Gunsmithing Bullet curving left after 400 yards. Crazy?

[Snakum]

While shooting my little Marlin sporter (amazingly accurate after bedding/stiffening) about 500 rounds we are starting to notice a trend. No matter what the wind is doing, the POI seems to head left once we get past 400 yards. By the time we are at 700 yards (as far as we've shot)we are off the board to the left (we can see where the bullet nicks the board on the left side.) And this has happened with both Federal Match and with Georgia Arms BTHP match ammo. It will usually hold MOA to 500 yards unless it gets hot, and we've even gotten 6.5" groups at 600 on no-wind days. So the shift to the left is really weird. It seems perfectly centered from 100 to 400 yards.

Any ideas? Do barrels actually do stuff like this. Are we crazy, or are we canting the gun and just not realizing it?

 

[zenbiker]

Re: Bullet curving left after 400 yards. Crazy?

This is not barrel. What is your scope/base setup? What caliber are you shooting? I'm laboring under the impression the Marlin Sporter is a lever action .44 cal, in which case, you likely don't have enough scope to shoot that far.

My first thought is if you are shooting with a scope, your scope is not tracking truly vertically, either because of a defect in the scope, a lack of sufficient elevation above your zero point, or both. Knowing what your 100 yard zero setting is in relation to the optical center of the scope would help answer this question.

If you're shooting with open sights, I'd say you're running up agains the limits of the cartridge you're shooting. If you want to go Quigley, you might need a little more cartridge

<span style="font-style: italic">Anyways</span>, I doubt its your barrel. More likely a question of the limits of your cartridge or the sighting system, or maybe both.

 

[Greg Langelius *]

Re: Bullet curving left after 400 yards. Crazy?

Wind at significant distances generally has little or nothing to do with wind where you're shooting. There may be a prevailing wind out there that you have no indication of at the line. Try to spot foliage out there to see what winds are working where the deflection is occurring.

There is no such thing as a phantom force. Coriolis and spin drift are real, but I sincerely doubt that you'd be seeing major influences from them that close to the line.

It's probably not the rifle or sights. I'd be fairly certain it's some sort of wind effect.

<edited to add>:

I am assuming you are shooting higher BC bullets at significantly quick velocities. If we are talking about lobbing pistol caliber stuff way beyond design distances, then the comment about coriolis and/or spin drift being inconsequential <span style="font-style: italic">may</span> have less relevence.

Greg

 

[Snakum]

Re: Bullet curving left after 400 yards. Crazy?

Rifle is XS7 in .308, Devcon bedded, forearm stiffened with aluminum rod. Scope is 12x Redfield (excellent scope compared to all my others ... love it). The bases and rings have all been thoroughly cleaned, threads chased, and blue Loctited on. Ammo was Georgia Arms BTHP Match and Federal BTHP GM Match - always 168gr.

Again ... the rifle shoots amazingly well for a 22" sporter barrel. We've even shot 1/2 MOA at 500 when the planets were aligned, though it is a one off deal, of course, and will likely never happen again. The rifle is a helluva shooter. 600 yards it generally runs 6.5 - 7.5 inch groups with the good Match ammo.

About the wind: I am usually spotting because my bro shoots better than I. I check the wind speed and direction at the shooting site and at each target location as they are set up. Before we shoot I check again on the line, halfway to our max distance, and then at the max distance. The days we're noticing the left curve tendencies we basically in a no wind situation.

The POI is dead on at 100 ... 200 ... 300 ... 400 ... then 1" - 3" left at 500 ... then 2" - 4" left at 600. Very odd. And you can definitely see it starting to bend. So I was just curious. Never heard of it before.

 

[Snakum]

Re: Bullet curving left after 400 yards. Crazy?

I dunno about Coriolis Effect (studied it in pilot training) ... seems like 600 yards is no where near far enough for it to be any kind of factor.

I wonder if we're getting an intermittent wind running thru the target zone that stays rather consistent in vel and dir but only shows up periodically? Gotta be.

 

[colchester]

Re: Bullet curving left after 400 yards. Crazy?

As stated above I have wittnessed this occur when you get near the end of the adjustment limits on the scope. I noticed on my rifle as I got near the end of my elevation adjustment as I added elevation I would also get windage correction. It has to do with the way the internal crosshair is constructed

 

[Hellbender]

Re: Bullet curving left after 400 yards. Crazy?



You can eliminate the scope variable by shooting at 600 with the 400 yard (known good) dope on your scope and using a long piece of paper to catch your shots below the target, if they hit directly below the target, your scope is shifting w/elevation change. If they are still hitting to the left, take the Snipers Hide on-line training course and this problem will probably disappear.

Are you shooting left or right handed?

 

[Snakum]

Re: Bullet curving left after 400 yards. Crazy?

I guess I am sorta getting near the limits of the elevation turret, was using 76 of 125, iirc I have about 250 clicks full width both elevation and windage, and the zero is pretty close to centered on the knobs. There could be something to that.

I'm shooting left and right-handed, and my partner is shooting left handed (we already correct with proven dope for his weird right-shooting tendency). With the weapon shooting beautifully to 400 yards and the groups staying pretty tight past that, I don't think a course will fix this particular problem. Had a couple of those courtesy of Uncle Sam.

Thanks for all the help. Much appreciated. I'll try a few of the suggestions today when we go back out.

 

[zenbiker]

Re: Bullet curving left after 400 yards. Crazy?

OK. Good information. Let's assume your rifle is a 1 MOA rifle (probably better, but bear with me). Your error as you state is
1-3" at 500 yards, 2-4" at 600 yards; in other words, 1/2 moa; not huge, and well within the range of wind drift. Forget coriolis and spin drift at this range.

Wind effects are going to be very noticeable at 600 yards. Even a 1mph full value wind will give you 3" of drift at that range, and depending on the layout of your range, brief exposure to a 3mph crosswind past 400 yards will produce this much error.

so you've got two possible sources: wind and scope. Test the scope like Hellbender recommends: Get a 36" tall piece of cardboard, draw a staight vertical line on it with a cross at the bottom of the line for your aiming point. Set up the target at 100 yards using a plumb line or level to get the vertical line straight up and down. Holding on the bottom aiming point for each shot, shoot successive shots with your scope set for 100-800 yards. The impacts should walk up the line within the limits of your normal 100 yard group. If you see the impacts suddenly start walking left at 600 yards and you're near the limits of your scope elevation, your scope may be part of the problem, but it doesn't necessarily rule out uncompensated wind drift.

As far as the wind goes, set up wind flags at all your target points. Watch them. 600 yards is when bad things start to happen to bullets with 308 ballistics because the bullets begin to slow down and are exposed to gravity and wind vectors for a longer period of time for a given range traveled.

I was under the impression you were missing by feet. Inches at those ranges can easily be wind. Keep shooting, you'll figure it out.

 

[Snakum]

Re: Bullet curving left after 400 yards. Crazy?

Thanks for the info. Will do.

 

[Snakum]

Re: Bullet curving left after 400 yards. Crazy?

Did as instructed re: traversing up a straight line, the scope eventually started veering left the tiniest amount and just before where the 500 yard come-up begins at 50 clicks it looks like it would be enough to start throwing noticeably left at longer ranges. I simply didn't have it absolutely dead level. Rookie mistake. I actually use a level when installing, I just goofed this one a tad.

Thanks all!

Boy ... tiny deviations or weak points in the system really start showing up past 400 yards.

 

[ArcticLight]

Re: Bullet curving left after 400 yards. Crazy?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Snakum</div><div class="ubbcode-body"> at 50 clicks</div></div>

50 what? LOL -

So you have 11 MOA dialed in roughly, assuming 1 "click" is 1/4 MOA -

Sounds to me like it's either a scope issue - or a canting issue.
Especially because it's consistent.


But - on the other hand - if I CLEAN the heck out of my barrel - it shoots 1"left at 100 yards on a CBS - hence why I don't clean very often or very thorough.

And it's consistent out to 600, if and only if scrubbed clean.

 

[Snakum]

Re: Bullet curving left after 400 yards. Crazy?

50 clicks. It isn't a mildot scope so I work with a LRF and clicks. Tres simple and fast. as I said ... the scope is canted/not level. It has been fixed and will be tested tomorrow.

Thanks again for the tip on checking the cant, guys. Will use that this week when mounting the new scope. A level alone wasn't enough to ensure it was perfect due to lack of perfectly flat surface. Learn sump'n every day here.

 

[RTH1800]

Re: Bullet curving left after 400 yards. Crazy?

Spin drift and coriolois effect would both be to the right, given that the rifle has Right Hand twist and he is shooting in the northern hemisphere.

 

[oldgrayone]

Re: Bullet curving left after 400 yards. Crazy?

Or he could be shooting one of these

 

[zenbiker]

Re: Bullet curving left after 400 yards. Crazy?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: oldgrayone</div><div class="ubbcode-body">Or he could be shooting one of these

</div></div>

Ah, the WTF 7.65 select-fire Squid Automatic Weapon. Discontinued from foreign service due to the tendency to hit the operator in the ass. Also posed significant problems during Parade Formation Drills

 

[GCMxVeGeTa]

Re: Bullet curving left after 400 yards. Crazy?

Would bullet stabilization cause anything like this? If the bullet was unbalanced/keyholing after 400 yards it could turn towards the left or allow a right wind to affect it more since there would be more surface area on that side.

Just a thought, only way I know to check would be to look at the holes in the target.

 

[TCA4570]

Re: Bullet curving left after 400 yards. Crazy?

Straightness of hole may be coming into play.
Timing of arch of bore, may be coming into play.
Loppy bore may cause problems.
Setting to Go, No-Go is only part of the picture.
Where is muzzle pointed when set right?
Just more thoughts.

Which way does it buck?

 

[Scimitar]

Re: Bullet curving left after 400 yards. Crazy?

i'd bet the scope was slightly canted and when you dial "up" it
is showing up. Like someone posted above, could be the scope at the higher end in the adjustment range. Or wind

 

[scorehi]

Re: Bullet curving left after 400 yards. Crazy?

This is not a new problem. In fact it is quite common but usually to a lesser degree. I recently wrote an article about this very thing. It is on my web site at

http://www.scorehi.com/precision%20scope%20mounting.pdf

or www.scorehi.com under articles and videos
This may not be your situation but then again maybe it is. I learned from using the scope mounting fixture in this article that many scopes only track straight for a percentage of their total travel. Lots of scopes travel straight for about 25 MOA and then start to swing an arc.

Charley
Score High Gunsmithing

 

[jeffm]

Re: Bullet curving left after 400 yards. Crazy?

If you site a rifle in at 100 yards with the cross hairs level, and then shoot it at 600 with the cross hears leaning will it cause the affect listed above? A canted scope would cause the zero to move to the side, but I think you would see it progress as you extended further out.

 

[scorehi]

Re: Bullet curving left after 400 yards. Crazy?

You are correct indeed. What I am addressing in the article is a separate issue than level cross hairs. It is absolutely imperative that a the scopes internal movement follows a perfectly vertical line and that same vertical line perfectly bisects the center of the bore and the center of the scope. If the scope is rolled in the rings clockwise or counter clockwise the most minuscule amount it shows up as windage error. The further out the more error. This is something I think we all know and we deal with it the best we can. Another factor is that sometimes the movement or tracking using the turret does not follow the vertical cross hair. In that event one must choose, do I want to be able to accurately dial or do I want to be able to accurately mill hold, because with some scopes you will not be able to have both.


I believe the process of scope mounting will evolve as other segments of rifle building has evolved. I started blue printing actions back in the late 80s. Back then there was very few smiths doing that and most believed it to be a waste of time. Today no one builds a serious rifle on a commercial action with out blueprinting it first. The barrel work I was doing back in the late 70s and early 80s was at the time above par but by today’s standards it wouldn’t cut it. Barrel quality has never been better. The average rifle has never been as accurate as it is now. I think as a whole the industry needs to do a better job of uniting the rifle and scope so that they can both be all they can be. Every one wants to mount their own scope but that simple task most often taken for granted if not done perfectly produces a mediocre rifle. I predict that what is acceptable scope mounting today will not be good enough in 3 to 4 years

 

[frog5215]

Re: Bullet curving left after 400 yards. Crazy?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jeffm</div><div class="ubbcode-body">If you site a rifle in at 100 yards with the cross hairs level, and then shoot it at 600 with the cross hears leaning will it cause the affect listed above? A canted scope would cause the zero to move to the side, but I think you would see it progress as you extended further out. </div></div>

Cant gives you a frowny face distribution to hits. The more canted, the more to that side and low.

 

[jeffm]

Re: Bullet curving left after 400 yards. Crazy?

I have always struggled to get the scope straight when mounting. I would think higher quality scopes and rings should have some type of mounting grove or alignment marks to insure they are straight. It probaply isn't done becasue the scope would then have to be built with the cross hairs perfectly stright in the scope.