Re: Kimber 84M accuracy problems
Rick,
I spent the better part 10 years trying to get a 1" 5 shot group at 100 yards that I read about on the internet.
I do not have all the answers, but I know allot of the common folklore on accuracy and have opinions.
The best financial thing for YOU to do is sell your rifle on the internet to someone who does not know of the problems. Gunsamerica, gunbroker, or auctionarms.
Not as good for you financially, is selling to the small number of readers on this thread who know there is a problem and pay only a discount.
But you could keep trying to get accuracy.
1) The first thing I would do is check the scope mount to receiver screws for tightness. If the mount and rings are separate, the rings must come off the mount. Guys are reluctant to loose their zero, but have the bad groups I have ever seen are caused by this.
2) Go to the range when there is no wind. Those surveyor tape ribbons should be hanging straight down. If there is a 3 mph wind, shoot the 50 yard target, not the 100 yard target. Look on accuweather.com for wind predictions for the range area.
3) The receiver to stock fit should be checked. The mating surfaces can be cleaned with alcohol and a Q tip. Inspect the glass bedding for solidly supporting the action in front of and behind the front action screw. The action screws should be very tight, with the forward screw getting more toque first.
4) The bullet should be entering the bore concentrically, or the spinning from rifling will fling the bullet in random directions at the muzzle.
a) The inside of the neck of the case should be straight and concentric with respect to the taper of the shoulder. This usually means throwing away the expander ball, using Lee collet neck dies, partial neck turning and other reloading techniques.
b) The bullet should touch the lands. This can cause problems: 1) an increase in pressure, so lower the charge a grain. 2) The bullet can get stuck in the lands when an unfired round is extracted, so don't jam it so far into the lands.
c) The bolt face should be square with the bore, so when the case is pushed back, it does not push the rear of the bullet sideways. Sometimes the base of the brass is trued with a Wilson trimmer used with the case holder backwards.
d) The chamber must be cut concentric with the bore. The bore may not be straight. The dialing in of bore in the chambering lathe should use a spud and be test indicated over length and made concentric with the lathe over length with a gimbal and spider.
e) Reducing the twist rate reduces the effect of bullets entering the bore non concentrically. Reduce the choice of barrel twist to the slowest rate twist practical for the bullets to be used.
5) The bore should be free of excessive Copper fouling that could damage the bullet. I use a 20" Nite-Ize fiber optic flashlight accessory and a magnifying glass to get a bore scope view of the 1" of bore at the muzzle. Getting the Copper out may take days of effort, and if the bore fills with Copper again in two groups, get a new barrel.
6) The throat of the barrel should not be shot out. Used 22-250 rifles used on rodents will be good examples of this. There may be a 7mm-08 out there with a shot out throat, but it would be hard to find. It would take many thousands of rounds, which will not happen with a light 7mm-08.
7) The barrel should not warp from heat. Barrels can be stress relieved, and Douglas, Hart, Shilen.... do this. Cheap unrelieved barrels must be shot with waiting for cooling to get good groups. the thinner and longer the barrel the bigger the problem with warping.
8) The barrel can whip around when firing. The accuracy can be improved by: a) shortening the barrel b) getting a fatter barrel c) tuning the handload to the barrel d) turning the barrel to the factory load with a Boss system.
9) The barrel can be bent by the stock from warped stock or expanding hot barrel. This can be fixed by floating the barrel, which is having clearance between the barrel and the stock at all but the 2" near the receiver.
10) The recoil of the rifle moves the rifle before the bullet exits. If the reaction to recoil changes shot to shot, then the point of impact changes shot to shot. The placement in the bags or benchrest must be consistent. The pressure on the shoulder, hands, and bags, must be consistent. Heavy rifles and light bullets make less movement.
11) The trigger pull can move the rifle. This can be improved by; a) practicing dry firing at the range until the cross hairs stay on the bullseye b) install a hair trigger.
12) The scope power for groups on a target must be high enough. My human eye can resolve about 1 moa. To get a .1 moa group a 10X scope would be needed if everything else were perfect.
13) The range bench can be shaky. This shakiness may not be visible with low power scopes. Concrete is better than wood connected in parallelogram.
14) The case neck should not scratch the bullet. Pull bullets to check. This can be fixes with inside neck chamfering.
15) Bullet length can affect accuracy.
a) Short bullets are lighter bullets with less recoil and can improve accuracy.
b) Long bullets are more likely to get concentric despite other problems and can improve accuracy.
d) Bullets too long for twist rate may go unstable and key hole at long range or low velocity loads or all the time.
16) Watch out for benchrest techniques that will waste time and money, yet give no measurable improvement at the hunting level; weighing charges, weighing brass, neck turning, tight neck chambers, 6mmPPC, chasing action threads, etc.
