Changing POI through load development

[MSD MIke]

In the intrest of getting fixed sight guns to shoot to the sights I am interested in trying to figure out what changes POI with different loads. It is pretty well documentmented what changes POI up and down but the big mystery to me is what changes POI left and right. I know for a fact some loads will hit further left or right than others in the same gun. There is a reason for it, understanding what that reason is would be very helpeful in load development rather than just chasing it by trying different loads and seeing if you get lucky. I own fixed sight guns that shoot right to the sights. I also have a couple that shoot either a little left or right. I understand that if its way off the gun or my grip needs attention but if its only a little off I would like to try and correct it through load development if possible.
Anyway, any insight would be greatly appreciated,

Thanks
Mike

[Cowboy_Dan]

Just IMHO, but if the problem is caused by the shooter, then I would deal with those issues first.

[MSD MIke]

Thanks Dan, I am aware that technique plays a major role in POI. What I am interested in what are the technical reasons for certain loads hitting further left or right in the same gun. It seems to me if you could get a grasp on what factors play a role then it could be used with a positive effect in the relaoding room.

Thanks,

Mike

[Blackwater]

That's actually a pretty good question, but it's one with one of those answers "It depends." This can come from a number of factors, but all essentially relate primarily to barrel vibrations, I think. Barrels can never be perfect. They're as close as they are likely to get unless and until some supreme technology is invented, and I have my doubts this will happen. For now, even with our high uniformity and high accuracy steel alloys, no process is without hazards, and little bits of errant materials CAN still get into the steel alloys, and those small bits can affect how a barrel vibrates when fired. Deep hole reaming is better than ever, but still not perfect, so having a bore that runs slightly off center makes the barrel thicker on one side and thinner on the other. This can affect barrel vibrations, and throw a bullet high, low, left or right, depending on where the thicker part is located. I once cut a Contender barrel off, and it was visibly off center. In short, stuff happens that makes for slight differences in barrels as they come off a production line, and these slight irregularities make each and every barrel an individual, not quite like all others, though very closely similar. There was once a story about Shilen barrels that their rifling techniques produced some of the most consistent bores, but one of the "worst" rifling rate consistencies in the business. It was said that about 10% or so of their barrels were "hummers," meaning they'd seem to shoot through wind conditions, and grouped particularly well. It was said in that discussion that at least one person thought it was the ones that came out with very precise rifling twist rates. I'm afraid these mysteries produce explanations more on the order of theories than hard knowledge, though. The ones I've outlined here are theories based on what's been observed, and are merely efforts to explain it on some rational basis. All we really know for sure, though, is that most all barrels have to be shot to see what kind of POI they'll give with any given load. One friend's daughter has a .270 that seems to put all bullet weights and differing brands to the same POI at 200 or more yards, and this is a rare trait. She guards that rifle heavily! In actuality, if you find out why this is, let me know, will ya'? It's a haunting question, but barrels now are so good that you can mostly depend that most any quality bolt action will give you MOA or better if you learn how to glass bed it without inducing any stresses in the process, and learn how to get a good trigger pull without inducing safety issues. We'll probably never really know the answers to all these questions, but theories sure do keep the more curious among us busy trying to figure out at least plausible explanations for lots of things. I guess it keeps us out'a trouble, and out'a jail. Mostly. ;^)

[runfiverun]

in a rifle horizontal stringing is generally caused by ignition issues.

I know back in the old day's they would sight a revolver into the right height for the front sight then clock the barrel to adjust the left and right.

[country gent]

Harmonics in the firearm start with the trigger pull, release of the sear hammer/striker fall strke of primer, primer ignition and firing of the cartridge. the barrels vibrations increase at each step of the way in the brief milli seconds it takes. These harmoics cause the barrel firearm to oscilate in a distinct pattern and where the bullet exits in this pattern will show on the target as up down left or right. Change any component in the equation and the bullet can be exiting at a diffrent point in these causing a diffrent point of impact. The Audette ladder method of load development was a means of finding dead modes in the muzzles harmonics. By loading 1 round every 2 tenths of a grain from mininum to maximum and firing with same sight setting you get a string of shots with clusters of 2-4 shots at several points. a 3 shot grouping means your charge can be dead on .2 low or .2 above dead on and group in the same group. This shows how harminic works some what. By finding the load that exits in the oscilations where you want it you can adjust impact. Other wise its finding best load and adjusting sights to it.

[leeggen]

MSD Mike you ask a great question. I know we all deal with the harmonics of barrels, most of us work to adjust a load for up and down. Now you have asked the question above we also have harmonics left and right that we have to work on so we adjust loads in finer grains. Myself I know how to test for up and down, by shooting a ladder with rifles, but I now wonder how do we test for harmonics in pistol barrels. Maybe some of the longtime shooters can help us figure it out.
Thanks,
CD

[JSnover]

Like Blackwater said, it depends. I've had my POI move high and right just by changing lube.

[MSD MIke]

For example, I have a Springfield Mil Spec.If I load a Lee tumble lube 200 grain SWC it shoots centered but slightly low. If I shoot a Lyman 200 grain SWC IT shoots slightly high and to the right. Both with 4.2 grains of Bullseye.

Hmmm
Was it the lube, was it bullet shape, was it the small diff in FPS

[Echo]

Pure rationalizing here - but speaking purely about pistol shooting, grip can make groups wander. If the pistol is locked onto, wrist locked, elbow locked, then the gun is part of a system stretching from shoulder to muzzle. Barrel time then enters into the equation - if the shoulder-muzzle system has more play/resilience in it, then barrel time (fast burning powder w/light boolit vs. heavy boolit and slower powder) can cause a variance in POI, including a variance in l/r impact due to torque. Or so it would seem...

[Blackwater]

Echo, you hit on a fact that I too have observed often. Change grips on your handgun, and POI will very likely change also, and you'll have to adjust your sights to renew POI to the correct point with the exact same load for which it was originally sighted before you changed the grips. This generally applies to revolvers moreso than to autos since the grips of autos don't vary the contours nearly as much as most revolver grips can and usually do.

The size of one's hand also comes into play. One of my favorite shooting buddies has huge mitts for hands. His guns always shoot to the left for me, and mine shoot to the right for him. Not by a lot, but it's consistent, and the amount depends on the recoil level of the load. This is yet another reason one must sight in one's own guns, and not have someone else do it for them. With rifles, there is much less difference, but I have observed differences even with the long guns, too.

With shotguns, shooting turkey shoots at pitched targets will teach you very quickly that different people shoot to differing POI's with them as well. With these, stock dimensions and thickness of the comb sort'a substitute for your rear sight, and determine the position of your head when you mount it.

Ain't it funny how all this works? Sure keeps us humble .... and striving.

[376Steyr]

I say barrel dwell time (related to bullet weight and velocity) and grip are the biggest factors. The human hand is a pretty makeshift way to attach a weapon to a firing platform. I seem to recall a photo of a handgun being fired and a light on the muzzle traced out a figure-8 pattern as recoil interacted with the shooter's hand and arm. Somewhere on that figure-8 (which size and shape will vary with every different load tried) the bullet will leave the muzzle.

[rsrocket1]

I would agree that it is barrel harmonics and changing POI with loads is not a bad thing if you stick with the load, zero your scope on the POI and keep using that load or develop other loads in your arsenal to have the same POI. My .308 788 can get close to MOA accuracy, but when I was experimenting with different loads, the groups tended to remain the same, just the POI was moving. So when I found the most comfortable effective load with my main bullet, I simply made it a point to load to the same POI with other plinker bullets and cast bullets even if the group could have been a little better with a different load which would result in a more shifted POI.