Why does the ZERO on a rifle change so much in a year?

Are you perhaps hitting a different batch of ammo over time - perhaps with different powder or powder charges? If it's all the same cartridges the zero shouldn't drift unless the scope or mounts are bad.

The only rifle I "target shoot" with a lot is a 9mm AR, but for my hunting rifles they all hold perfect zero from year to year.

Under less than static conditions things move. Wood dries, cracks, shrinks, swells. Metal expands, contracts. Plastic ages, warps, dries out. Without a solid foundation for a rifle you’re fighting nearly every part on it from barrel to stock fit to action screw torque.

The first step in mitigation imo is bedding and floating. Some claim their rifle is a tack driver so why mess with perfection and to them I say check back in 5 years. If done correctly the above won’t decrease accuracy. Incidentally it won’t improve it either, it simply removes certain inhibitors. Bad barrels and twisted actions won’t ever make for accurate firearms.

Overcleaning?

I'm guessing that its a bedding problem. Its not to hard to free float a barrel or to bed an action. Its also not terribly expensive to have a Smith do it for you. Check your scope rings and bases for tightness. Its also possible to have a bad scope. Not real common but possible.

Its usually one of two things or both. The first being temperature affecting ammunition performance. Most gun powders are temperature sensitive no matter what the manufacturer claims. It is not uncommon to see 100 FPS loss in performance going from say 70 degrees to 20 degrees. Now in many rifles this will not make a noticeable difference in impact. I was extremely surprised when I chronographed my 25-06 in -20 deg temperatures because it had lost 300 fps, yet the point of impact did not change at all at 200 yards. Other rifles such as a couple that I have ship point of impact dramatically with velocity changes. My 1898 Krag will change point of impact by a full foot (a foot!?!?!) at 100 yards depending on what you put in the tube. If you determine this is the case with your rifle you may need to find a less temperature sensitive powder and do some careful ladder load develpment. Typically you can find a node that shoots to the same POI even with a change in load of a grain or so of powder.

This is a ladder shot with my Krag. 38 grains shoots low, 42 shoots high, 39-41 grains shoots to the same relitive place. I did some more testing after this and settled on 40.5 grains. The rifle now shoots to the same place from about 50 to 15 degrees even though it looses 100 fps in that temp change. That covers the temp spread of my average deer season so its adequate.



The other common cause of this is the stock is swelling and contracting with temperature and humidity and is pushing on the barrel. My brother in law has a ruger M77 in 270 that had a mind of its own. Ever year sighting it in for deer season the scope would need 3 or 4 minutes of adjustment to get it back where it should be. It also shot really poorly to boot. I finally talked him into letting me free float and glass bed it. First thing I did was open up the barrel channel to free float the forend. The stock was putting so much upward pressure on the barrel that this alone changed the point of impact 10 inches at 100 yards.



Next thing was to fix the sloppy fitment of the action into the stock by glass bedding it. For this one I chose to only bed the front of the action and the rear tang.



The rifle now shoots well under 1 moa, and even more importantly the scope no longer needs adjustment from one year to the next. We just sighted it in last week and it was right where we left it from two years ago.

This was a group before I started



This is the last group I shot before giving it back. It was blowing about 15 mph left to right when I shot this hence the horizontal stringing.

I've had the same issue, but not as extreme as 4"-5". My Rem 541S hasn't needed adjustment for several years, seemingly unaffected by numerous trips to the woods and much handling. My varmint rifles seem to need a slight tweak before every trip. It never occurred to me to see which rifles need the most adjustment. This thread has given me something to observe and think about. If I had a rifle that was off by 4-5 inches, I would be concerned.

Ammunition changes

potential stock issues

do you clean the rifles squeaky clean every time you shoot?

maybe you are adding 50%of the fluctuation as well

I believe you are experiencing the frustration of wood stocks. I went through this years ago before going to pilar bedded synthetics.

Your CZ has a wood stock which likely touches the barrel. The channel for the recoil lug is obviously wood as well. Temperature and humidity changes are swelling or shrinking areas which are too small to see either adding or subtracting pressure. A good bedding job will likely clear that up.

The old pump Remington might be wood buttstock and forend, but the barrel and receiver are metal-to-metal and the forearm doesn't contact the barrel when firing. So no change in scope alignment.

Wood was once a living breathing organic material that was at least 60% water by weight. Even after it is cut, dried and made into a stock it is roughly 15% water by weight, any less and the wood is brittle and breaks easily. Even if the stock is completely sealed the water trapped inside will expand and contract as temperature, humidity and altitude change. There is no way to prevent this except scrap the wood for synthetic. As the wood changes shape it puts pressure on the receiver and barrel differently. You can glass bed the action and give the stock a generous free float and it will lessen the changes, but not stop them. Sealing the stock only makes it harder for more moisture to get inside. It does little to really fix the problem.

How much it changes zero depends on a lot of things. One stick of wood may simply be more stable than another stick of wood. If you live in an area of the country where the humidity is pretty stable, you'll see less impact. Hunters who travel see the greatest changes. Last year I left GA at 90% humidity, 80 degrees, and 900' elevation. Two days later I was hunting in 15% humidity, 10 degrees and at 11,000' elevation in CO. Chances are good a wood stock would have been off quite a bit.

I appreciate a decent walnut stock as much as anyone and have several lever actions that way. But I haven't hunted with a wood stocked bolt rifle since 1983. I saw too much of that prior to making the change.

So the gun and the loads shoot super accurate all the time, except the scope needs to be adjusted on a regular basis.

Sounds like the scope. Call Leupold for an RMA and tell them the scope wont hold zero and see what they say. Consider it the first step in a process of elimination.

Your eyes and eye position changes as well.

I just shot my 308 at 100 and 200 yards. I was expecting it to shoot 1” high at 100. It shoot about 4” low.

It is wood stocked, but I just got glasses for the first time as well.

Either way I know it will kill a deer next week.

It’s why they say “guns are like women”. You gotta pay attention to them.

While the general consensus is that zero change is all due to mechanical variation of the gun, in my opinion, the human is the greatest source of error and the primary reason for apparent wandering zero's.

I can look at my Smallbore Prone data, complied over a decade now, and I am shooting around 4000 rounds a year in competition, and each and every match, even with the same ammunition, there are slight differences in elevation, and of course windage. I could blame temperature (which I do believe could make a difference in elevation), and wind direction makes radical changes in windage, but when temps are nice and wind is absent, my nice day xero's are close, but never exact.

And that is due to me. It is due to my position, my flinch, my reaction to the shot, how stiff I am, etc, etc, etc. If you ever seriously shoot in competition shooting, you will find out just how important position is. And how tiny, itty bitty changes in position, stock weld, trigger pull, move groups around. If you are one of those who thinks they are ready for 600 yard game hunting because you shoot 200 rounds a year, well, every time you hold a gun, the zero will be different. You just do not shoot enough to be able to break position and resume it without a zero change. I am going to say, even highly experienced competitive shooters hate breaking position in a match. They have seen that when they get back into position (and I am talking sling shooters) the next bullet may or may not go where the last one went. If I have to break position during a Smallbore Match, I am thankful I get all the sighter's I need. Because time and again, the zero has changed. If you are in a centerfire match, you don't get sighter's after the first two shots, and breaking position can cost you a match.

I think there's a fairly repeatable formula to NOT getting zero shifts over time. When people do see shifts, inevitably they seem to be missing at least one piece:
- Fix all the stock mechanical issues, no barrel contact with anything forward of the receiver and no strain in the glass bedding. Leave plenty of gap in the free float.
- Get your scope mounts nailed down
- Use an optic with ocular and objective focus, and get them both right. Prove it to yourself by moving your head behind the optic through the full eyebox with the rifle bagged, and see if the crosshairs move on target. If they do, it's not right.
- Use an optic that retains zero.
- Do your zeroing where there is no wind (ideally indoors with controlled HVAC)
- Zero no farther out than 100y to avoid humidity and air density effects
- Shoot a load that is on a velocity node. Prove it with a velocity ladder.
- Shoot a load with demonstrable low extreme spread.
- Prove your load's accuracy with a large number of rounds, not 3-5
- Chrono your load consistently (or ideally every shot), to prove you are still on the node and the barrel has not sped up/slowed down

My experience is that "classic" high power type rifle competition is full of people with shifting zeros and it's because they aren't doing what you need to do to eliminate them.

someguy2800 said:

It is not uncommon to see 100 FPS loss in performance going from say 70 degrees to 20 degrees.

Click to expand...

This is very true, and I should have included it in my list. If you want to avoid trouble, use Hodgdon Extreme, IMR Enduron, or the 3 temp-stable Alliant powders (RL-16, RL-23 and AR-Comp). And then put it on a velocity node (or even a tenth or two of a grain above one). If you're using something else, don't complain when you accuracy goes away or your zero moves because the temp changed.

Llama Bob said:

This is very true, and I should have included it in my list. If you want to avoid trouble, use Hodgdon Extreme, IMR Enduron, or the 3 temp-stable Alliant powders (RL-16, RL-23 and AR-Comp). And then put it on a velocity node (or even a tenth or two of a grain above one). If you're using something else, don't complain when you accuracy goes away or your zero moves because the temp changed.

Click to expand...

One thing I will add to that is don’t take the manufacturers word on being temp stable or not. In the testing I have done some of the powders that the manufacturers claim to be temp stable are even more affected by temps than other standard powders. For example I’ve personally found some ball powders to perform quite well at low temps which is contrary to popular opinion. Likewise I once tested H4831sc and RL19 back to back and found much less velocity loss in cold temps with RL19. And of course some primers play better with some powders than others.

If the metal moved enough to cause a 5 inch shift from POA to POI in 12 months and that’s as good as it gets, most precision machined devices, that are not brand new, would be locked solid.

taliv said:

i would NOT zero indoors.
and i don't think low extreme spreads would change your zero, just the size of your group

Click to expand...

Care to explain your reasoning as to why zeroing and checking zero outdoors in variable atmospherics and wind is better than indoors in controlled conditions?

The point of low extreme spread is that it proves you have a consistent powder burn and neck tension. Without those things, you can very easily get an apparent zero shift due to different barrel timing or having some shots off the velocity node. You can also find this by shooting lots of rounds, but you can know instantly by chronograph.

taliv said:

i would NOT zero indoors.
and i don't think low extreme spreads would change your zero, just the size of your group

Click to expand...

Why not indoor? No wind to deal with you get an actual zero. For my last PRS match I zeroed indoors at 200 yards and with my dope was getting good hits out to 1000 in the real world. I had to come off my drops by a 0.1mil for the additional temperate. Read the wind as usual.

taliv said:

wind has a pretty small effect at 100 yards. and if you can't call a wind at 100, you're prob not gonna have much luck calling wind at any greater distance.


there's a 200 yard indoor range in alabama??
having shot PRS since before there was a PRS, you'd be the first shooter i know of to use a 200 yard zero. everyone zeros at 100.

i dont doubt you got good hits out to 1000 in the real world. i've occasionally had to show up to a match without a good zero and still got good hits.


i've never heard anyone call dope or come ups "drops". and the affect of temp would not be linear. you'd have to come off more than .1 at distance and less up close. if it was linear, it probably wasn't the temp. it was prob the light.

Click to expand...

Still does not explain why you won't zero indoors. Indoor the shooter is out of the elements and very comfortable. The rifle could care less.