In my .308 I normally load 168gr bullets and that is what my rifle is sighted for at 100 yards. When I shoot some cheap 150gr or 180gr bullets just to foul the barrel, my shots aren't even on the target backer. I would say I'm off by about 12-18" high and 12-18" left. Is this just because of the different bullet profile and weight or is there something else going on? I would normally see an impact shift in my .223 or 7mm rem mag, but only maybe 2-3" at 100 yards. This is very frustrating when I am testing several loads with different bullets.
Huge POI shift with bullet grain change
- Thread starter jojo4711
- Start date
That sort of change is definitely due to more than just the difference in bullet weights.
since the rounds didn't even hit the target backer, did you see them impact somewhere off the target to be sure that they even reached the target?
What rifle are you shooting?
What powder is loaded with the "normal" 168 gr bullets?
What brand and configuration are the "normal" 168 gr bullets?
Same questions again for the 150 gr and 180 gr bullets?
Were the 150 gr and 180 gr loads based on 1) Starting Loads per the manual, 2) loads you previously developed or 3) did you just pick a powder and charge from somewhere in the manual?
You also cannot ignore psychological factors that may have impacted your aim.
Just a guess, but if your barrel bedding is less than good you may have found the only "sweet spot" in your barrel harmonics with the load that shoots well. If barrel is free floated, can you slide a dollar bill all the way back to the recoil lug? If not free floated, stick a piece of index card of something similar into the barrel channel on your stock (to change pressure points) and see if your fouling groups improve.
Again, just a guess, but barrel harmonics play a large part in accuracy.
Again, just a guess, but barrel harmonics play a large part in accuracy.
The rifle is a mossberg patriot action with a Boyd's pro varmint stock and a 12x42 SWFA scope.
The 150gr and 180gr are just Winchester or Remington factory loads. The 168gr amax or sierra matchking, with AR-Comp powder for this particular test.
I do see them hit the dirt to the left and high of the backer. Basically I need to just do a cipro fouling shots with the billet and powder I am using and use them to make sure I'm zeroed at the same time.
The 150gr and 180gr are just Winchester or Remington factory loads. The 168gr amax or sierra matchking, with AR-Comp powder for this particular test.
I do see them hit the dirt to the left and high of the backer. Basically I need to just do a cipro fouling shots with the billet and powder I am using and use them to make sure I'm zeroed at the same time.
"...some cheap 150gr or 180gr bullets just to foul the barrel..." Those loaded as meticulously as the 168's? You shooting 'em the same way as the 168's? Makes difference.
"...Dollar bill goes all the way up to the lug..." Shouldn't go to the lug. It should only go to the chamber area of the barrel. Glass bedding should be out to the end of the chamber area. Free floating doesn't guarantee anything anyway. And a Mossberg Patriot is still a hunting rifle.
Paper matchsticks can make a C1A1 rock and roll. Works better with wooden matches though. snicker.
"...Dollar bill goes all the way up to the lug..." Shouldn't go to the lug. It should only go to the chamber area of the barrel. Glass bedding should be out to the end of the chamber area. Free floating doesn't guarantee anything anyway. And a Mossberg Patriot is still a hunting rifle.
Paper matchsticks can make a C1A1 rock and roll. Works better with wooden matches though. snicker.
That is correct, no part of the barrel should be in contact with the stock.
I've never seen one this far off with different bullet weights. There is nothing about the ammo alone that would cause this. You have a rifle, or stock problem.
Does it do this with the factory stock?
Sometimes you can buy an aftermarket stock and they bolt right to the gun with no issues, other times they need to be fitted by someone who knows what they are doing. This often costs more than the stock.
Jojo,
I have seen this reloading my wife's 243. It is a Rossi single shot with 23" heavy barrel.
My reloads shoot a 10tgr. Hornady Match about 12" higher and 3" right of and factory 100 gr loading.
Velocity and barrel harmonics.
Your factory loads are probably leaving the barrel on a down swing while reloads are leaving on an up swing. Or vice versa.
I have seen this reloading my wife's 243. It is a Rossi single shot with 23" heavy barrel.
My reloads shoot a 10tgr. Hornady Match about 12" higher and 3" right of and factory 100 gr loading.
Velocity and barrel harmonics.
Your factory loads are probably leaving the barrel on a down swing while reloads are leaving on an up swing. Or vice versa.
JJ,
What is your barrel length and profile? Hollow cylinder transverse stiffness increases as the difference in the square of the square (4th power) of the inner and outer diameters. So adding very little thickness stiffens it a lot. A thin, ultralight barrel or one that is very long (moment of inertia increases as the square of length at a given diameter) or some combination of the two can whip substantially under recoil near the muzzle. If the bullets are exiting the muzzle at substantially different positions in its whipping action, as induced by recoil and pressure deformation of the barrel, then there can be sometimes surprisingly different points of impact. This can be addressed by changing barrel time by changing the load level. If changing load level makes a big change in POI, the bullet is exiting during a high rate of change in the swing. If it doesn't, the barrel should be near one end or the other of the swing. The latter loads are usually most accurate.
For tuning methods, take a look at Dan Newberry's OCW method. However, with a big vertical POI, you may find the older Audette ladder works best.
What is your barrel length and profile? Hollow cylinder transverse stiffness increases as the difference in the square of the square (4th power) of the inner and outer diameters. So adding very little thickness stiffens it a lot. A thin, ultralight barrel or one that is very long (moment of inertia increases as the square of length at a given diameter) or some combination of the two can whip substantially under recoil near the muzzle. If the bullets are exiting the muzzle at substantially different positions in its whipping action, as induced by recoil and pressure deformation of the barrel, then there can be sometimes surprisingly different points of impact. This can be addressed by changing barrel time by changing the load level. If changing load level makes a big change in POI, the bullet is exiting during a high rate of change in the swing. If it doesn't, the barrel should be near one end or the other of the swing. The latter loads are usually most accurate.
For tuning methods, take a look at Dan Newberry's OCW method. However, with a big vertical POI, you may find the older Audette ladder works best.