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Author Topic: How much jump before pellet hits the grooves?  (Read 3033 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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Offline oldpro

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Re: How much jump before pellet hits the grooves?
« Reply #105 on: November 30, 2021, 06:15:45 PM »
 having tested this exact thing in the past(two years ago) I found pellets shot from a smooth bore had far less accuracy then ones shot with even the slightest twist. Modern pcp guns are getting slower and slower on twist rates as compared to years past though as we are finding you do need twist but not nearly as much as once thought. The fx smooth twist is a great example of a smooth bore with just a slight twist at the very end. Very accurate with pellets and horrible with any solids. 
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Offline lennyk

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Re: How much jump before pellet hits the grooves?
« Reply #106 on: November 30, 2021, 09:01:31 PM »
What smoothbore barrel did you use and what caliber?

having tested this exact thing in the past(two years ago) I found pellets shot from a smooth bore had far less accuracy then ones shot with even the slightest twist. Modern pcp guns are getting slower and slower on twist rates as compared to years past though as we are finding you do need twist but not nearly as much as once thought. The fx smooth twist is a great example of a smooth bore with just a slight twist at the very end. Very accurate with pellets and horrible with any solids.
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Offline Roboslug

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Re: How much jump before pellet hits the grooves?
« Reply #107 on: December 11, 2021, 10:46:41 PM »
In a past life before I had to use prescription glasses I would shoot shotgun slugs as a hobby.  My shotgun is a bone-stock Browning B-2000 auto with a smoothbore slug barrel.  Other than a .22 I don't have any rifles and never considered myself a rifleman, I was always just enamored with the idea of shooting slugs.  When I found a site that hosted postal matches I had to find a barrel.  I chased this for a couple of years trying countless slugs in the name of accuracy.  It was 18 months of shooting/searching (a slug) that grouped adequately before I adjusted the sights.  Though I never won, I did have fun. 

So yes, pellet/bore fit does matter.

50 yds
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #108 on: December 27, 2021, 05:19:40 PM »
I'm having a hard time finding motivation to work on this. I keep seeing videos on the new AEA Big 9 and wondering if I should just stop messing with this thing. It's going to take a large pile of work, and a bunch more money to get close to what you can get out of the box with many of these AEA products.  :-[

The spiral flute reamer is spiraled the wrong way, so it is worse. Still trying to make something to help me with the straight flute reamer. I have an idea to try, and after this I might quit.
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Re: How much jump before pellet hits the grooves?
« Reply #109 on: December 27, 2021, 10:17:35 PM »
Finding motivation to get motivated can be difficult :)
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Re: How much jump before pellet hits the grooves?
« Reply #110 on: December 30, 2021, 11:59:08 PM »
having tested this exact thing in the past(two years ago) I found pellets shot from a smooth bore had far less accuracy then ones shot with even the slightest twist. Modern pcp guns are getting slower and slower on twist rates as compared to years past though as we are finding you do need twist but not nearly as much as once thought. The fx smooth twist is a great example of a smooth bore with just a slight twist at the very end. Very accurate with pellets and horrible with any solids.
     "  The fx smooth twist is a great example of a smooth bore with just a slight twist at the very end. Very accurate with pellets and horrible with any solids. "
        I'm Shooting NSA, FX, and ASA slugs out of a .25 cal. 600mm STX barrel and and I wouldn't call the results, horrible. I have no experience with any of the designated FX slug barrels but if extreme bench shooters are tagging bullseyes and silhouettes at 300 yrd.'s and out, I'm guessing they are doing OK with them. ;D
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Offline nervoustrigger

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Re: How much jump before pellet hits the grooves?
« Reply #111 on: December 31, 2021, 12:26:08 AM »
Quote
I'm Shooting NSA, FX, and ASA slugs out of a .25 cal. 600mm STX barrel and and I wouldn't call the results horrible.

FYI, Travis was describing the original Smooth Twist in which only a portion near the muzzle is rifled.

The Smooth Twist X (STX) is a different design in which the entire bore is rifled.

While their twist rates on paper may be comparable, the former produced an actual rate much, much slower on account of the pellet already having reached near max velocity before hitting the rifling.  It would therefore skid through the rifling and pick up only a small fraction of the twist rate...sufficient to stabilize a pellet but far too slight to stabilize slugs.
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Offline Spin

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Re: How much jump before pellet hits the grooves?
« Reply #112 on: December 31, 2021, 05:20:52 PM »
Quote
I'm Shooting NSA, FX, and ASA slugs out of a .25 cal. 600mm STX barrel and and I wouldn't call the results horrible.

FYI, Travis was describing the original Smooth Twist in which only a portion near the muzzle is rifled.

The Smooth Twist X (STX) is a different design in which the entire bore is rifled.

While their twist rates on paper may be comparable, the former produced an actual rate much, much slower on account of the pellet already having reached near max velocity before hitting the rifling.  It would therefore skid through the rifling and pick up only a small fraction of the twist rate...sufficient to stabilize a pellet but far too slight to stabilize slugs.
  Got It! Thanks Jason.
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #113 on: April 03, 2022, 09:52:19 PM »
Finally got around to doing this on the good barrel, had to make an alignment tool to go over the end of the barrel out of a PEX pipe fitting. The round of concern now loads better, but no real increase in power, I was figuring it wrong before, they are really 155 grain which puts me right at 200fpe when I hit the sweet spot at around 2700psi. Going to compare the new valve to the old valve and maybe open the port on the barrel a slight amount, but other done.

Next mod is higher pressure tube, regulator at around 2700psi and be done.

5 shots as follows, start at 3000 and end at 2500:
620fps
732fps
761fps
758fps
758fps

Again with a 155 grain soft lead, powder coated Lyman bullet. Shot into bucket of denim.
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Re: How much jump before pellet hits the grooves?
« Reply #114 on: April 03, 2022, 10:01:36 PM »
Thanks for the update, Greg.
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Offline Greg_E

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Re: How much jump before pellet hits the grooves?
« Reply #115 on: April 04, 2022, 12:54:59 PM »
All I really accomplished was to knock the higher spots of the ramp down, I didn't really push the leading edge forward at all. So there is still some engagement, just not displacing as much lead now.

Pushing the 155gr up to 800fps would be ideal, but I think that may be farther than I want to go without using a lot more air than I want. Goal is slightly better than stock power (200-250fpe with 145 to 155 grain ammo), 10 shots per fill, and good accuracy. The 84gr stuff should be really moving, and the 125gr should be in that 900fps range where we really like things to stay.
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Offline Howler

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Re: How much jump before pellet hits the grooves?
« Reply #116 on: June 23, 2022, 09:41:14 AM »
Down in my velocity after valve thread, I discovered something that I'm not sure about.

When I chamber a round in my Bulldog, it embossed the rifling pretty far back. Should the projectile be able to travel a short distance before encountering the rifling?

Or do we want the projectile engaged in the rifling to help seal the barrel for optimum performance?

With firearms you normally want at least a small jump distance, or else chamber pressures can get high. Not sure if the same is true for airguns, just without the pressure spike since the pressure is the pressure with air.

I cut the lead in mine for the 25 caliber 43.5gr NSA slug. I get a loose fit (seats deep) with JSB KO slugs but the 55.5gr NSA seats very hard into the lands. FWIW, I shot mine a lot before deciding on a slug I wanted to cut it for.
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