I agree that a slam fire OOB with an AR is highly unlikely due to the firing pin due to the way the bolt works as Redshooter describes.
I have always used magnum primers in semis due to the cups being thicker and harder. The bolt can slam into the rear of the cartridge with some force and as it heads up the feed ramps it gets tilted. So I suppose the lugs could potentially hit the primer which is why I am curious about the rounded mark on the primer around the firing pin detent in your picture.
Or did the primer flow into the Filipino hole? Have you looked to see if it's a crater or protruding?
Was the brass new or factory once fired?
Are you full sizing?
"Slam fires
Slam-fires are an ever-present danger with Service Rifles (particularly with the M1 and M14 family). A slam-fire occurs when a round discharges as the bolt is closed. This can result in an accidental discharge with no other damage, or it can virtually destroy the rifle and injure the shooter. The deciding factor here is whether the rifle is fully in battery. Unfortunately, most slam-fire incidents are due to improperly assembled handloads, sometimes combined with poor gun-handling techniques. A few simple precautions can decrease the chances of a slam-fire.
Virtually all U.S. military Service Rifles utilize firing pins that rest freely within the bolt. Referred to as a floating firing pin, it will actually strike the primer lightly when the bolt is closed. This results in a slight dimple in the primer, which is plainly visible if the unfired round is extracted. This isn’t a problem with military ammunition because they use primers with thicker cups specifically because of this. However, it can be hazardous with the more sensitive commercial primers. The risk increases with high primers, headspace problems, and poor gun-handling technique. These risks are covered in greater detail elsewhere in this section. Please pay particular attention to the sections about rifle manipulation, sizing and priming.
Out of Battery Fires
An out-of-battery firing occurs when the bolt is not fully closed and locked. This is less of a problem in the M16/AR-15 series of rifles due to their bolt design, but it can easily happen with the M1 or M14/M1A rifles. Several safety features are built into these rifles specifically to prevent this from happening. However, these safeties can be defeated, and the condition needs to be understood."
"Another often-attempted technique is to place the cartridge directly into the chamber before seating the bolt. This poses two serious potential problems. If the bolt is allowed to slam home on a cartridge that is fully inserted into the chamber, there is a real possibility of a slam-fire. This is rare with military ammo, but the potential increases with reloaded or commercial ammunition. (The reasons for this are covered more extensively in the sections on primers and headspace.) "
"Benchrest or Match primers are favored by many shooters, with an eye toward achieving the ultimate accuracy. Such primers generally contain a somewhat milder pellet mixture that contributes to improved accuracy potential. Some makes may also utilize a thinner cup to guarantee reliability, although this is not always the case. The Remington 7 1/2 Benchrest primer, for example, uses a fairly thick cup. Sensitivity can become an issue here and may increase the risk of a slam-fire. The Federal 210M (Match) primers, a very popular choice among competitive shooters, have developed a reputation for being rather sensitive. While this may not be the case, Federal 210M primers have developed a reputation as a potential source of trouble when used in Service Rifles. Over the years, we have used literally thousands of 210M primers in both M1 and M14/M1A rifles without incident. We feel that the single most significant cause to such slam-fires can be traced back to improper seating, not the primer itself."
"Primer Pockets
With the exception of Match brass, such as that produced at Lake City, most military primers are crimped in place. This crimp must be removed prior to the case being reprimed. Failure to do this makes the case extremely difficult to prime, leading to the potential for high primers—a very dangerous condition. While there are several tools on the market to correct this, Dillon’s Super Swage 600 is one of the best we’ve worked with. Whether with swage or cutter, the remnants of the original crimp must be removed.
Removing the primer crimp is a one-time operation, but keeping the pocket clean is an ongoing process. The goal here is the same — to eliminate the potential for a high primer. It’s rare that enough residue would build up to prevent a primer from fully seating, but it is a possibility. Keep a close eye on the primer pockets during your case prep, and clean them if there is any question about how well the next primer will seat."
I have always used magnum primers in semis due to the cups being thicker and harder. The bolt can slam into the rear of the cartridge with some force and as it heads up the feed ramps it gets tilted. So I suppose the lugs could potentially hit the primer which is why I am curious about the rounded mark on the primer around the firing pin detent in your picture.
Or did the primer flow into the Filipino hole? Have you looked to see if it's a crater or protruding?
Was the brass new or factory once fired?
Are you full sizing?
"Slam fires
Slam-fires are an ever-present danger with Service Rifles (particularly with the M1 and M14 family). A slam-fire occurs when a round discharges as the bolt is closed. This can result in an accidental discharge with no other damage, or it can virtually destroy the rifle and injure the shooter. The deciding factor here is whether the rifle is fully in battery. Unfortunately, most slam-fire incidents are due to improperly assembled handloads, sometimes combined with poor gun-handling techniques. A few simple precautions can decrease the chances of a slam-fire.
Virtually all U.S. military Service Rifles utilize firing pins that rest freely within the bolt. Referred to as a floating firing pin, it will actually strike the primer lightly when the bolt is closed. This results in a slight dimple in the primer, which is plainly visible if the unfired round is extracted. This isn’t a problem with military ammunition because they use primers with thicker cups specifically because of this. However, it can be hazardous with the more sensitive commercial primers. The risk increases with high primers, headspace problems, and poor gun-handling technique. These risks are covered in greater detail elsewhere in this section. Please pay particular attention to the sections about rifle manipulation, sizing and priming.
Out of Battery Fires
An out-of-battery firing occurs when the bolt is not fully closed and locked. This is less of a problem in the M16/AR-15 series of rifles due to their bolt design, but it can easily happen with the M1 or M14/M1A rifles. Several safety features are built into these rifles specifically to prevent this from happening. However, these safeties can be defeated, and the condition needs to be understood."
"Another often-attempted technique is to place the cartridge directly into the chamber before seating the bolt. This poses two serious potential problems. If the bolt is allowed to slam home on a cartridge that is fully inserted into the chamber, there is a real possibility of a slam-fire. This is rare with military ammo, but the potential increases with reloaded or commercial ammunition. (The reasons for this are covered more extensively in the sections on primers and headspace.) "
"Benchrest or Match primers are favored by many shooters, with an eye toward achieving the ultimate accuracy. Such primers generally contain a somewhat milder pellet mixture that contributes to improved accuracy potential. Some makes may also utilize a thinner cup to guarantee reliability, although this is not always the case. The Remington 7 1/2 Benchrest primer, for example, uses a fairly thick cup. Sensitivity can become an issue here and may increase the risk of a slam-fire. The Federal 210M (Match) primers, a very popular choice among competitive shooters, have developed a reputation for being rather sensitive. While this may not be the case, Federal 210M primers have developed a reputation as a potential source of trouble when used in Service Rifles. Over the years, we have used literally thousands of 210M primers in both M1 and M14/M1A rifles without incident. We feel that the single most significant cause to such slam-fires can be traced back to improper seating, not the primer itself."
"Primer Pockets
With the exception of Match brass, such as that produced at Lake City, most military primers are crimped in place. This crimp must be removed prior to the case being reprimed. Failure to do this makes the case extremely difficult to prime, leading to the potential for high primers—a very dangerous condition. While there are several tools on the market to correct this, Dillon’s Super Swage 600 is one of the best we’ve worked with. Whether with swage or cutter, the remnants of the original crimp must be removed.
Removing the primer crimp is a one-time operation, but keeping the pocket clean is an ongoing process. The goal here is the same — to eliminate the potential for a high primer. It’s rare that enough residue would build up to prevent a primer from fully seating, but it is a possibility. Keep a close eye on the primer pockets during your case prep, and clean them if there is any question about how well the next primer will seat."
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