"Bullet shear"

[Slater]

I read somewhere that the original AK-74's were designed with the older AK-47/AKM gas block, but had to go to the familiar 90 degree gas block because of a phenomenon called "bullet shear". Can anyone shed any light on this?

[TOrrock]

The 45* gas blocks allowed the port to come through on a land, versus the groove. The Soviets experienced bullet shear when this happened. The bullet would shed jacket material into the gas port as it passed the sharp edge of the port on the land.

They found that if they drilled the port through a groove, this didn't happen.

Going with a 90* port straight down did away with the elliptical gas port and the issue of bullet shear.

[Slater]

Gotcha. Thanks.

[Massoud]

Templar,
Can you post any supporting evidence of what you say? I would really like to read where you found this.

Many semi-auto rifles today have the port drilled through a land, or half through a land and half through a groove. It is so common that Krieger (and maybe some others) used to specifically advertise that they drilled their gas port holes only in a groove. This is considerably more difficult than just drilling the gas port wherever. Basically you have to locate the groove, then drill the gas port hole, and plan everything around that hole location. You can't even cut the shoulder for the gas block until that hole is drilled. Then you have to cut your breech face and chamber the barrel all of that hole location that is 10-16 inches down the barrel, and on top of that you have to time the barrel extension so that the hole you drilled ends up pointing exactly vertical. After trying this myself, I asked a well known gunsmith how he does it, he said he doesn't even worry about getting the port only in a groove.

Given two gas ports in two barrels drilled in the same spot relative to a land/groove, the port that is drilled at a 45 degree angle rearward should actually have less jacket shear.

The 45 degree angle will give less gas port erosion, so I suspect that is why they initially went with that design. I have no idea why they switched to a 90 degree design, but if you think about all the extra bs involved in drilling a 45 degree gas port, I would guess the switch was ease of manufacturing.

Justin

[TOrrock]

This is what I've heard from multiple sources over the last couple decades.

The best reference book I've found on the Kalashnikov was started by Edward Ezell in the late 80's and then updated by Blake Stevens after Ezell passed away with more current information.

Kalashnikov, the Arms and the Man

http://www.collectorgrade.com/bookshelf7.html

[Massoud]

I just edited my above post to add a couple things. I'll check that book out, thanks,
Justin

[TOrrock]

I'm 100% certain that ease of manufacturing entered into it as well.

[GTifosi]

Basically you have to locate the groove, then drill the gas port hole, and plan everything around that hole location. You can't even cut the shoulder for the gas block until that hole is drilled. Then you have to cut your breech face and chamber the barrel all of that hole location that is 10-16 inches down the barrel, and on top of that you have to time the barrel extension so that the hole you drilled ends up pointing exactly vertical.

Couple ways to get around all that song and dance:

1) Fixed jig that the barrel sits in where the rifling tool is set in such a way that the lands and grooves end up in the same place from barrel to barrel, which means the hole can consistantly be drilled at a known fixed point. Barrel extension if present or breach face work is in turn indexed off the port hole in another jig, or the same one with different tooling. Probably the fastest/easiest for production purposes and easily accomplished with CNC equiptment.

2) 'Yoke' type tool that goes into the cut grooves internally and has an outer arm that indicates where the tip of that inner arm is.
Fiddle from groove to groove until things line up appropriately for length. Best for very low volume or even one-off type work as its incredibly tedious.

3) Rifling is done after port is drilled. Again with a jig indexed off the port hole. Doing so also clears any crap left in the bore from that port drill penetrating into the interior.
Probably easiest when using old manually operated machinery and mabe not so skilled labor production line techniques.

Any of the jigs could alternatively be indexed off the step in the breach face instead. As long as everything is consistantly positioned relative to everything else and the barrel blank not removed before most processes are done, it'll be just fine. Mearly a question of specifically you want to index off.

In all three methods barrels are already cut to relative length and bored through. Chamber would come later in production as you'd not want to destroy it dragging a rifling tool through there, though a relief in the basic chamber shape could be cut in at the time to make chamber cutting and/or reaming a finishing process.

As long as you know the rate of twist and how the barrel blank and tool align, you can quite easily predict where a port hole should be located radially to meet both a groove and fixed distance for the gas block location.
All it takes is a little numbers crunch once or twice to get it right, then run hell out if it down on the line.

Additionally, ever notice that gas blocks tend to have a larger hole than the port itself?
That is where some of the linear tolerance is achieved as that 'extra' size allows for the block to go fore or aft a smidge to also assist in getting the block the correct distance from X while the port hole in the barrel controls rate of flow.