Barrel Blank Clocking, Up or Down...

[thane]

Setting up another Lothar Walther slow twist poly in .177 and since there is no barrel transfer port in this rifle I can "clock" the barrel to my advantage.

At first it seems natural to fit the barrel with any POI offset in the twelve noon position, reducing the vertical compensation needed from the scope or rail.
And then there is the thought that the "nose up" might contribute to a lower BC at target. Clocking the barrel down would decrease this angle a minuscule amount.

Is there a known convention, not speculation, for the orientation?

 

[Ca_Varminter]

Have alway setup my new barrel blanks in the Up position.....
Less rail or scope compensation.....

 

[BC]

Up or down whatever works for you. It only takes a few minutes when you have a gun like a Thomas or USFT, when you can loosen a couple screws, rotate the barrel and tighten it up and try again. Once you have it figured, make sure you put a witness mark in the barrel, so it can be realigned again when it is taken out of the gun. I usually take a small drill and make a mark near the gap in my barrel clamping blocks. Good luck INDEXING the barrel.......BC

 

[Airgun-Revisions]

I’m trying to understand why you would offset the barrel? Or leave it offset from OD? Why not true the whole OD to the bore?

 

[thane]

A trued bore at 50 yards would still likely have an unplanned bias seen in groupings when the barrel is rotated 12-3-6-9.
Since some rifles, Thomas, USFT (in my case) and others can be rotated to some advantage, or ignored and compensated soley with scope adjustment... maybe found to be so minor as to be negligible.

 

[AirSupply]

Fx has 2 transfer ports, one pellet and one slug opposite each other. Doesn’t seem to affect performance at all. You could do the same with 2 identical ports, one at 12 and 1 at 6 and see which works better for you.
I do remember Harry Fuller stating that there was an advantage with the 6 o’clock position but I’m afraid I can’t remember why.

 

[BillG]

When I cut the port, I typically orient the deviation at 12 o'clock.

 

[Revoman]

While the FX does have two transfer ports at 180° from each other, you can/should still index each one to the 12:00 o'clock position. The liner doesn't have to point down on one and up on the other as long as you index for each side.

mike

 

[AirSupply]

While the FX does have two transfer ports at 180° from each other, you can/should still index each one to the 12:00 o'clock position. The liner doesn't have to point down on one and up on the other as long as you index for each side.

mike

Hi Mike. Most people seem to think 12 o’clock is the preferred position. Do you have a reason for doing this?
If there isn’t any gain in accuracy, zeroing at 6 o’clock will give you an increase in the vertical adjustment of your scope.
Zeroing at 12 o’clock you will loose some upward adjustment.

 

[BillG]

Setting up another Lothar Walther slow twist poly in .177 and since there is no barrel transfer port in this rifle I can "clock" the barrel to my advantage.

At first it seems natural to fit the barrel with any POI offset in the twelve noon position, reducing the vertical compensation needed from the scope or rail.
And then there is the thought that the "nose up" might contribute to a lower BC at target. Clocking the barrel down would decrease this angle a minuscule amount.

Is there a known convention, not speculation, for the orientation?

Unless the bullet is "over or under stabilized" the nose should follow the trajectory. the mentioned flight characteristic is arguably the only reason to not spin a bullet with a really fast twist rate.

 

[OldSpook]

Unless the bullet is "over or under stabilized" the nose should follow the trajectory. the mentioned flight characteristic is arguably the only reason to not spin a bullet with a really fast twist rate.

That is true of pellets. It isn't really true of bullets even ones which are not over stabilized. That "nose up" attitude is one of the sources of tumbling at long range as the projectile maintains gyroscopic stability in it's downward path to the target. There is very, very little drag stabilization in a bullet and if the CG is far enough backward (which it usually is) the nose will eventually get kicked up and the tumbling will begin. JMO and your mileage will most likely vary.

 

[BillG]

Over stabilization does promote nose high attitude. It been published in multiple books and studies. Litz wrote a book detailing it. There are direct correlation between spin, velocity, CG, center of form and center of pressure. Velocity and center of pressure are strongly associated. The center of pressure changes throughout the flight as velocity drops. Typically, a bullet has a cg that is behind center of form. As the bullet slows the spin remain very high in relation. Eventually the bullet wants to act like a top. A 1.5 stability will Typically give a very long, nose oriented to trajectory, flight. Even when sent vertical at angles greater than 45, the bullet impacts nose down. Over stabilized bullets will be tumbling right before or at apex and thereafter.

 

[thane]

24" LW Slow Poly is 1:36" twist rate, too slow for 12fpe, much better for 20fpe running higher velocities.

After a few hours of lapping and testing for 10 shot group size, clocking at 90° sectors (no transfer port on USFT) is not making any measurable difference, must be very concentric.

So it is making 0.426" (0.74 MOA) ten shot groups at 55yds using JSB 10.34 @ 930FPS, best barrel to date! At 880 fps the groups open up to 5/8", must be some harmonics at work... next session.

 

[OldSpook]

... A 1.5 stability will Typically give a very long, nose oriented to trajectory, flight. Even when sent vertical at angles greater than 45, the bullet impacts nose down. Over stabilized bullets will be tumbling right before or at apex and thereafter...

Theory works every time in a lab. I suppose if you get the balance just perfect, velocity just perfect, and the other physical conditions just perfect, the assertion that you will consistently achieve perfect stabilization could be true.

 

[Franklink]

So it is making 0.426" (0.74 MOA) ten shot groups at 55yds using JSB 10.34 @ 930FPS, best barrel to date! At 880 fps the groups open up to 5/8", must be some harmonics at work... next session.

Slow twist poly in my USFT likes the 10.34s between 910 and 930 also (mines unregulated so that's roughly the ES over the course of a "fill").

0.426" 10 shot groups at 55 yards just sounds like cheating Thane. Just kidding-sounds like a helluva good barrel! Nice work!!!

As for the clocking question, I've never tested it in my USFT. Before I removed the barrel the first time I created a witness mark so it's always been shot in the same orientation as it was supplied by Tim.

However, I did some pretty extensive testing with clocking the barrel in a .20 JSAR Raptor that I requested a thimble for, specifically so that the barrel COULD be clocked. With that barrel/gun/platform the impact point would rotate around "center" with each barrel orientation, and there was a definite relation to point of impact and barrel orientation. I eventually settled on leaving it at the orientation that produced a 12 o clock impact point.

 

[thane]

I need to shoot more see how it progresses untouched and then come back to it.

Time working on forced position shots would definitely be more productive for my current game...

 

[IRBent]

Hi Mike. Most people seem to think 12 o’clock is the preferred position. Do you have a reason for doing this?
If there isn’t any gain in accuracy, zeroing at 6 o’clock will give you an increase in the vertical adjustment of your scope.
Zeroing at 12 o’clock you will loose some upward adjustment.

Wouldn't orienting the barrel towards 6 o'clock be akin to inducing barrel droop, or the same effect as barrel droop? To me it seems the 12 o'clock position would help get the projectile back to the line of sight faster as well as help offset gravity.