Question to the colleagues:
With two of my rifles (a CZ in 30-06 and a Voere in 7x64) I consistently shoot good groups at 100 m (15-20 mm) and rather large groups at 200 m.

The loads are:
30-06: Sierra HPBT 165 grs with 57 grs of Norma 204, CCI LR Bench Rest Primer, MV 782 m/s= 2566 fps (low speed due to short 20” barrel)
7x64: Hornady SST 139 grs with 55 grs of Norma 204 , CCI LR Bench Rest Primer, MV 875 m/s = 2871 fps

I exclude that the large groups are due to my shooting skill, as with my third rifle (Weatherby Vanguard Sub MOA) in 7mm R.M.) I consistently shoot groups of 20-30 mm at 200 meters.
IMHO the causes could be:
-parallax; while the Weatherby is fitted with a Zeiss with parallax adjustment the other rifle are equipped with Zeiss (the CZ) or Swarowski (the Voere) without adjustment
-bullet precession, due to improper stabilization

Any suggestion?

There are a lot of issues that affect groups.
Audette developed this method and a lot of shooters use it to develop loads. For long range I use the method it may work for you at 200 but I like 300 to 600 yrds.
http://home.mindspring.com/~45.../the_ladder_test.htm

It's the loads + your rifle(s) more than it's you IMO. That's why we actually shoot to see what our individual results are, rather than relying on ballistic calculators and so on. That you have different results with a third rifle tells me it ain't you...

Adjust your load(s) to group at 200 (or better yet 300).
It's not uncommon for a rifle to group well at close range, and fall apart a little further out, or to even group well enough at a distance but poorly at close range (the second is often the result of a bullet being spun to fast).

Ok, you are shooting 20mm groups at 100; that is sub moa. How large are your groups at 200?

I would have thought the most obvious reasons for 200-yard groups to be more than twice the size of those at 100 would be wind and loss of bullet stabilisation. I notice the bullet weights you mention are not over-heavy in those calibres.

A 30-06 that won't shoot a good 165gr bullet has some serious issues. What is your twist in the '06? Have you tried some other bullets.
and, as posted, what is your group size at 200?

Doesn't seem like you are pushing the 30/06 very hard, if it were me I would load that a little hotter and see what happens.
By the way I've had several 30/06's that didn't like 150's and 165's so I try 180's in them and they are tack drivers.

try heavier bullets in the 7x64 and zero at 200 --

139 is REALLY light for the '64

Many thanks for thesuggestions, my friends.
The groups at 200 m are to be measured in inches. They are so wide I didn't care tp measure them.
The '06 has twist of 1 in 10 (standard, as far as I know).
I cannot increase the load: the case is full and I don't want a compressed charge.
One suspectarallax, which cannot be adjusted on neither scope.

IIRC it's not uncommon for non-adjustable scopes to be set parallax free out to 200 yds/mt. Parallax may not be the culprit.
I don't know if this helps but I had a very accurate Sako .270 Win that started to lose longer range accuracy. At that point it still shot good groups at 100 yds but further than that shots were all over the target board. A gunsmith looked it over and told me the barrel was shot out ! I had put barely more than 1,000 rounds through that barrel. There was also some metal decay of the bore so maybe I should have looked after it better. Anyway I had it rebarreled and it now shoots very nicely again.

In order to shoot good groups at 100 meters the shooter must maintain a constant position on the stock and a consistent sight picture with the cross-hairs centered and on target. If the same is done at 200 meters then groups will remain tight. Parallax may shift the point of impact from the sighting line but the shift will be consistent and will not effect grouping if the shooter uses a consistent hold and a consistent sight picture. Even if you don't shoot with everything centered up the margin of error from parallax out to 500 yards is less than 1 MOA with a standard sporting scope.

So, unless you are using a 25 meter air rifle or 50 meter rimfire scope, parallax is not your problem.

Pick this up at the 5:00 minute mark:

Check the focus on the 2 scopes that are giving you problems. Also what magnification are they? You can't hit what you can't see.

Then I have no clue. Usually, with a good bullet, giving good 100 yard groups, you will at least get a useable group at 200 and sometimes a fantastic one, as bullets sometimes take that far to fully stabilize. In your case, the only other variable, is the shooter and you aren't the issue.

Seems to me it has to be something to do with stabilization, an one inch rifle at 100 should be a 2" rifle at 200 and a 3' at 300, give or take an inch or so..

Sometimes we shoot better than other times, and that just happens to everyone..A change of any kind in reloading can have detrimental effect.

A hundred or more reasons for your problem, can only be fixed by investigation and a process of illumination..Unfortunately that may take a wealth of hard earned knowledge.