Bullet strings moving horizontal??

[dshowan]

I'm relatively new to reloading so I'd like to get some opinions on something I've noticed on occasion. During my load developments for my 300 win mag (Browning A-Bolt with Boss) some of the 3 shot groups have a odd pattern to them. Most groups (9 out of 10) have random spacing. But occasionally, I notice a string of three that is almost perfectly aligned horizontally but each shot moves a little more to the right (about 1/2" on each shot). When it does this it's always more to the right for each successive shot for 3 shots. I've been changing something after each 3 shot group, usually power charge amount or type or Boss setting or bullet etc. I know I need to load up more rounds than 3 of the same thing and leave the Boss alone so when I run into this I can see what the 4th, 5th ect. shot would do.
I should add that these horizontal strings happen on the first shots with a clean barrel. But other first shot groups with a clean barrel do not do this. Barrel temp does not seem to affect it. Some of the best groups of 3 I've obtained @ 100yds. are under 1/4" with a clean barrel and those groups do not string to the right nor do 90% of the other groups. I'm just about satisfied with what I've worked up so far and now have accurate low and high power 165 gr. loads and good high power 180 gr. ones.
I guess it's the clean barrel that does it, but it's strange that it only happens to specific loads.
Any ideas????

 

[DWARREN123]

Usually but not always this is caused by the shooter. Either breathing or slightly different sighting pictures.
Just my .02.

 

[U.S.SFC_RET]

Breathing causes you to move the bullet either up or down from the center of POI. If you are right handed and pulling the trigger your hits will travel to the right. As the barrel warms up will cause the group to widen if the barrel is not free floated or bedded.

 

[GardDog223]

Are you shooting from a bipod, rest, etc?

I noticed the same problem during my last range endeavor, shooting from a bipod & monopod. I noticed that my breathing was causing right to left movement of the crosshairs, so I had to focus hard on my trigger pull at the same rhythm in my breathing.

GD

 

[Art Eatman]

Realizing there's no such thing as "always", but, generally, vertical strings result from forearm bedding problems. Horizontal strings generally result from a slight canting of the rifle from the vertical.

If you sight in without canting, you get a known point of impact. If you shoot a group with uniform canting, you'll get a tight group but off to the side.

Using targets with one-inch grid lines allows alignment of the scope's crosshairs uniformly for all shots.

FWIW, Art

 

[dshowan]

I use a lead sled and a steady bench. I don't think it's a trigger pull problem as I'm left handed, and kick is almost nill with the lead sled. My targets have 1" grids and I do line up with them. Thanks for the responses so far..

edit: Maybe I need to be more careful in leveling out the targets when I put them out?? Canting - that does make some sense.

 

[U.S.SFC_RET]

dshowan: I need to ask you a question. when those shots are breaking to the right are they somewhat quick successive shots or slow in between? If the barrel is heating up it might cause them to group in that direction, just maybe.

 

[dshowan]

The shots that have done this have all been with a almost completely cold barrel (I've been going for max accuracy). I also use a parallax adj. Nikon Monarch.

 

[Grump]

Watch the consistency of your cheek pressure on the comb, and whether your right hand/elbow contact is just resting on or wedged against the wrist of the stock.