Browning A-Bolt .300 Ultra Mag - Can't get good groups?

[browning ultra mag]

I am working up a load for a Browning A-bolt II .300 Ultra Mag & can't get good groups at 300 yards...

Barrel Twist: 1:10
Barrel Length: 26"
Bullets: Nosler AccuBond 210 grain
Powder: Hodgdon H1000
Brass: Shoulders .001" back
Brass is full-sized - not pushing shoulder back
Neck trimmed
Bolt shuts good
Distance to lands: .153" (can't go less in order to use clip & eject shells out of gun)
Bullet run-out: .001"

Shot gun at 100 yards to find best load...
88 grain powder- shot dime sized groups
90 grain powder- slightly larger group than 88 grains

Shot both loads at 300 yards. Got 3" groups with both & then got stray shots up to 6" above or below groups.

Shot factory Remington Scirocco 180 grain bullets at 300 yards and groups were even larger groups. (About 10" group scattered)

Any suggestions as to why this is occurring?? Any help would be appreciated!

 

[Ledd Slinger]

Bad rest, improper shooting form, flinching from recoil, optical parallax not properly adjusted, extreme speed spread between rounds...etc. Who knows?

Try some different powders and primers too. Ive had tight groups at 100 go to hell at long range, then ive had so-so groups at 100 hold tight at long range. You can have tight groups at 100 yards and all the rounds are 50-100+ fps apart. That dont work out so well when shooting long range.

 

[Mastershot]

I have the same basic rifle.

Adjust the boss system and allow the barrel to cool down between shots.
If your doing this on a hot barrel it will change the harmonics.

 

[gunnermhr]

My Tomahawk (300 RUM Improved) didn't like h1000 at all. I use a 240 smk with military surplus WC 872 and it works awesome. It hammers at long range and wc872 is or used to be super cheap.

 

[fourpower]

i have 2 ultra mag they both shoot retumbo very well. i tryed the noslers when they first came out and both guns would not shoot them. i couldnt get rid of them fast enought. i have never been able to get accubonds to shoot! i shoot only bergers and if gun wont shoot them then down the road. both guns shoot 210 berger -90grs retumbo great. built one of them for trip to africa and ph couldnt get over how gun shot and how great the bergers did on game. 6 big animals with 6 shots all went right down with no bullet leaving body! as for any a-bolt i have seen you must let gun cool down after 3 shots or they start shooting all over the place. factory says sightin 3 shots let rifle cool!

 

[XTR]

What does the gun weigh? If this has a light contour hunting barrel it will walk all over the target after a couple of shots. The barrel doesn't have enough mass to be a heat sink for strings of fire.

Assuming it is a hunting contour, those types of rifles are designed to shoot a group of one(1) on target, not strings of fire.

I've seen a factory 243 trying to shoot in a 600 yard F class match start to melt the factory stock after 30+ shots trying (and was still trying to get it to get into the 10 ring)