Parallax Question??

[scsims]

I have 2 Nikon Monarch UCC 6.5x20x44 AO scopes on 2 of my rifles. I having issues with POI moving to the right past 100 yards. Mainly I've been shooting at 200 and 300 yards. I started another thread about hold and trigger control that seems to help the groups move closer to the center when I do it correctly.

In the meantime I just finished reading a good article on parallax and it got me thinking could this be part of my problem. I do my best to try to adjust for parallax but I can't seem to get perfect. I can get it adjusted to where the reticle does not move on the target when moving my head up and down, but I can't seem to adjust it to remove the side to side movement. Is there something I'm doing wrong?? And could this be the reason for the groups moving to the right??

 

[SBruce]

Parallax should be gone when moving from side to side too if it's done correctly.
Hard part about side/side is it's easier to move the gun accidentally when moving our head side-side. Especially on a portable bench that moves when our body position moves..........Dont know if that applies to your situation or not, but parallax could certainly be the issue or part of it.

Speaking of trigger position on finger; Are you shooting with very light hold on rifle? If so, and the trigger is touching the middle of the finger (especially with a heavy trigger pull) it's possible that you're inadvertantly torquing the rifle some as the trigger breaks, which could in effect move your shots slightly to the right. Just an idea, but light hold on the gun makes it more critical to use the same trigger technique on each and every shot. Some bench shooters dont even grip the grip, but they "pinch" the trigger toward the rear of the trigger gaurd with the pad of the finger and the thumb.

Another possibility that I've seen quite often is the scope not mounted perfectly top dead center above the bore. If the scope is mounted ever so slightly to the right of top dead center, your bullets will (in effect) have to go right to intersect the line of sight (IE your 100 yd sight in). The further the distance the further to the right the bullet will continue to go. Same effect can be seen if the scope is pointed toward a different azimuth than the barrel is.......we're talking very very small amounts here of course.

Just some ideas, hope something mentioned will help.

FWIW, when I've seen a rifle consistantly throw shots to the right or left as distance increases, I've countered this with a 300 or 400 windage sight in rather than the 100 yd typical sight in. In your case, sure you might end up shooting 1/2 or 3/4 inch left at 100, but that is a small distance, and unless we're attempting eyeball shots at 100 yds, it simply doesn't matter. I'd rather be off by 3/4" at 100 than 4" at 400.

 

[silvertip44]

Also look at the possibility of canting the rifle when shooting or that the scope reticle may not be properly leveled.

 

[Bullet bumper]

Another possible issue is , do you crank up the power as you move to a longer range . If so then the reticule could be moving over due to the scope tube being in a bind in the mounts. To test this install a shot saver and move the power up and down while watching the reticle against the grid . If it moves around away from one single POI then it's not tracking straight.

 

[bruce_ventura]

I agree with SBruce about the reticle alignment/canting being a likely culprit. How far off is the POA at 300 yds?

 

[scsims]

SBruce,

I usually use a firm grip with my right hand (trigger hand) and hold the rear bag with my left hand. The trigger pull isn't that good on one of the rifles, it has a Rem Xmark at about 4lbs. The other is better, an older style Remington at about 2 lbs.

To answer the scope mounting question I used a Wheeler scope leveler when installing the scopes and I tested for cant at 100 yards by having a verticle line on the target. Then adjusting the scope up 20 moa and shooting a group. Both rifles grouped around the line.

I just checked the parallax on both again this morning..... and your right I must have been moving the rifle for the side to side reticle movement I was getting. When I have it at 20x there isn't much movement you can do before you lose the site picture in these scopes. Maybe I shouldn't always target shoot at 20x???

I also noticed that when I shot at 300 yards in the morning with a heavy frost on.... about 28 degrees my goups were lower than expected by about .5 moa. Then at luch I shot again at 50 degrees and they were back on target. I'm using IMR 4831 in one and varget in the other.

 

[SBruce]

SBruce,

I usually use a firm grip with my right hand (trigger hand) and hold the rear bag with my left hand. The trigger pull isn't that good on one of the rifles, it has a Rem Xmark at about 4lbs. The other is better, an older style Remington at about 2 lbs.


To answer the scope mounting question I used a Wheeler scope leveler when installing the scopes and I tested for cant at 100 yards by having a verticle line on the target. Then adjusting the scope up 20 moa and shooting a group. Both rifles grouped around the line.


I just checked the parallax on both again this morning..... and your right I must have been moving the rifle for the side to side reticle movement I was getting. When I have it at 20x there isn't much movement you can do before you lose the site picture in these scopes. Maybe I shouldn't always target shoot at 20x???

I also noticed that when I shot at 300 yards in the morning with a heavy frost on.... about 28 degrees my goups were lower than expected by about .5 moa. Then at luch I shot again at 50 degrees and they were back on target. I'm using IMR 4831 in one and varget in the other.

Heavy triggers can cause misalignment when the shot breaks. Competition shooters who are after ultimate precision generally use triggers that are anywhere from 1.5 oz. to 1 lb. Not saying you need to do this, but lighter crisper pulls do certainly help. Be careful not to toque the grip when firing.

Sounds like you did all that we as shooters can do when it comes to scope mounting. Only a machinist/gunsmith or someone with specialized measuring tools can determine if the scope is mounted a few degrees off of top dead center or pointed at a slightly different azimuth. Most of use use an ACI or ACD level to insure that we're still holding the reticle level with gravity when firing the gun. Do you have one of those?

Parallax shows up best on highest power unfortunately. It really is best to use the highest power unless the heat waves and mirage are giving you grief. I am not saying that you couldn't shoot just fine on 14X, but for group testing and such I always use max power............."we can only shoot as good as we can see" type of thing. Parallax is tougher to dial out if we can't see it as well. Which is what happens on lower powers IMO, it's still there, we just don't see it.

Were you seeing mirage and heat waves when it warmed up to 50? I've seen ground mirage throw off poi by a full moa at 300. Granted, it was very bad mirage, but it darn sure happened. The temperature alone should not have that much effect on bullet trajectory at 300. Combination of cold dense air, cold ammo vs warmer lighter air and warmer ammo and possibly heat waves would certainly be enough in some cases for 1/2 moa at 300 though.

Hopefully with a couple more range sessions you'll have the bugs worked out. Remember, if nothing else you can always sight in your windage to be dead on at 300/400 yds (on a calm day of course).