RPK issue: hammer/carrier interface

[Lone Eagle]

Posted this on a couple of forums over the weekend.

As I was shooting my CAI "Polish" M64 RPK yesterday, I noticed something. I was running the following loads:

Yugo M67
Tulammo
Wolf
Arsenal brass
Federal Fusion
Hornady SST

I noticed some loads didn't eject as far as others. Tula/Wolf just seemed to dribble out a couple of feet from the rifle, some just barely clearing the ejection port. The Arsenal, Federal, and Hornady would go straight out about 6' to my left. Yugo brass would enter a low earth orbit. recoil felt the same with most of them, but the Yugo and Arsenal seemed the "hottest". The same loads in my Saiga would fly between 6-9' forward at a 45 degree angle from the ejection port. Could this be because of the longer RPK barrel and what seems to be a heavier ejection spring, or just the newness needing to be shot off?

UPDATE:

OK, broke it down, and I have found the issue. The hammer is contacting the carrier at an odd angle.   This is with the carrier against the hammer.





Here is a shot of where the carrier catches on the hammer on the way back. It's the shiny spot above the bolt.



Here's a side shot of the carrier.  The hammer is catching right on the flat spot at the rear of the carrier "ramp".


The FCG is a Tapco G2. Should reprofiling the hammer in that area clear it up, or do I need another FCG entirely? Maybe a reprofile on the rear of the carrier? I checked my Saiga carrier(Bulgarian), and it has a slightly different angle in that area.  It is dragging the carrier enough if I ride it, the carrier will hang.

I know not to touch the flat that hits the firing pin, but what about taking the hump out a touch where the bolt rides?  I haven't pulled out my spare FCG parts and compared hammers yet, but the spares are all Polish/Romanian.

[AKBLUE]

It is functioning. Powder/load  variations can cause ejection differences. Lube the firearm., it looks bone dry.
Nothing to fix IMHO.

[Lone Eagle]

It is functioning. Powder/load  variations can cause ejection differences. Lube the firearm., it looks bone dry.
Nothing to fix IMHO.

LOL...yeah, I had just finished cleaning it when I snapped those pics. 


ETA:  Bud and I found the issue, and corrected. I borrowed a Romanian hammer from a friend, and compared it to the Tapco. The "hump" the carrier was contacting on the Tapco was slightly too high, causing drag on the carrier. We test-fired it with the Romy hammer, and it ran like a top with no hiccups or dribbling steel cases. He took my Tapco and re-profiled it to match the Romy hammer, and all is well.

[Mikeym_us]

Apparently the Tapco hammer causes some function issues with AK's because of that hump. And the fix is exactly what your friend did.

It is functioning. Powder/load  variations can cause ejection differences. Lube the firearm., it looks bone dry.
Nothing to fix IMHO.

LOL...yeah, I had just finished cleaning it when I snapped those pics. 


ETA:  Bud and I found the issue, and corrected. I borrowed a Romanian hammer from a friend, and compared it to the Tapco. The "hump" the carrier was contacting on the Tapco was slightly too high, causing drag on the carrier. We test-fired it with the Romy hammer, and it ran like a top with no hiccups or dribbling steel cases. He took my Tapco and re-profiled it to match the Romy hammer, and all is well.

[AKBLUE]

seldom a problem., and OP's firearm is not malfunctioning. ??

[Lone Eagle]

Other than the dribble issue, it went bang every trigger pull.  After removing the hump(only about 1.5MM of material), it tosses the empties in the same spot, no matter the load.  Before, finding my brass was a chore.  Now, I know where to look.

[Mikeym_us]

With the M64 having a 1.5mm receiver the amount of material removed from the hammer was equal to the thickness of the receiver. I believe the thicker material of the receiver is a contributing factor as I have not heard of the G2 hammer causing this problem on the 1.0mm receiver AK's. Maybe the carriers on the 1.0mm recievers ride over the hump more freely due to wiggle room .

Other than the dribble issue, it went bang every trigger pull.  After removing the hump(only about 1.5MM of material), it tosses the empties in the same spot, no matter the load.  Before, finding my brass was a chore.  Now, I know where to look.

[Mancat]

There seems to be a pretty wide casting variation in the G2 FCG as far as this "hump" is concerned. Some are more severe than others.  FWIW I have never had to reprofile any G2 group that I own.

The only time I try to avoid the G2 FCG is in any AK that uses an AK74 carrier/bolt. The FCG and carrier geometry is a bit different and they tend to not cycle very smoothly with the G2, but will otherwise work fine with it.

[Mikeym_us]

There seems to be a pretty wide casting variation in the G2 FCG as far as this "hump" is concerned. Some are more severe than others.  FWIW I have never had to reprofile any G2 group that I own.

The only time I try to avoid the G2 FCG is in any AK that uses an AK74 carrier/bolt. The FCG and carrier geometry is a bit different and they tend to not cycle very smoothly with the G2, but will otherwise work fine with it.

I always thought the difference in the 47 and the 74 was that the bolt and barrel were different being a .22 caliber bolt and barrel instead of a .30 caliber bolt and barrel.

[Mancat]

There seems to be a pretty wide casting variation in the G2 FCG as far as this "hump" is concerned. Some are more severe than others.  FWIW I have never had to reprofile any G2 group that I own.

The only time I try to avoid the G2 FCG is in any AK that uses an AK74 carrier/bolt. The FCG and carrier geometry is a bit different and they tend to not cycle very smoothly with the G2, but will otherwise work fine with it.

I always thought the difference in the 47 and the 74 was that the bolt and barrel were different being a .22 caliber bolt and barrel instead of a .30 caliber bolt and barrel.

there are a lot of minor internal differences. one of those is that the '74 carrier and trigger group have slightly different geometry than a typical earlier AK-74/AKM design. the differences aren't enough to prevent the G2 trigger from working in a '74, but a trigger group with the correct geometry (e.g. Arsenal FCG) will operate smoother in an AK-74

[Danjal]

I have a fix wrote up here on a "sticky bolt fix". That should cover this.

[AKBLUE]

With the M64 having a 1.5mm receiver the amount of material removed from the hammer was equal to the thickness of the receiver. I believe the thicker material of the receiver is a contributing factor as I have not heard of the G2 hammer causing this problem on the 1.0mm receiver AK's. Maybe the carriers on the 1.0mm recievers ride over the hump more freely due to wiggle room .

Other than the dribble issue, it went bang every trigger pull.  After removing the hump(only about 1.5MM of material), it tosses the empties in the same spot, no matter the load.  Before, finding my brass was a chore.  Now, I know where to look.

the rails and configuration on a 1mm and 1.5mm receiver are the same.  Steps and dimples etc., create the same contours from milled to 1.5 to 1mm. This is done for parts interchangeability.

[Lone Eagle]

there are a lot of minor internal differences. one of those is that the '74 carrier and trigger group have slightly different geometry than a typical earlier AK-74/AKM design. the differences aren't enough to prevent the G2 trigger from working in a '74, but a trigger group with the correct geometry (e.g. Arsenal FCG) will operate smoother in an AK-74

Now you got my gears rolling.  Is it possible I got a RPK74 carrier?  The "Polish" M64's are mismatched, with only the front trunnion and bolt numbers matching.   I know Bulgy carriers interchange, as I'm running a SGL31 carrier in my SGL21 right now.

[Mikeym_us]

Actually I was talking about the bolt carrier rails not the bolt rails. I know the bolt rails are the same.

With the M64 having a 1.5mm receiver the amount of material removed from the hammer was equal to the thickness of the receiver. I believe the thicker material of the receiver is a contributing factor as I have not heard of the G2 hammer causing this problem on the 1.0mm receiver AK's. Maybe the carriers on the 1.0mm recievers ride over the hump more freely due to wiggle room .

Other than the dribble issue, it went bang every trigger pull.  After removing the hump(only about 1.5MM of material), it tosses the empties in the same spot, no matter the load.  Before, finding my brass was a chore.  Now, I know where to look.

the rails and configuration on a 1mm and 1.5mm receiver are the same.  Steps and dimples etc., create the same contours from milled to 1.5 to 1mm. This is done for parts interchangeability.

[Mancat]

there are a lot of minor internal differences. one of those is that the '74 carrier and trigger group have slightly different geometry than a typical earlier AK-74/AKM design. the differences aren't enough to prevent the G2 trigger from working in a '74, but a trigger group with the correct geometry (e.g. Arsenal FCG) will operate smoother in an AK-74

Now you got my gears rolling.  Is it possible I got a RPK74 carrier?  The "Polish" M64's are mismatched, with only the front trunnion and bolt numbers matching.   I know Bulgy carriers interchange, as I'm running a SGL31 carrier in my SGL21 right now.

Nope. RPK74 carrier would not even allow an AKM/RPK bolt stem to be inserted, and the lockup cam surfaces in the carrier are wrong, etc.

AK-74 carrier works in the SGL-21 because the 21 has a 74 spec bolt, carrier, and front trunnion/chamber.