A new rifle's poor accuracy can be caused by several things. I'm gonna get a little bold now.....
Have you ever shot a rifle with a .300 Win. Mag.'s recoil into sub MOA groups at 100 yards before? If so, then your rifle or ammo probably has a problem. If not, try shooting slung up in prone with a bag under the stock toe and fore end. Most folks shoot rifles designed for firing off ones shoulder more accurate from a good prone position than holding it against their shoulder as it rests on somthing atop a bench. Or get brave like I did years ago when I couldn't shoot worth a darn from a bench and asked a friend to shoot my rifle and he did; very accurate and impressive proving to me I had a lot to learn.
Was that Hart button-rifled barrel fluted before it was gundrilled, reamed, rifled and lapped? If not, then the fluting probably changed the bore and groove diameters enough at the flute ends to cause accuracy problem. Seen that happen with several button rifle barrels.
I went with the 1-10 twist on the recommendation of Hart, as they said it would be the best all-around twist for my style of hunting. Is my twist to fast for lighter bullets????.
After seeing a lot of folks shooting .300 Win Mag's in 1:12 twist barrels for 190's and 200's in long range matches and doing very well accuracy wise, a 1:10 may well be too fast. Note that virtually all modern 30 caliber magnums are steroided children of the .30-06 wildcats and original magnums based on 1:10 twists and most folks think that's what they need. The first 30 caliber magnums shot a given bullet weight out only about 100 fps faster than a .30-06. And they were mostly used with heavier bullets so their 1:10 twist worked well. I had a .300 Win. Mag. made from a shot out 30 inch .308 Win. barrel with a 1:13 inch twist that shot Sierra 180 HPMK's and SBT's well inside 2 inches at 300 yards.
If the bullet weights you listed are what you asked Hart about, I'm surprised they suggested a 1:10 twist When it's well known among other high-end barrel makers that 200's do great from a 1:12 twist in 30 caliber magnums and those around 170 grains you listed would be much better off with a 1:12. Spinning bullets too fast tends to make the more unbalanced ones jump too far off the muzzle axis when they leave. And the centrifugal force on their jackets sometimes causes those with very deep rifling grooves to break apart.[/QUOTE]
I've never seen nor had an issue with a 10" twist 30 cal., mag's or otherwise. I've owned three 300 win's, a 30 ultra, and four '06 rifles and all were 10" twists that shot any bullet they liked at all into 1/2 moa. My brother and father own or have owned more than me and the only real problem child is a hawkeye 300 win that only likes hornady 150 grain fb bullets and won't shoot heavier bullets worth a darn. Not to say they aren't cranked up just a bit from minimum rpm. The worst trouble is always spinning to slow so I'll take a bit fast any day.
I agree that if all you were ever shooting was 150-180 grain, a 12 twist is better but as others have stated, I have had several 10 twist 30's that would shoot well under an inch consistently even down to 130 grain so I don't think twist is your problem.....Rich