Why are AR gas block dimples conical?

Z

zukiphile

New member
After using only clamp-on gas blocks for a long time, I've gotten a couple set screw blocks and dimpled barrel.

The gas journal dimples are drilled with a conical tip and the set screws have a truncated cone shape that centers in the bottom of the conical hole. I read that not too many inch pounds should be used on these set screws so the bore isn't distorted.

Why aren't the dimples drilled with a flat bottom and flat set screws so they function more like the pins in a FSB with the stress borne by the sides of the screw and hole? Wouldn't this would permit the screw to secure the block with almost no torque?
 
It is for centering, I think. If the flat lands on flat, the gas block will wiggle. I don't believe a #10 set screw driven by a Allen wrench can ever generate enough force that can crush a barrel. A tapered pin pounded in sideways would probably do it first.

I did have the set screw, even though torqued down pretty hard, backing out on me. A bit of blue loctite fixed the issue. But still checking the screws has become a routine. That's drawback of the design, not as reliable and rugged as the pin.

-TL

Sent from my SM-N960U using Tapatalk
 
If i remember right, my aero presicion gas blocks used 40 in-lb. The ends of the screws were also serrated and actually bit into the barrel steel.

But as to your question on the conical hole. Think about it as a double taper. Or a red solo cup. The tapers match up allowing them to self center when tightened down.
 
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Centering makes sense.

tl said:
I don't believe a #10 set screw driven by a Allen wrench can ever generate enough force that can crush a barrel.
Click to expand...

That would have been my intuition as well.

I've used clamp-on gas blocks because I understood them to spread stress evenly over that part of the barrel, a trait that might contribute to better accuracy. I saw a Criterion video about how the set screw makes the block about as secure as a pin, and that sounded useful for an end cap and clamshell handguard arrangement.

Later, I saw a School of the American Rifle video in which he noted that he checks set screws for torquing beyond 20 inch pounds (iirc) because he has seen overtorquing result in a tight spot in the bore.

Even a set screw that is flat and functions like a peg that doesn't perfectly center in the flat bottomed hole would locate the large gas hole over the barrel's gas port well enough. Of course, it isn't as if every barrel on which I used a clamp-on gas block has been remarkably accurate.
 
I have always just had trouble getting clamp ons to fit under handguards.

I would note that JP rifles, who i feel makes some very accurate high quality rifles, only makes/uses clamp on style gas blocks.

I would personally love to see a test where a clamp on gas block is installed and shot fir groups. Then a set screw type. Then a pinned gas block. See how it effects the accuracy of yhe gun with yhe same ammo.
 
even hard steel has some flex to it, when dealing with .0001" or less tolerance just holding with pliers might be noticeable,

when i was rebuilding engines i tested a strange report that i had heard. "if you take a new (or in this case rebored) v8 engine block and lube a piston without rings and start it down the bore it will free slide all the way through, but if you slap your hands on (outside of the water jacket, seeing that you couldn't reach the actual cylender) you can stop the piston from sliding and hold it until you release the pressure. i know that the blocks were not harded steal, but the pressure wasn't being applied dirrectly and it was not high pressure. i tested and it is a fact. so i extrapolate that a set screw screwed in too tightly will distort the bore of a rifle also. but you might consider that it will depend on "how" the force is distributed. the double taper distributes more force in a radial direction than in a forward (relative the force) direction. a flat bored hole with any shape screw should exert more forward force. thus the answer to your question "why".

edit: additionally, when accuracy and endcap style forearms are mentioned together, i have to think "what sighting system will (or can) work with that combo?"

it's kinda like resting the very end of a barrel on a block to improve accuracy, it may reduce movement in the overall weapon; but, it introduces a variable poi shift according to variable pressure applied. it is not a desirable thing when searching for accuracy. you either want a free floated barrel (best) or a very ridged chassis that the barrel and action both simply rest in.
anything that will change with the way you hold the gun or how much you might pull on the grips will generate a "miss" when shooting small thing from different positions "or large things that are far enough away to look small" every time.
 
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by the way, the only sighting system that will remain "close" to accurate when actually applying variable pressure to a gun barrel, is sights that reside on the barrel. and the further from the action the back sight is the less affected the system is. even then there may be a noticeable poa - poi shift.
 
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