Author Topic: How much jump before pellet hits the grooves?  (Read 1460 times))

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Re: How much jump before pellet hits the grooves?
« Reply #100 on: November 21, 2021, 04:30:00 PM »
So, Duy; did your groups shrink after cutting the shorter leade?


And for general speculation:  I wonder if smaller caliber slug guns are more sensitive to free-bore, because they have more projectile circumference in bore contact (friction), compared to the area that air pressure acts on to generate engraving force (and accelerate the projectile moving); because: 

Circumference is PI x D;
while area is PI x D X D / 4

As such, larger projectiles have engraving forces that increase with the diameter, but available air pressure forces that increase by the square of the diameter.  The inverse means that small calibers slugs with significant weight (flank length) may bog down more during land engraving. 

This obviously also depends on the how close the bearing diameter is to barrel groove diameter.  Dubber and his buddies have been shooting .22 cal slugs that are under groove diameter for a long time to good effect.  Members on this forum have recently noticed that large bore slugs sized 0.0005" under groove diameter shoot faster, with no loss in precision.  That approach seems perhaps better because it reduces friction all the way down the barrel, than slamming the slug into the rifling with a lot of free-bore. 

Of course, it depends how close the slug diameter is to the free-bore diameter.  If the free-bore is a gentle cone then it will align the slug with the barrel axis before the slug slams into the rifling.  What is the optimal free-bore for velocity, without degrading accuracy is the million dollar question.  Somehow, better precision seems more valuable than a few more FPE.  A trade-off one can't effectively speculate about, without absolute numbers.
« Last Edit: November 21, 2021, 04:34:39 PM by subscriber »
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #101 on: November 21, 2021, 08:53:50 PM »
In theory, pellets can be shot accurately out of smooth bore.
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Re: How much jump before pellet hits the grooves?
« Reply #102 on: November 21, 2021, 09:02:08 PM »
In theory, pellets can be shot accurately out of smooth bore.

As long as the pellet head is adequately supported by the tube ID.

How do you know if a chamber is cut "too loose"?  The loaded pellet or slug falls out if you invert the open breech...  If the projectile fit is snug enough not to fall out, it should be pre-aligned adequately to shoot straight.
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Offline rsterne

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Re: How much jump before pellet hits the grooves?
« Reply #103 on: November 21, 2021, 09:04:01 PM »
Quote
In theory, pellets can be shot accurately out of smooth bore.

if the pellet is PERFECT (concenricity, balance).... Any imperfections will require at least a slow twist rate for accuracy....

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Offline lennyk

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Re: How much jump before pellet hits the grooves?
« Reply #104 on: November 23, 2021, 09:17:23 PM »
in my country we are only allowed smoothbore air guns in 177.

at 25m i would say they are very close

beyond 40m the key difference is stability in wind.
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