No it wasn't wind, dead calm day and my other guns were producing nice round groups.

I was shooting a new to me 6.5 x55 and it was consistently shooting groups 2 inches wide and about half an inch high, using very widely varied loads that have proved very good in my old swede

Same issue 2 weeks ago!!! every rifle I shot was great except my new one and it was 3-4" groups, horizontal, ended up being the scope. BUT, A trigger job tightened it up some and new mounts/bases, finally swapped scopes and she's shooting fine.

No real one issue with horizontal, could be alot of things or a combinations of little bits here and there.

CastleRock,

Does the horizontal string walk from right to left or left to right?

Or,

Is the pattern random?

If the pattern is random I'd guess scope

If the pattern walks the stalk may be pressing against the side of the barrel.

Shod

I agree could be few different things.
If it were me I'd check scope mounts and action screws first.
Since rifle "new to you" you don't know if it always shot like that or something changed.
Next I would try free floating barrel, I have seen where barrel touching stock causes horizontal stringing. When ya fire a shot barrel may move slightly in its channel and "rest" in a different spot on each shot.
Eliminating barrel contact with the stock has made some of my hunting rifles real tack drivers.
Ive also had some with "featherweight" barrels that would not group well over three shots due to barrel heating up. They will put 2-3 almost touching then forth shot on up group widens.
Just my thoughts...Great cartridge..Good luck to you

Thanks for the ideas
Barrel is well floated, heaps of clearance , and it is about a Remington sporter or No 3 I guess
The only trend is probably one shot seems a bit further to the left as well as stringing
Edited to add thanks Nick scope mounts I am pretty confident about Leupold PRW

Shodd's comment is the probably the most pertinent.

Does the stringing occur randomly or does the generally rise with each succeeding shot?

I had an un bedded scope base that caused horizontal stringing. Bedded the base and all was good.

Every time I have experienced horizontal string with a rifle, the culprit has almost always been the scope mounts/rings. Make sure everything is seated and all screws tightened to spec. Good luck.

Maybe the barrel is shot out or just no good. Does that cause stringing?

Even a new barrel can strong horizontally if it wasn't correctly stress-relieved. It can also be caused by loose threads in the barrel/action connection.

Or as some people have pointed out, by the mounts. I've also seen it a number of scopes, especially new scopes or those that have been shot a lot. Once ended up several days into an African safari with the .375 H&H that had shot sub-inch groups when checking the zero at the beginning of the safari shooting horizontal groups several inches wide and less than an inch high.

It is random, my gut feeling is the scope is to blame, but I would have thought if it failed grouping would be even worse

I will change it and try again

I am happy with the scope mounting and the barrel is a fairly new Shilen

Scopes fail in many different ways,I once had a scope that would walk the bullets right about 1/4 moa every shot.Not saying it's the scope but don't discount it.
I find scopes fail quite frequently if you shoot enough.

Lock the rifle in a vice, use a culminator to ensure there is no side to side movement as you apply pressure to the scope. If the bases do not fit the receiver perfectly the culminator will show the movement. Even with tight screws movement can occur if a less than optimal fit is achieved with the scope bases.

I am guessing automatic spell-check is the reason for "culminator" and "culmination."

Yes, you are correct, I should have proof read. I changed one and missed the second.

How's it bedded??

sometimes this occurs with a heavy trigger and the direction is determined if you shoot Left Hand or Right Hand

You didn't mention the model of the rifle you were shooting. The only time I ever experienced horizontal stringing
was when I was sighting in a new to me hunting rifle. Each sucessive shot walked horizontally away from the previous impact.

I later found out from the previous owner of the rifle, that this particular model was designed to be lightweight for field carry
and the lighter barrel was extremely heat sensitive - thus causing the string pattern.

Once I sighted it in, barrel cold, I never had a problem using the rifle as intended - at least for the 1st shot.

Had a 300wsm that had a "crooked" crown (cut more to one side of the bore than the other) that would string horizontally in a very random order, no pattern to it. Even shot some nice sub-MOA groups, immediately followed by a 2-3MOA group with very little vertical spread.

cas..,

Happened to me twice. Both times it was a cracked stock. One split behind the recoil lug the other cracked behind the tang.

O

Pulled a few to the left have you....

Looks to me like you missed them both.

I think collimator is the word you're looking for.