Last year, I was working on some microwave parts. I was measuring for phase change at approx 25 gHz.
I took a break and when I came back, the phase point had changed a bit. It was as if the
waveguide had gotten shorter..
Then, I laid my hand on top of the waveguide. Within a matter of seconds, I could see the phase was
shifting again. The aluminum waveguide part I was touching seemed to be getting longer!
Realizing it was my body heat, I fired up a heat-gun and wow! It really expanded when it got warm!


So, the last few times I have taken my rifle out back to shoot a little, it needed to be re-zeroed.
Today it was shooting a bit off zero, but slowly came back into zero after about 10 minutes of shooting.?.

Then a little LED when on over my head.

The stock is made from cast aluminum, but the laminated wood under it is bolted on with with a screw at each end.

When the temperature changes a few degrees, the aluminum changes length, but the wooden underbelly
doesn't expand at the same rate(if at all), so it sock is going to curve up or down, depending on the direction of the temperature change!

The barrel band is bolted to the stock. The barrel band is putting a slight down pressure on top of the barrel.
Which will change the POI with any temperature change!

I'm going to leave the front screw holding the laminated for-end loose as a goose for a while
to see if I'm right..



Cheers,
Rich

Believe nothing, no matter where you read it,
or who said it, no matter if I have said it,
unless it agrees with your own reason and
your own common sense. ~Buddha~

.

It is what it is, because it is!!!

Aluminium expands at about 4 times the rate of steel. If you have a steel reciever on your gun and fit an aluminium scope to it, on a hot day the expansion of the scope will cause a slight bend or tension in the reciever. This isn't so bad in a centrefire due to the strength of the components but in a rimfire you can have some real problems. This is why so many of the older benchrest shooters stuck with their steel-tube scopes. The scope and action expanded at the same rate, therefore balancing out the tension in the action / reciever of their rifles.
The same would apply to aluminium and wood. Wood is fairly temprature stable and Aluminium is not. Add to this that wood is moisture sensitive and Aluminium is not and you have a real problem.
Can you free-float the barrel band so it can slide independantly of the wood?
Dennis.

The group opens up too much with a full free float (well, the last time I tried it).

Now, the barrel is pointing just a tad high. So, the barrel band is pulling it down a little bit.

And now that screw 23 is loose (into block 5), the wood should be out of the 'expansion' picture.

Now, I just have to hope the casting itself doesn't bend. It's kinda like a U-channel, so it
should be pretty stable when the sun hits it.

If I'm right, the stock will lengthen in the direction towards the target and the barrel band will
slide along on the outside of the shroud bushing.

Yeah, The way the beats down on a black scope, maybe a little slope in the rear screws would let
it slip-n-slide a bit during expansion.

Haha.. Once when I was tweaking the crosshairs to vertical on the indoor bench (using the hanging fishline)
I forgot to tighten the front screws down firmly. The rifle shot fine. They stayed that way for weeks
and I never noticed.

Cheers,
Rich

Believe nothing, no matter where you read it,
or who said it, no matter if I have said it,
unless it agrees with your own reason and
your own common sense. ~Buddha~

You have rediscovered why most accurate rifles have a single point of attachment for the barrel. The M16/AR-15, for instance, started out with the barrel attached to the breech and the handguard -- very quickly it was found that having the barrel float in the handguard (only be attached at the breech) resulted in a far more accurate gun.

The same principle applies to scopes, I have found. At the 2007 Nationals, there were huge temperature changes from morning to mid-afternoon. Many people were finding that their zeros had changed fairly dramatically each morning. I remembered a suggestion by LD to loosen one of the scope mounts (at the gun) a bit to let it slide and my temperature problems went away.

Best,

Joe McDaniel
DIFTA Match Director
AAFTA Webmaster

.

It is what it is, because it is!!!

Rear mount is tight, front is just tight enough so there is no sideways slop but can slide on the tracks. I use BKL mounts and the rear one is a double-strap. This was more by happen-stance than design but seems like a logical choice.

Joe

I assume this isn't a free slide but a resistive sliding fit? I have BKL doubles from and rear. Also I assume POI may shift quite a bit one you untighten the from straps , since ur allowing the scope some movement from where it may have been positioned. Good or Bad assumption?

Thanks again forthe reply.. Great Info!!!

It is what it is, because it is!!!

Yes, there may be some initial shift depending on the temperatures when it was locked. After that, I have not noticed any major POI changes other than a few clicks after a flight, etc. Of course, you can still have temperature shifts relating to pellet velocity -- I check my Steyr with a portable chronograph or use the "poor man's chrony" (zero at 30 then adjust power at 55 until POI and POA match).

Best,

Joe McDaniel
DIFTA Match Director
AAFTA Webmaster

.

It is what it is, because it is!!!

.

It is what it is, because it is!!!

Then tighten evenly til scope JUst slides then tighten rears up firm. O'rings keep proper tension on front cap so it doesnt loosen.

"Just Slides" is a snug sliding fit.

Thanks again!

It is what it is, because it is!!!

Float tubes were added to AR15's becasue sling tension, while in the prone position, would bend the barrel. Not because it made the rifle any more or less accurate.

All center fire guns have a singe point of attachment for the barrel. Does that make them all "accurate?" No it doesn't. Some shoot better with forearm pressure, some don't...

Brian S. Johnson

Doesn't bending the barrel, and thus changing it's poi, affect the rifle's accuracy?

No, It effects where the barrel is pointing.

Brian S. Johnson

Good point. There is definitely a difference between inherent accuracy and repeatable accuracy. Thanks for the new point of view! =]

Why not manufacture a small insert with elongated holes that you could inlet into the laminated wood forend. The insert could be made of some type of nylon material (Delrin?) that is slippery enough that the the aluminum could expand and contract without binding. The screw tension would be critical but once adjusted, you would probably never have to touch them again.
John

And I designed my rifle the way it is to account for that as well as a raft of other issues I had seen on popular FT guns back in the late nineties.

One loosened screw and rifle is just about dead-on.
I was just outside cutting some clover leafs (in leafs) at 42 yards.

Yesterday, I got a false start, because I had not re-zeroed AFTER loosening
the fore-end screw (and the forward scope strap screws).

So, I tweaked her back in and shot some. Came back 6 hours later
and it was still zeroed. This afternoon too! (very low wind today).


So, I'm going to see if I can dig up some gas money and head for Westfield this Sunday!




Cheers,
Rich

Believe nothing, no matter where you read it,
or who said it, no matter if I have said it,
unless it agrees with your own reason and
your own common sense. ~Buddha~

Hats off to you, and Rich kick sum booty!!

It is what it is, because it is!!!

There has not been a big variation in temperatures today, so I'm not too surprised it was still zeroed ok this morning at the Westfield match.
Took the gun out of the car and the 1st shot was very close to POA. It was off a few clicks windage
wise, but the elevation was still locked it nicely.

Before I made the lose screw mod, the elevation knob would be off a full 1/4 to 1/3 a turn..


One new thing I learned today about counting your shots, even if the rifle shoots 50 shots
on a 22 yard target, that doesn't mean it can shoot the last few shots of a 50 shot match
at 55 yards!
The longest range target in the whole course was on the last lane! I filled it to almost 300 bar
before hand.. I found out the velocity starts dropping around the 150 bar mark.. 6 Oclock misses.
From now on, I'll swap cylinders when I hit the mid-match mark..

After all this time, this is the first time I could sit down and shoot this Walther
and be somewhat confident the POA=POI..

Cheers,
Rich

Believe nothing, no matter where you read it,
or who said it, no matter if I have said it,
unless it agrees with your own reason and
your own common sense. ~Buddha~

Well good deal, but You should have searched past posts(yellow forum) concerning Barrel support: Re: **Dominator FPE increase November 27 2007, 10:06 AM


If you like, you can "leave the barrel support in place and ream it out for clearance around barrel". I would try to adjust the reg pressure without a guage. Unless your free time is limitless. It's louder cause of some wasted air. You get a lot of it with the shorter barrel (wasted air). No I have never been deep into the trigger assy. I wouldn't do that either. You should be able to get the 3 oz by following the trigger adjustment instructions that came with the gun.

Not trying to take anything away from your discovery but How come you didn't try the above first? Just curious...

>If you like, you can "leave the barrel support in place and ream it out for clearance around barrel".

Since the barrel will point more 'up' if the barrel band is not holding down a little,
I fail to see what I would gain by reaming the band ID larger..
(And if the POI moves too high, I'm going to need different a scope mount).


>I would try to adjust the reg pressure without a guage.

Since my FPE is fine where it is, why would I need to adjust the reg? (without a gauge)..


>Unless your free time is limitless.

Since I'm retired and like to tinker with my airguns (it's a hobby) my time is limitless.


>It's louder cause of some wasted air. You get a lot of it with the shorter barrel (wasted air).

True, I learned that years ago when I got my Talon SS. Short barrels stink.


>No I have never been deep into the trigger assy. I wouldn't do that either. You should be able to get the 3 oz by following the trigger adjustment instructions that came with the gun.

I adjusted the trigger to my liking on the day I got the rifle a couple of years ago.
It's one of the best triggers that I've ever used. Only my 40X match .22 rimfire has better.
(Of course that trigger cost more than the rifle).

>Not trying to take anything away from your discovery but How come you didn't try the above first? Just curious...

Which "above" do you mean?

FYI, I installed a DIY Carbon fiber shroud this past winter and just got it tweaked in this spring.
(My indoor range is only 10 yards. I need at least 40 yards(and no snow) to check for accurateness).

The temperature expansion problem with the stock was 'new' to me, but it was just one problem
in many that I have had with this rifle.

Cheers,
Rich

I meant reaming the barrel support band. That's what I meant by "the above". I Didn't know you had a gun with big problems though. I didn't have any extraordinary problems on the 4 Dominators I have come across. Other than stuck Regs from sitting too long.I have come across many airguns and ALL needed a little something or another.I hate to see Dommies put down. They are a LOT better as a whole out of the box than a lot of other airguns.......I free floated all the Dommie barrels and played with reg pressures till satisfied with velocity,SD,ES,and shot spread. I use the lightweight "junior" cylinder and want more shots from it. That's why I fooled with the regs. I use adjustable mounts for all my rifles so a "slightly pointing up" barrel is workable for me. I am typing this on me cell phone so sorry if it seems choppy . I also built a reg pressure guage to help with adjusting them. Sounds like You've got yours figured out. What's your shot count and velocity? Do you machine parts?

I've considered reaming the barrel band. But when I test fired it without the barrel band
(free floating) (and also with just the bare barrel), in both cases, it failed to group as tight as I wanted.

But, I have yet to check grouping with the new CF setup free floating. I'll have to try that soon.
It will have to do very well to beat it's current accurateness.

Mine came 'new' with a old reg (non-adjustable) that worked very poorly. It also had a poorly
designed exhaust(firing) valve and seal. Once those items were replaced, (with newer technology)
I had to deal
with the off-set of the air cylinder (use shims under the reg mounting face),
a barrel that wasn't pointing the same direction as the rifle. (I had to bend to the right a bit).

and it was also an extra loud barrel.

Note:
I actually didn't have to deal with any problems, since the dealer wanted me to return the rifle
for a full refund. But, I wasn't real busy and I like to tinker..

One of the many problems I had at the 2006 NATs (came in 50th) was my fancy adjustable mounts.
It was at that match, that I was advised by some old time FT shooters to dump those mounts.
I checked them over a while and decided they would be just fine for my vermin rifle..

I have a little shop, and I was thinking about building a test rig and even got the oil filled 2000 psi gauge,
but since I've been shooting the new reg (and have not made a wrench for it yet),
I've put the gauge project on hold..

Here is my idea for the test rig..


What do you think?

I think this is a fantastic mod, but I'm too chicken to do it..

I have not even looked inside my new reg to see if there is an adjuster screw!
(If it works, don't break it)..

Thanks,
Rich

Do you still have the old reg and valve? I happen to like them and would like a spare setup.It sounds like you had a labor of love to get your gun where it is now. I know you had a good time with the process I have not had a problem with adjustable mounts yet ( Tasco on just about everything I shoot and one BLK? mount). But I have not dropped or bumped my rifles yet during competition. I kinda like using stuff that is not supposed to work .... It makes things more interesting at times...I think that test rig will work for fine tuning a newer reg. You wont be able to get a large range of adjustment with the screw on the back of the reg though. It sounds like you dont need it anyway . Besides,the newer regs tend to leak sometimes if taken apart so I'd leave it alone if it stays like it is now. Good shooting at your match the other day by the way. I thought about that "reg pressure guage on the side Mod" too. Might do it next, Just for kicks. Will you shoot at the NATs this year? If so I would love to shoot your Dommie a bit....

That reg was mailed back to the dealer before they would send the new one..
(Don't know why they wanted it).

The valve pin, I saved. It has a convex face, instead of flat, like the new style.
With it's soft 'green' concave German seal, it's one temperature sensitive combo.
I found out about that one on UK sites. That's the first thing the airsmith replaces over there.

I saved it for my buddy who still has the old style reg (and valve I think).
In case he wanted one to turn down for a flat seal surface.


"You wont be able to get a large range of adjustment with the screw on the back of the reg though."

Is the screw under the cylinder attachment nipple?



The only one I've ever seen adjusted was made in the USA..

Nope, no more NATs for me. Unless they have another one around here.

Bedtime for me..
Cheers,
Rich

Well kinda but it is inside the reg housing. But I wouldn't mess with it at this point. You can only get small adjustments out of it and it might leak on you. If you have to go into it later We can talk on the phone about it.

.

It is what it is, because it is!!!