Bob what about Barrel indexing thimbles on a PCP ???

[TOM aka critter99]

Does it help...

[rsterne]

I have no idea what indexing a barrel is....

Bob

[plinker81366]

have had experience with barrel indexing and true it helps with accuracy.. with this type of pcp its easy to index the barrel,

[Lee]

I'm not trying to sound smart here Bob, just from what I read for the last 3 days.

Apparently when the barrel is rotated in the breech (say 90 deg), the POI and grouping will change. You pick the barrel position that produced smallest and lowest grouping as the best barrel placement in the breech.

[Motorhead]

Seems to me, if rotating barrel changes group size ... ya got some lousy machining going on & parts needs to be better in TIR tolerances   

[plinker81366]

my experience with indexing.. the group size change is somewhat nil.. but the poi changes at 75 yards. by 1 to 1/2 inch..depending on barrel quality.. i just use this to compensate zero adjustment.. instead of scope shimming..

[Lee]

Seems to me, if rotating barrel changes group size ... ya got some lousy machining going on & parts needs to be better in TIR tolerances   

beats me. Until few days ago I have no idea that someone purposedly made mechanism to rotate barrel in breech. Now that I've read what I can, it seems the logic is sound.

my experience with indexing.. the group size change is somewhat nil.. but the poi changes at 75 yards. by 1 to 1/2 inch..depending on barrel quality.. i just use this to compensate zero adjustment.. instead of scope shimming..

plinker, at what distance barrel indexing is done? is it for specific range (bench rest competition) ?

[plinker81366]

Seems to me, if rotating barrel changes group size ... ya got some lousy machining going on & parts needs to be better in TIR tolerances   

beats me. Until few days ago I have no idea that someone purposedly made mechanism to rotate barrel in breech. Now that I've read what I can, it seems the logic is sound.

my experience with indexing.. the group size change is somewhat nil.. but the poi changes at 75 yards. by 1 to 1/2 inch..depending on barrel quality.. i just use this to compensate zero adjustment.. instead of scope shimming..

plinker, at what distance barrel indexing is done? is it for specific range (bench rest competition) ?

50 to 75 yards Lee.. its a bit hard work.. zero in at 50 then shoot to 75 or 100 see if how much is the poi shift.. its just for hunting config

[TimmyMac1]

...except at the highest level of precision. It is a mechanical reality that there will be some barrel position the gun/barrel likes best in terms of where it groups the tightest. The starting point for most is the lowest POI the barrel produces as the theory is the arc matching the gravity forces acting on the pellet are not in conflict with the barrels downward path. We must start somewhere in the 360 degree circle most barrels print and we must choose a speed of the pellet to tune the gun. My starting point is 885 FPS and the lowest POI with relatively centered windage.
Sometimes there is a horizontal component to the group that goes away when you turn the barrel in the barrel mount. If you can't index you can't do a thing about it. Cleaning from the back, indexable & swappable barrels/calibers, access to valve, reg, trigger and inlet systems independent of one another and ease of service are all features that adorn the better Bench Rest systems available.
The more BR pleasing features you have the more tools in your box you can use to improve. Thimbles can serve two purposes as they can capture an o-ring that acts as a M-Rod like bolt seal on (transfer port from the bottom systems) as well as allow the barrel to spin because the thimble has the transfer port in it and that is pressed onto the back of the barrel, so the barrel is able to turn independent of the port.
My Rifle is a design that allows indexing of barrel & swapping calibers by simply reducing the torque value on two of the four primary action fasteners allowing a slit in the barrel mount to release the barrel. When the air comes up through the Swiveling transfer port there is no transfer port in the barrel and indexing is childsplay. Cleaning from the back with a Dewey rod requires cheek piece removal, open transfer port and hammer cocked.
Indexing, Dampening and Firelapping are areas where incremental improvements can make for the final installments of accuracy for a barrel.

TimmyMac1

[Lee]

Seems to me, if rotating barrel changes group size ... ya got some lousy machining going on & parts needs to be better in TIR tolerances   

beats me. Until few days ago I have no idea that someone purposedly made mechanism to rotate barrel in breech. Now that I've read what I can, it seems the logic is sound.

my experience with indexing.. the group size change is somewhat nil.. but the poi changes at 75 yards. by 1 to 1/2 inch..depending on barrel quality.. i just use this to compensate zero adjustment.. instead of scope shimming..

plinker, at what distance barrel indexing is done? is it for specific range (bench rest competition) ?

50 to 75 yards Lee.. its a bit hard work.. zero in at 50 then shoot to 75 or 100 see if how much is the poi shift.. its just for hunting config

wow, never thought
it requires shooting that far. As I don't have access to even 50m indoor range, it would be hard to do. Thanks for the info.

...except at the highest level of precision. It is a mechanical reality that there will be some barrel position the gun/barrel likes best in terms of where it groups the tightest. The starting point for most is the lowest POI the barrel produces as the theory is the arc matching the gravity forces acting on the pellet are not in conflict with the barrels downward path. We must start somewhere in the 360 degree circle most barrels print and we must choose a speed of the pellet to tune the gun. My starting point is 885 FPS and the lowest POI with relatively centered windage.
Sometimes there is a horizontal component to the group that goes away when you turn the barrel in the barrel mount. If you can't index you can't do a thing about it. Cleaning from the back, indexable & swappable barrels/calibers, access to valve, reg, trigger and inlet systems independent of one another and ease of service are all features that adorn the better Bench Rest systems available.
The more BR pleasing features you have the more tools in your box you can use to improve. Thimbles can serve two purposes as they can capture an o-ring that acts as a M-Rod like bolt seal on (transfer port from the bottom systems) as well as allow the barrel to spin because the thimble has the transfer port in it and that is pressed onto the back of the barrel, so the barrel is able to turn independent of the port.
My Rifle is a design that allows indexing of barrel & swapping calibers by simply reducing the torque value on two of the four primary action fasteners allowing a slit in the barrel mount to release the barrel. When the air comes up through the Swiveling transfer port there is no transfer port in the barrel and indexing is childsplay. Cleaning from the back with a Dewey rod requires cheek piece removal, open transfer port and hammer cocked.
Indexing, Dampening and Firelapping are areas where incremental improvements can make for the final installments of accuracy for a barrel.

TimmyMac1

Tim, thank you for the explanation. So basically in your opinion indexing only needed for competitive shooting. In terms of group size, what kind of improvement in, say, 50m indoor, one can expect from indexed barrel vs non-indexed ? I realized it is difficult to quantify but is 1" group to 3/4" possible ?

I also noticed that on USFT rifle the barrel is free floating. Does indexing barrel also works on barrel locked by bands?

[rsterne]

Thanks for adding yet one more variable to the myriad of things I need to think about.... *LOL*....

Great explanation, Tim....

Bob

[plinker81366]

hope this helps also...  after zeroing in the rifle at lets say 55 yards..and i find that theres a shift of poi at 75.. one thing i find also is to shoot when the sun is at the back .where one can see the pellets flight.. i index till i get to see the pellets travel/flight are mostly or most of the time aligned with the vertical of the crosshair.. the rifle should be firmly rested

[rsterne]

As I understand it, that is what Tim@Mac1's setting for maximum drop does.... aligns any barrel curvature with the vertical....

Bob

[PakProtector]

As I understand it, that is what Tim@Mac1's setting for maximum drop does.... aligns any barrel curvature with the vertical....

Bob

I suppose it would not hurt to align it pointing UP too...
cheers,
Douglas

[TimmyMac1]

As I understand it, that is what Tim@Mac1's setting for maximum drop does.... aligns any barrel curvature with the vertical....

Bob

CORRECT! Tim

I suppose it would not hurt to align it pointing UP too...
cheers,
Douglas

That would be plan B

WE START WITH THE LOWEST poi AND THEN GO FROM THERE TO SEE IF ANY OTHER POSITION IS BETTER. WE HAVE THE BEST SUCCESS AT THAT lowest SETTING and 885 FPS. If the gun gives a horizontal or Verticle stringing, turning it will often correct the issue.

TimmyMac1

[eeler1]

Do you end up milling flats on the barrel, once you find the best position?  Or has rotation not been a problem?

[PakProtector]

Do you end up milling flats on the barrel, once you find the best position?  Or has rotation not been a problem?

Index-able barrels will have a groove, or at least the ones I have seen pix of all had a groove. If no groove, there will be a burr that makes it impossible to get barrel out of a properly fitting breech.
cheers,
Douglas

[Bryan Heimann]

I had no idea such a level of attention to detail went into building a competition rifle.  I knew it was high, but I wonder who figured this out about barrels and how he came to the conclusion.  I imagine a mad scientist with half a dozen guns connected to regulated tanks at a hundred meter range and comparing the groups lol

[TimmyMac1]

Do you end up milling flats on the barrel, once you find the best position?  Or has rotation not been a problem?

Index-able barrels will have a groove, or at least the ones I have seen pix of all had a groove. If no groove, there will be a burr that makes it impossible to get barrel out of a properly fitting breech.
cheers,
Douglas

The barrel mount has a slit in it that we mark adjacent to a little mark with a pointed punch once we have decided which position to go with. The slit in  the barrel mount allows the barrel to be turned when two of the four primary action fasteners are loosened. Once a witness mark is put on the barrel the barrel can still be removed as long as you keep the witness within the slit.

TimmyMac1

[TimmyMac1]

I had no idea such a level of attention to detail went into building a competition rifle.  I knew it was high, but I wonder who figured this out about barrels and how he came to the conclusion.  I imagine a mad scientist with half a dozen guns connected to regulated tanks at a hundred meter range and comparing the groups lol

The Guys who figure that indexing is advantageous is a long list of Benchresters that came long before me. We all stand on the shoulders of giants and that means we do it because we learned it could improve things. The Range the mad scientist uses is a 50 yard long tunnel made of 24" Pipe. Larry Durham designed my rifle and he has most of the features Benchresters demand in the rifle as delivered. Ease of barrel changing, cleaning from the back and indexable barrels are elements that a competition shooter needs in his bag of tricks.

TimmyMac1