Contact areas in barrel?

[M Volle]

Rookie Questions...

I'm fairly new to PCP and airguns - I am trying to get a handle on the tuning process and understand some parts but am struggling with others.

I am about 2000 rounds into learning and have been keeping the distance at 50M

I purchased an Air Venturi Avenge-x in 25 cal (bottle and classic synthetic stock)


I have opened up the barrel port to match the area to the barrel bore - oval opening now - increased speed for same settings but no other significant impact - just as consistent as when it was a 4mm round port. Get a few more shots per fill for same FPS

Checking that there weren't any burs left from milling the port, I chambered some pellets and pushed them back out the breach to look for scrapes or scars - there were none visible.


What I did see was that the contact area and the depth of engagement with the rifling was very different depending on the projectile.

I then quickly seated most of the ones I have been trying to see how each looked - summary in the photo below.

The deepest markings (and stiffest to load) is the H&N Slug HP Heavy while the Slug HP MK II in 0.249 barely marked at all.

Is this amount of variation typical?

For my gun, the Zan 30 grain slugs never group all that well.
The Barracuda Hunter Extremes are completely wild.
The Wildman is only moderate.
The HP II Mk II's and the JSB Exact King Heavy will shoot same hole at 50M - even with the same tune for both -
The Zan Slug HP in 26.5 grain works at this tune as well - just flys a lot faster.

I can tune all of these to fly at very tight speeds - typically +/- 1 FPS but am struggling to find the most accurate speeds for each (if even possible)

I would like to find a tune that delivers 60+ ft lbs and is accurate at 100M so I was favoring slugs - don't know if that is good thinking or not but they do fly flatter and with less wind drift.

My (rookie) understanding is that the BC will max out sub sonic somewhere in the 925-975 FPS range so I was targeting those speeds but my groups seem a bit larger over 900 FPS.

These are what is working -

ZAN Slug HP 26.5 @ 175 Bar and 950 FPS - tight groups .05 STD on FPS
HN Slug HP II Mk II 30 @ 200 bar and 940 FPS - tight groups and 0.9 STD on FPS
HN Slug HP II Mk II 34 @ 175 bar and 852 FPS - tight groups and 1.0 STD on FPS
JSB Exact King Heavy MK II 33.95 @ 175 bar and 860 FPS - tight groups and 0.6 STD on FPS

Will some projectiles just never like a specific gun?
Is there a sweet speed for each and you just have to find it? ( just getting the FPS consistent clearly isn't the whole answer)
How significant is the "advantage" of a slug over a pellet at 100M? Some manufacturer information suggests maximum ranges and tops out most pellets at 50 yards.

Any advice will be appreciated.

Mark

 

[Back_Roads]

Diameter of the projectile has a roll, a,long with weight and speed they are shot. finding the right land and groove dimensions of the barrel is the first place to start, then select projectiles that claim to be a match. From their what speeds do they like. One can get sizing equipment to properly size the ammo, many who cast their own size to the bore specs.

 

[Troy Bakel]

When actually shot, the projectile will be swaged into a different shape in three steps:

1. The initial very rapid, very high, differential pressure across the projectile acting against the inertia and drag that is resisting acceleration.

2. Bore dimensions as the projectile accelerates down the bore.

3. The amount of choke at the muzzle.

The final swaged shape determines:

1. The trajectory of the projectile as it crosses the threshold of the crown.

2. The external ballistics of the projectile.

I have now tested 17 different diabolos of varying weights and deminsions in my 0.177 FX Impact M3 at various tunes for each and only two are accurate from that barrel.

 

[Ranchibi]

Yes, each barrel is different and will most likely perform better with certain ammo. Remember, slugs are spin stabilized for accuracy so a loose fitting slug will not be accurate from the get go, slugs need to contact the lands. You found the Zan 26gr to be accurate so I would stick with those.

 

[Troy Bakel]

Yes, each barrel is different and will most likely perform better with certain ammo. Remember, slugs are spin stabilized for accuracy so a loose fitting slug will not be accurate from the get go, slugs need to contact the lands. You found the Zan 26gr to be accurate so I would stick with those.

Have you seen how badly deformed a loose fitting diabolo looks after it wobbles down the bore at high speed and hits the choke wth its nose off center?

I have hard copy pictures of such diabolos that were arrested by a soft catcher and have their nose bent back into their waste area.

A brick spinning at 30,000 rpm would probably exhibit better external ballistics.

 

[M Volle]

When actually shot, the projectile will be swaged into a different shape in three steps:

1. The initial very rapid, very high, differential pressure across the projectile acting against the inertia and drag that is resisting acceleration.

2. Bore dimensions as the projectile accelerates down the bore.

3. The amount of choke at the muzzle.

The final swaged shape determines:

1. The trajectory of the projectile as it crosses the threshold of the crown.

2. The external ballistics of the projectile.

I have now tested 17 different diabolos of varying weights and deminsions in my 0.177 FX Impact M3 at various tunes for each and only two are accurate from that barrel.

Troy -

I've noticed that several pellets I have utilize a significant hollow on the back side of the pellet - the probe on the Avenge-X 25 cal is quite narrow pointed and does not push these pellets as far into the barrel -

The JSB Exact King Heavy pellets have a fairly flat back and after loading, even the skirt edge shows the impression of the barrel rifle grooves - the other pellets do not - the skirt isn't seating into the rifled portion and presumably is barely past the port in the barrel. How significant is this in the total process of firing?

Is this typical of the probe on other guns?

I am also finding that slugs vary in length within a batch much more then they vary in weight and diameter - how critical is the slug length to the performance? Working between various brands, the same weight slugs vary in length and my gun seems to like the shorter lengths for the same weight - but the longer the contact surface, the slow the slug flys for the same weight and pressure...

Lots of questions but trying to wrap my head around how this all works at the detail level.

 

[PumaCarl]

Really good post.

You've answered your own question pretty well, shoot the Zan 26.5gr.

 

[Troy Bakel]

Troy -

I've noticed that several pellets I have utilize a significant hollow on the back side of the pellet - the probe on the Avenge-X 25 cal is quite narrow pointed and does not push these pellets as far into the barrel -

The JSB Exact King Heavy pellets have a fairly flat back and after loading, even the skirt edge shows the impression of the barrel rifle grooves - the other pellets do not - the skirt isn't seating into the rifled portion and presumably is barely past the port in the barrel. How significant is this in the total process of firing?

Is this typical of the probe on other guns?

I am also finding that slugs vary in length within a batch much more then they vary in weight and diameter - how critical is the slug length to the performance? Working between various brands, the same weight slugs vary in length and my gun seems to like the shorter lengths for the same weight - but the longer the contact surface, the slow the slug flys for the same weight and pressure...

Lots of questions but trying to wrap my head around how this all works at the detail level.

The deep hollow allows the skirt to expand deeper into the grooves providing a better seal with the bore and reducing the amount of air flowing past the pellet as it accelerates down the barrel.

How snug and centered rhe pellet fits into the chamber and how far it has to jump before reaching the lands determines the alignment of the pellet with the bore when the swaging process begins which in turn determines the shape of the pellet and its orientation when it crosses the crown threshold.

The section of Internal Ballistics that studies how a bore reshapes a projectile is " The Effect of In-Bore Yaw on Lateral Throwoff and Aerodynamic Jump"

If the pellet enters the lands centered, no yaw, and stays centered down the bore and maintains a good Aerodynamic shape then Aerodynamic jump is the main factor in determining the pellets final trajectory crossing the crown threshold.

If however the pellet slams into the lands off centered (yawed, such as nose down) the swaging process could deform the pellet to such a degree that Lateral Throwoff at the muzzle becomes dominant and is why you get extreme fliers. The pellet was so badly deformed by the bore that it crossed the crown with the Aerodynamics of a brick and a bad trajectory.

You can also have a pellet that is too small for your chamber and bore and wobbles (yaws) down the bore and will produce a lot of Lateral Throwoff when it crosses the crown threshold.

So having said all that you need to test various slugs/pellets with a specific probe-barrel setup to determine:

1. Internal Ballistics are good for the combination.

2. You can tune that combination to have the External Ballistics you desire

 

[Airgun-hobbyist]

@M Volle The few thousandth's of an inch difference in length of a given manufacturer's slug or pellet is probably only in the tip or skirt, caused by extrusion/flashing during pressing/swaging. This is evidenced by your slugs being fairly consistent in weight. The head or shank diameter and a consistent grain weight will have a much bigger influence on your group size, unless the diffence in length is significant which will effect consistent speeds and long range group size.

Weight sorting your projectile's is a good way to eliminate flier's and get an even clearer picture of what works best out of your barrel. As an experiment with accuracy, try weight sorting out a tin of each of your three most accurate brand and type of projectile's. Sort them in piles down to one tenth of a grain in weight. Then, of those brand's and their respective piles, shoot 100 yard long range groups on the calmest tine of day you can. To get a good picture of before and after accuracy, also shoot a couple of groups of each brand of them, unsorted and loaded directly from the tin.

 

[Troy Bakel]

@M Volle The few thousandth's of an inch difference in length of a given manufacturer's slug or pellet is probably only in the tip or skirt, caused by extrusion/flashing during pressing/swaging. This is evidenced by your slugs being fairly consistent in weight. The head or shank diameter and a consistent grain weight will have a much bigger influence on your group size, unless the diffence in length is significant which will effect consistent speeds and long range group size.

Weight sorting your projectile's is a good way to eliminate flier's and get an even clearer picture of what works best out of your barrel. As an experiment with accuracy, try weight sorting out a tin of each of your three most accurate brand and type of projectile's. Sort them in piles down to one tenth of a grain in weight. Then, of those brand's and their respective piles, shoot 100 yard long range groups on the calmest tine of day you can. To get a good picture of before and after accuracy, also shoot a couple of groups of each brand of them, unsorted and loaded directly from the tin.

Weight distribution across (side to side) the projectile has an impact on Lateral Throwoff in that the heavier side of the projectile exerts more & more force on the bore as the projectile spins up causing the projectile to Throwoff to its heavy side when it is free of the bore.

EDIT - My statement was wrong. If the heavy side of the projectile exits the crown at 12 o'clock from a right hand twist bore it will Throwoff right and down towards 3 o'clock. If from a left hand twist, left and down towards 9 o'clock.

Because Throwoff is totally due to the mechanical characteristics of the projectile and not its Aerodynamics , pellets and slugs will Throwoff in the same direction. - END EDIT

The higher the spin rate the greater the Throwoff per amount of weight imbalance.

This could be another issue facing large caliber heavy slugs and pellets.

 

[Airgun-hobbyist]

Weight distribution across (side to side) the projectile has an impact on Lateral Throwoff in that the heavier side of the projectile exerts more & more force on the bore as the projectile spins up causing the projectile to Throwoff to its heavy side when it is free of the bore.

The higher the spin rate the greater the Throwoff per amount of weight imbalance.

This could be another issue facing large caliber heavy slugs and pellets.

It for sure is an issue, likely due to the hollow point pin being off-center causing an imbalance.

On another note of interest, I have seen proportionately long shanked slug's of almost exact fit bore diameter where the contact patch is slightly lacking on one side of the shank. I have not pin-pointed it to either the slug die or barrel land and or groove issue of not being cut perfectly round, but have seen this on several barrels and suspect that the bore cut depth on a particular land and or groove side is deeper and thus throwing the projectile out of the bore on an imbalance.

 

[M Volle]

This is all extremely helpful -
And now I have an entirely new rabbit hole to dive into and try to understand.

" The Effect of In-Bore Yaw on Lateral Throwoff and Aerodynamic Jump"

I also "over complicated" my situation a little with some mods to the rifle -

The barrel port is now oval and the same cross sectional area as the bore - no issues feeding slugs or any damage from the edges but a change.

The "Power select" high power port has been opened up by a 1/2 mm in diameter.

The main spring is replaced with a slightly stiffer and straighter one - I can now reach a point where increasing the hammer spring pressure a full turn does not result in any additional speed at 175 bar - so I am guessing that I am dumping the entire plenum - still achieving +/- 1 FPS results over 8 rounds.

I replace the poppet valve with one made from Peek plastic - hopefully it will wear and seal better at higher pressures.
I also found that there is a free flight distance between the hammer and the poppet stem at low spring settings and it diminishes as you increase the hammer spring tension - but it is visible through a vent in the block when the stock is off.

I am a rookie to shooting but I'm a semi-retired (not working) engineer with a shop so I'm not likely to "leave it alone" as a significant part of the fun for me is messing with the variables.
Having an Avenge-x, the only part I can't replace inexpensively is the main block - so I can even mess up a barrel completely and it's only $69 to replace.

I have found that the o-rings on the bottle pressure side of the system are struggling at 300 bar - some of the tolerances are too loose and even the shore 90 o-rings are extruding - so I may try ordering some of the key o-rings in alternate formulations or slightly larger cross sections -

The forums have been a great help due to the knowledge of everyone participating -



Future experiments -

Additional plenum -
True ballistic chronograph (have the FX Pocket II but it only measures at muzzle)

At 175 bar, the energy is sitting right around 60 FP. I am trying to get the velocity between 940-975 (best BC?)and find a "happy" slug in that range.

I can just manage 100 yards in my backyard "range" but need a much sturdier bench to shoot from - that will be next - as my current setup is preventing me from seeing some of the finer results (unless shooting prone with a bipod)

Credit to Air Venturi, with a tune from the web and running JSB Exact King Heavy Pellets at 33.95 gr. the rifle shot 3/8" groups at 50M from day one - and the same tune shot just as tight with H&N Slug HP II 34 gr and ZAN Slug HP 26.5 gr.

 

[Airgun-hobbyist]

This is all extremely helpful -
And now I have an entirely new rabbit hole to dive into and try to understand.

" The Effect of In-Bore Yaw on Lateral Throwoff and Aerodynamic Jump"

I also "over complicated" my situation a little with some mods to the rifle -

The barrel port is now oval and the same cross sectional area as the bore - no issues feeding slugs or any damage from the edges but a change.

The "Power select" high power port has been opened up by a 1/2 mm in diameter.

The main spring is replaced with a slightly stiffer and straighter one - I can now reach a point where increasing the hammer spring pressure a full turn does not result in any additional speed at 175 bar - so I am guessing that I am dumping the entire plenum - still achieving +/- 1 FPS results over 8 rounds.

I replace the poppet valve with one made from Peek plastic - hopefully it will wear and seal better at higher pressures.
I also found that there is a free flight distance between the hammer and the poppet stem at low spring settings and it diminishes as you increase the hammer spring tension - but it is visible through a vent in the block when the stock is off.

I am a rookie to shooting but I'm a semi-retired (not working) engineer with a shop so I'm not likely to "leave it alone" as a significant part of the fun for me is messing with the variables.
Having an Avenge-x, the only part I can't replace inexpensively is the main block - so I can even mess up a barrel completely and it's only $69 to replace.

I have found that the o-rings on the bottle pressure side of the system are struggling at 300 bar - some of the tolerances are too loose and even the shore 90 o-rings are extruding - so I may try ordering some of the key o-rings in alternate formulations or slightly larger cross sections -

The forums have been a great help due to the knowledge of everyone participating -



Future experiments -

Additional plenum -
True ballistic chronograph (have the FX Pocket II but it only measures at muzzle)

At 175 bar, the energy is sitting right around 60 FP. I am trying to get the velocity between 940-975 (best BC?)and find a "happy" slug in that range.

I can just manage 100 yards in my backyard "range" but need a much sturdier bench to shoot from - that will be next - as my current setup is preventing me from seeing some of the finer results (unless shooting prone with a bipod)

Credit to Air Venturi, with a tune from the web and running JSB Exact King Heavy Pellets at 33.95 gr. the rifle shot 3/8" groups at 50M from day one - and the same tune shot just as tight with H&N Slug HP II 34 gr and ZAN Slug HP 26.5 gr.

It seems you.like to.tinker, as do a lot of us! I'd focus on the H&N slug that produced the tightest group at 50m to meet your 100m goal. Also, being that the plenum pressure is really high at 175 bar, maybe try backing it down to 150 bar with the same hammer spring setting to see if the valve opens up longer and pushes the slugs faster towards your goal fps in the mid 950+ range. (As an experiment with valve lock).

 

[Troy Bakel]

It for sure is an issue, likely due to the hollow point pin being off-center causing an imbalance.

On another note of interest, I have seen proportionately long shanked slug's of almost exact fit bore diameter where the contact patch is slightly lacking on one side of the shank. I have not pin-pointed it to either the slug die or barrel land and or groove issue of not being cut perfectly round, but have seen this on several barrels and suspect that the bore cut depth on a particular land and or groove side is deeper and thus throwing the projectile out of the bore on an imbalance.

The critical dimensions of a rifled bore that have to remain identical its full length or the pellet/slug will not swage uniformly and will exit the bore with a non symmetrical shape and/or weight distribution.