Seabeeken
Well-Known Member
I had a question for Manson Reamers regarding a measurement on my 244AI HS gauge and Ray provided the full AI explanation so I thought I'd share it here for those new to AI chambers and headspacing. I use Manson HS gauges because they have the .004" crush fit built into them. Hopefully it helps someone. Here is the email.
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The Ackley method is a well thought out and well proven process, but too often misunderstood, even amongst professionals in the industry.
Proper Ackley gages have the same shoulder angle as the parent, un-fire formed brass. The gage is acting as a standard for proper virgin/unfireformed cartridge fit and thus maintains the original parent shoulder angle.
Due to the difference in shoulder angle between the un-formed brass and the improved chamber, the brass and the gages will only register against the chamber neck/shoulder junction. This is why you are getting the discrepancy. The standard 244 Rem (aka 6mm Rem AI) neck diameter is .277. If you had a datum that was close to .277 diameter, your measurements would be a lot closer.
To properly headspace an Ackley chamber, the Ackley GO gage acts as the true GO gage. Typically we make these gages .004 shorter than the standard GO. The parent caliber, standard GO gage acts as the NO GO. Sounds confusing, but it works.
The reason to use the shorter 'Ackley GO' gage, is that you want to ensure a slight crush fit when fireforming. Your goal is to have some reasonable amount of crush fit on the un-fire formed brass. You should be able to fully close the bolt by hand, with moderate resistance
Occasionally the gages are made to different dimensions based on known differences in the chamber and brass lengths. For the vast majority of projects the .004 setback is the norm. Even so it is a good idea to send your gunsmith a few pieces of the brass you plan to use, so that the moderate crush fit can be confirmed. Brass differs from lot to lot and manufacturer to manufacturer. Your setback may need to be adjusted to suit your brass.
The reason for the crush fit is that you want the base of the brass in good contact with the bolt face. The crush fit avoids a headspace problem that is possible in all chambers, but especially when using un-fire formed brass in an improved chamber. If there is a gap between the brass shoulder and the chamber neck/shoulder junction when the primer detonates, the primer will push the entire brass case forward until it reaches the chamber neck/shoulder junction. As the powder ignites, the case expands and grips the chamber wall. As pressure builds the base of the case will be pushed back to the bolt face, stretching and weakening the case wall.
When the brass shoulder angle and chamber shoulder angles match (such as normal SAAMI spec brass & chamber or with an already fireformed case in an improved chamber), a .001 -.002 headspace gap between brass shoulder and chamber shoulder is desirable for good functionality and any stretching is well within normal limits. Because the Ackley chamber is a very different shape from the un-fireformed brass, it is much more important to keep it trapped against the bolt face to ensure that the fire forming process remains safe and results in a usable, fire formed case.
If you are fireforming a bunch of cases at once, you should re-lube your bolt lugs often to avoid the possibility of galling the lugs. Another good rule of thumb is to dedicate a sufficient lot of the same brass to a barrel with an improved chamber. The barrel and brass can wear out together and then you can start fresh with the next barrel.
If you already have a bunch of fire formed brass that you want to continue to use, it would be a good idea to confirm that the this brass will fit properly while setting chamber headspace. In theory, if the first chamber was headspaced properly, as described above AND the new chamber has the same neck diameter; then the brass will fit and there is no need to confirm it. However, it is always best to check."
"
The Ackley method is a well thought out and well proven process, but too often misunderstood, even amongst professionals in the industry.
Proper Ackley gages have the same shoulder angle as the parent, un-fire formed brass. The gage is acting as a standard for proper virgin/unfireformed cartridge fit and thus maintains the original parent shoulder angle.
Due to the difference in shoulder angle between the un-formed brass and the improved chamber, the brass and the gages will only register against the chamber neck/shoulder junction. This is why you are getting the discrepancy. The standard 244 Rem (aka 6mm Rem AI) neck diameter is .277. If you had a datum that was close to .277 diameter, your measurements would be a lot closer.
To properly headspace an Ackley chamber, the Ackley GO gage acts as the true GO gage. Typically we make these gages .004 shorter than the standard GO. The parent caliber, standard GO gage acts as the NO GO. Sounds confusing, but it works.
The reason to use the shorter 'Ackley GO' gage, is that you want to ensure a slight crush fit when fireforming. Your goal is to have some reasonable amount of crush fit on the un-fire formed brass. You should be able to fully close the bolt by hand, with moderate resistance
Occasionally the gages are made to different dimensions based on known differences in the chamber and brass lengths. For the vast majority of projects the .004 setback is the norm. Even so it is a good idea to send your gunsmith a few pieces of the brass you plan to use, so that the moderate crush fit can be confirmed. Brass differs from lot to lot and manufacturer to manufacturer. Your setback may need to be adjusted to suit your brass.
The reason for the crush fit is that you want the base of the brass in good contact with the bolt face. The crush fit avoids a headspace problem that is possible in all chambers, but especially when using un-fire formed brass in an improved chamber. If there is a gap between the brass shoulder and the chamber neck/shoulder junction when the primer detonates, the primer will push the entire brass case forward until it reaches the chamber neck/shoulder junction. As the powder ignites, the case expands and grips the chamber wall. As pressure builds the base of the case will be pushed back to the bolt face, stretching and weakening the case wall.
When the brass shoulder angle and chamber shoulder angles match (such as normal SAAMI spec brass & chamber or with an already fireformed case in an improved chamber), a .001 -.002 headspace gap between brass shoulder and chamber shoulder is desirable for good functionality and any stretching is well within normal limits. Because the Ackley chamber is a very different shape from the un-fireformed brass, it is much more important to keep it trapped against the bolt face to ensure that the fire forming process remains safe and results in a usable, fire formed case.
If you are fireforming a bunch of cases at once, you should re-lube your bolt lugs often to avoid the possibility of galling the lugs. Another good rule of thumb is to dedicate a sufficient lot of the same brass to a barrel with an improved chamber. The barrel and brass can wear out together and then you can start fresh with the next barrel.
If you already have a bunch of fire formed brass that you want to continue to use, it would be a good idea to confirm that the this brass will fit properly while setting chamber headspace. In theory, if the first chamber was headspaced properly, as described above AND the new chamber has the same neck diameter; then the brass will fit and there is no need to confirm it. However, it is always best to check."
