could be dangerous. Use your brain.

        Your assertions about length are incorrect. The .014″ difference in length is a result of the establishment of a datum line on the shoulder for the SAAMI version of the cartridge. The traditional version of the chamber had a datum point at the junction of the neck and shoulder. Mathematically, that is .014″ further forward. We are talking about two different methods of measuring. Unfortunately, this is not explained on the chamber prints.

  5. Adam

    If that is the case what accounts for the fact the original dies were 0.014″ longer? That’s the one aspect not addressed here still in saying there is no difference. The die makers clearly are saying there is a difference between the two. The reamer explanation made sense but if that isn’t the source then what is?

    The article did a good job of showing with the same reamer the two gauges will yield the same chamber. But hasn’t even Dave Kiff stated the reamer prints were altered from what he initially suggested?

    Not trying to be argumentative there just seems to be a hole in the explanation that needs clarification.

  6. reamerrentals

    When the chamber prints came to the public it was not explained that the datum line was moved. I have yet to see a print that clearly states things so that a novice can understand it. (No negative connotation intended here.)

    I pulled prints from every source available. Because there in no notation on the prints for the traditional chamber about the datum being at the neck and shoulder junction confusion abounds. In fact the prints I have seen for traditional chambers indicate an datum point along the shoulder, this is simply an oversight. They would have to call out the detail or make a special print just for Ackley chambers.

    Don’t overthink it. Its just two different ways to measure the same exact chamber.

  7. Adam

    Nothing negative taken.

    But to circle back around you agree there is an actual difference in the traditional dies versus the new ones right?

  8. Adam

    So the major die makers were making oversized (and/or now undersized) dies is being suggested?

    • reamerrentals

      I have no evidence that supports the idea that the die makers were ever wrong. This is internet chatter, which I discount 99.999%.

      Let’s back up, using simple logic, at the link below to the Nosler page, Nosler clearly states you can fireform thier version of the 280 AI from 280 Remington brass. http://www.nosler.com/280-remington-ackley-improved/

      This fact alone proves that there is little or no difference between the Traditional and and the SAMMI chambers. Otherwise you would have problems with miss-fires or case head separations do to excessive headspace in the traditional chamber.

      The .014″ that everyone keeps touting comes from the fact that the chambers are being measured at points .014″ apart from one another. Not because there is a difference in length. The gauges used in this article your are commenting on are made the these very specs. Look at how they interact with the chambers in the article. It’s that simple.

      • Adam

        Its not internet chatter, the manufacturers have different dies. RCBS makes a “280 Ackley Improved” and “280 rem improved 40 degrees”. Redding also has the “280 Ackley Imp” and previously made the “280 Rem Imp 40°”. Their words: “These older dies will not bump the shoulder of cases for a SAAMI chamber. In other words, the old dies are too deep for the current SAAMI chamber.” http://www.redding-reloading.com/tech-line-a-tips-faqs/133-280-changes

        The nosler link doesn’t really prove there is no difference in the chambers in the end, it just proves both can fire form 280rem. IF the previous Dave Manson info was correct the interference fit where the chamber contacts the case shoulder is slightly larger due to the radius thus it would hit slightly lower on the neck and when the case walls get blown out the distance from the bottom of the case to the shoulder would be a bit shorter than if the interference was higher up on the neck (ie less radius on the reamer and chamber set with a traditional gauge).

        If you’re discrediting the radius on the reamer aspect I’m still quite interested in the rational for why the die makers themselves are stating their original dies are 0.014″ longer?

        Basically this was a good test in showing the new reamer (if different) will yield the same chamber length no matter which gauge is used, which makes complete sense as the geometry of the 280rem cartridge and reamer radius would be accounted for in designing the sammi gauge. The issue out there is whether an older reamer will yield a sammi chamber with a traditional gauge (a sammi gauge should still result in a sammi length chamber even if there is a radius difference since it doesn’t head space off that). The fact redding is saying the old dies are longer, redding offers a different shell plate and also you have Dave Manson giving a rationale for why the old chamber is also longer shows this isn’t necessarily a cut and dry issue. I’m not saying you need to go redo this test with an older reamer I’m just cautioning dismissing it as a non-issue and saying anyone with a long chamber has a lazy or inexperienced gunsmith (unless redding, rcbs, etc. fall into that camp). A diligent reloader can work with either chamber and jamming the lands when fire forming should remove casehead seperation concerns even in a long chamber as that will hold the case against the bolt face while the shoulder is blown out.

        Basically in the end anyone with a 280AI chamber would be wise to check their fire formed brass against commercially produced brass/ammo before firing it to make sure they don’t have an excess headspace condition (regardless of what you want to attribute the cause to).

  9. reamerrentals

    I did some further checking with my industry contacts, as I wanted to be sure what I say here is accurate.

    It is very important to understand that over the years gunsmiths, reamer makers and die makers have all offered versions of the 280 AI that may or may not match up with the original Ackley concept. I will not address the fact that various people do whatever they please, but it is a simple fact.

    In short, every comment I have made to this point along with the full content of the posts on this blog concerning the 280 Ackley Improved are accurate and correct.

    In speaking with industry folks who make tooling for most manufacturers they say all makers are utilizing the SAAMI prints at this point. Therefore, you might find a set of old dies that do not follow the dimensions we have discussed. But anything manufactured since Nosler took the cartridge to SAMMI will work exactly as I have stated.

    If you find a set of dies you are worried about the headspace can easily be checked. Normal specifications for a resize die has the “go” gauge for caliber protruding .130″ form the base of the die. A tolerance of +.005″ -.000″ would be within safe limits. This set up allows the end user to “bump” the shoulder when needed. That all assumes that the guy who built the gun followed the rules.

    • Adam

      Nice follow up. I hope I was clear that yes I fully concur that anyone using anything new (dies/reamers/etc) should all end up with SAMMI and not have issues. Part of my main contention was the claim if you had an older chamber that was longer it was a lazy or inexperienced smith (if they used a traditional gauge with a slightly different reamer and properly chambered otherwise that’s not them).

      The main thing I was trying to flag is older chambers (or chambers still being cut today with older reamers) COULD be longer so check before blindly firing ammo and/or reloading, even if a good smith cut the chamber. That was basically my intention in my original second paragraph.

  10. Chris

    Adam, PO Ackleys intention was for the improved case to be .004 shorter than the parent ( 280 rem) referenced at the neck shoulder junction. If, in the example shown above and also as I have found when physically comparing my original Ackley chamber with SAAMI spec cases, there is no difference in head space then surely any die / chamber that is longer cannot be representative of an Ackley improved design?
    Regarding the dies, the .014 difference that’s spoken about is less than 1/4 turn in/out when setting up in a press, has anyone actually measured the two different dies and established the difference?
    I have original dies from Redding and can bump the nosler brass back to saami spec with standard shell holders.

  11. S&S Precision Rifles

    I agree with Criss and Reamer rentals. I have been chambering 280 AI for 25 yrs and do so with the .004 short method by using the standard 280 go guage as my nogo. My fire formed brass measure exactly the same as the new nosler brass. And yes the older dies will still bump the shoulder.

  12. Scott

    I bought a Kimber Montana 280ai about 2 years ago and use Nosler 280ai factory ammo (I’m not a reloader). These rounds chamber with much effort and occasionally jam. How do I tell if I’m chambered in the traditional Ackley or SAAMI? I suspect that my rifle is chambered to the traditional 280 Ackley specs which would cause my factory ammo to have excess headspace and lead the problems I mentioned above. Any thoughts would be appreciated.

    • reamerrentals

      The whole point of this post is that there is ZERO (0) difference between the two chambers. Its just a math problem that was not clearly explained or understood by gun writers and keyboard commandos. I would talk to Kimber if the gun has feeding issues.

  13. Porter

    Lee makes .280 Ackley improved pacesetter dies they say are for the original .280 chamber. If I am understanding everything I have read here correctly, I should have no problems reloading ammunition with these dies for a SAMMI 280 Ackley improved chamber. Correct?

  14. reamerrentals

    Correct. Its all about you setting the dies to match your gun. There is no real difference between the Ackley Traditional and the SAAMI measurements. Just how they are described mathematically.

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