Barrel Slugging an Air Venturi Avenger

[Reidy]

And the rest.

[Reidy]

Bright light!

[subscriber]

Wayne,

Improvements are always good.  As is an action that does not make matters worse.

One of you targets intrigues me - enlarged below.  Are those three clusters marked with red arrows made by two pellet each, or is it one projectile cutting an oblong hole?  The fourth one is not so obvious, so lets just look at the three.

[Reidy]

One of you targets intrigues me - enlarged below.  Are those three clusters marked with red arrows made by two pellet each, or is it one projectile cutting an oblong hole?  The fourth one is not so obvious, so lets just look at the three.

I had not noticed that. I just saw that they grouped badly and moved on. This rifle isn't pushing these anywhere near fast enough so tumbling at 50 yards would not surprise me. I had to hold 4 mil dots to get these on target! I have not opened up the barrel or transfer port in this yet. Everything is still factory. Even then the plenum is just too small for heavy pellets / slugs.

The griffins look like there's only 4 shots but the middle hole of the tight group has two pellets through it.

[subscriber]

I had not noticed that. I just saw that they grouped badly and moved on.

Understandable.  I have a strange way of noticing subtleties.  It is noticing patterns, and when patterns are broken.   Good for spotting game in tall grass.  Been that way since I can remember.

[Reidy]

I got around to slugging the barrel tonight. It's a tight one!

The restriction (I'm not calling it a choke) is still definitely there to. It is less BUT it's still really tight compared to the rest of the bore. The bore felt quite nice though.

I put pellet tin tape where the restriction starts. It ends  at the top of the lettering. That's the reason I'm not calling it a choke. I think you'd agree that a choke is supposed to go to the end of the barrel?

I tried 3 different brands and sizes.

AVS .2213" 25 grain hollow tail, which were really tight, but you can also see that they're very uniform in how they contact the rifling.

NSA .2165" 20 grain flat tail. These were also really tight, even though they're supposed to be on the small side of slug diameters. They also held the rifling very evenly.

Griffin .218" 22 grain hollow tail. These were fairly loose in comparison to the others, even though they're on the larger side of diameters, as can be seen by the uneven grip markings from the rifling.

Fx hybrid slug .217 22 grain. Also very tight andn gripped the rifling evenly BUT you can see that the skirt isn't square. This isn't a trick of the light either. I took a few different pictures to get this one and it was noticable in all of them.

I'm definitely going to fire lap it more but I need to find a slug that isn't SO tight. The NSA. 218" ones used were sometimes sticking and I needed to fire twice to clear them.

[subscriber]

Wayne,

The choke can be parallel, after its initial cone shape towards the muzzle.  This can make the cone feel like a restriction (because it is); then the force required to drive the projectile can be "flat" to the muzzle. 

What would be bad is if the choke point were back from the muzzle, and the bore actually opened up in diameter towards the muzzle.  That would feel like the slug suddenly had no resistance to being driven towards the muzzle.

If the bore were to "flare" after the choke, then cutting off that portion of the barrel and recrowning would probably shrink groups.  There may be reasons why one does not want to do that.  Boring out the muzzle to create a deep crown can sometimes work to improve such a barrel (although I would think that air rushing past the projectile in the relief bored section would be a very bad thing).   

Now, as your Avenger is shrouded, cutting the barrel shorter seems to complicate matters - unless you install more baffles (or a spacer tube) to make up the difference.  Just cut off the equivalent length of 1, 2 or 3 baffles.  How the airstripper sits on the muzzle I am not sure.  I think it just presses on, in which case there is nothing complicated about this.  Certainly, cutting the shroud shorter and rethreading the end would be something I would avoid.  You may be able to single-point cut such fine threads.  I am not going to attempt it.

Folks who re-choke airgun barrels may be able to improve the existing choke.  They may insist in cutting it off first, so that any choke is "theirs"; and not a mix.

As you are shooting slugs, cutting off the choked section of barrel is an option.  If the bore diameter is even from breech to the new muzzle, it should shoot well - all else being equal.  Fire lapping will actually produce a very shallow, long taper from breech to muzzle.  This is a good way to never having the projectile rattle or yaw as it goes down the barrel.  Such taper lapping may be better achieved with a lead lap by hand, but it takes considerable skill.  Limited fire lapping is much easier.

If you don't mind losing 2" of barrel length (everything forwards of where the choke starts), cut, crown and then fire lap would be my suggestion - in that order.  Even a very good crowning job tends to leave a little disturbance.  This does not matter, if the "edge" is even all the way around; but, fire lapping knocks that off and evens it out.

A crown that is cut "flat" and lightly deburred at the bore with a brass round head screw lap will work fine.  The Avenger barrel appears to be saw cut with limited deburring, and most of them shoot well.

If you were hand lapping, then I would prefer that job be completed before the final crowning job.  Yes, it is possible to hand lap a finished, crowned barrel.  But, you must not allow the lap to exit the barrel (to any significant degree), or the crown will tend to end up "bell-shaped".

[subscriber]

I think that the Griffin slug you showed with rifling impressions did not have its nose shape and bearing diameter properly concentric.  The fact that the other two slugs look even tells me it is the third slug that has the anomaly - not your barrel.

It may be possible to start a slug yawed at the breech, but I think you would have to "work" at it.  Possible if the slug was very hard to get started and you somehow were not pushing coaxial with the bore.   If you roll more Griffin slugs on a flat surface, you might see the noses wobble - if there are more of them that are poorly made.  Else, the next Griffin you push through the barrel will probably look OK.  Unless they are so undersized that they yaw from the start.  Their hollow bases may make pushing them skew easier to do accidentally.

[Reidy]

The Griffin slugs, having a fairly long body, are difficult to start unevenly. It started easier than the others and I feel that they're either undersized or the NSA's are oversized. More likely the former.

The others should have been the ones possibly started unevenly as they take a decent bit of pressure to push home. They'r difficult to push into the leade with the cleaning rod, I needed to tap the rod to get them to start.

I expect the choke to be parallel after the initial cone or leade into the choke, I'm sorry if that didn't come across in my explanation. This restriction leads into and then back out into the regular sized bore. The pressure after the initial restriction isn't flat, it loosens off significantly.

I'm only using the slugs right now because of their abrasive holding capacity.  The fact that it's showing s liking for some of them is a nice bonus.

I'm currently trying to get some .216" and .2155" slugs and maybe some AVS .217" slugs that were designed for this barrel.

These newer barrels aren't as roughly finished as the original ones. This one has been properly lathe parted off, not saw cut and while the crown is small, it is there and it's obviously effective.

[subscriber]

Thanks Wayne,

Your measure of success will be how well your Avenger shoots your target projectiles - pun intended.  All the rest are details on the way there.

[Reidy]

Since my last post I've done a bit more tinkering.

I did more fire lapping as I still wasn't happy with how the restriction felt. I think I fired another 10 or so but not all at once. I fired a few then cleaned / slugged the barrel until I was happy. The restriction can still be felt but it's nothing like it was.

I figured while I had the barrel out that I'd strip the action down and take a look at other possible modifications.

I had the bleed cap off the plenum and took measurements of the cap and ordered some 7075 aluminum bar to make a plenum extension. I was taking with Duy (Darkcharisma) and the size I'm going to make will add about 14.5cc, more than doubling the available air.

I polished the hammer, put a light bevel on the barrel breech because it was sharp and nasty. I also drilled the barrel port out to .187" and cleaned up the hole.

I'm going to drill / tap the transfer port to M7 x .75mm and make an insert drilled to .187" to match the barrel port using aluminum bar.  I picked this up today. Taps & dies are on order for the insert and plenum extension.

I got the rifle back together to see how the barrel performs and also took the opportunity to turn down the moderator to 1800psi because I've seen many videos showing that it's set way to high.

What I didn't get to do was shoot it over the chrony to see where it's at speed / power wise.

Sooo........ my first 50 yards group's posted were 1-1/8".

The next ones after initial fire lapping were 3/4".

My latest group's are 5/8" and I was a rushed taking these because of fading light and without using my rear rest bag, so I'm going to now leave the barrel alone. I'm very happy with these results. You can see on one of the targets that 3 of the 5 shots are within 1/4" c/c!

So there's still some planned work to be done but a good deal of improvements have already been achieved.

[subscriber]

One has to know when to stop.  Where you are seems like a good place, Wayne.

[Reidy]

One has to know when to stop.  Where you are seems like a good place, Wayne.

Agreed! 

The thing is, after getting rid of my Hatsan, I said I wanted a rifle that I wasn't going to tinker with. I guess I like playing with these more than I realize, though this rifle is more accurate than the Hatsan was at first.