It's having a bubble level on my rifle a necessity?  Right now I have no way to see if it's canted.

For long range shooting yes. If I remember right 1 degree of cant will make you miss by about 5 inches at 1000 yards. Just make sure the bubble level is level with the reticle in the scope. Then Check it by doing a tall target test.

Yes, if you intend to shoot longer ranges.

The level must be matched to your reticle.  

Some people believe that they can match the x-axis of their reticle to the horizontal lines of the target to get "level".  This is incorrect unless your targets are leveled when you place them.

As the poster above mentioned, it's generally accepted that 1 degree of cant= 5" POI shift at 1k yards.

Why is this? What's the physics. or trig or whatever is the cause?

There's math to back it up, I just don't have it handy.

It's explained better here: http://www.accurateshooter.com/optics/canting-effect-on-point-of-impact/

Agree.

Yesterday I had a 2moa hold left and 3moa additional elevation fighting wind at 900.

Checking the level lets me know it's wind/temp/etc and not me.

It is simply because rolling the rifle left or right will turn you elevation adjustment into some windage and a little less elevation than needed.

Shooting on side hills and stuff like that fucks me up if I dont have a bubble. Cheap and easy to add so no reason for me not to add them.

^^^* This. If you are set up correctly to begin with and are good at repeatedly holding the rifle in the same manner.,and, or are very conscious of scope cant. Then you should be fine. People routinely shoot to 800+ yards with out them.

They are $35 ffs, it is ridiculous for a long range shooter to go without.

That thlr guy from Norway  doesn't use one.

He puts a digital angle finder on top of his turret and checks it when he thinks he is level and has come to the conclusion his spatial awareness and inner ear work to figure it out accurately enough.  (If I recall from his vids).

...
[youtube]https://youtu.be/gzX6DsJ5IrY [/youtube]

..another

[youtube]https://youtu.be/iChDlTNmk0I[/youtube]

Hes a stud. Something Ill never be accused of. Shooting off unlevel surfaces at unlevel targets having a bubble is super cheap insurance for me.

this

His little test is horse shit.

I haven't needed one yet. Farthest I shoot out to is 1580 yards. Rarely. But typically 1,000 yards as that's what my local range has. I've found my own sense of level is good enough and when comparing to a bubble level, I can get it spot on no problem. I haven't noticed any real need for one at least, for the type of shooting I do.

The argument that the MK 1 Mod 0 Human ear is good enough to sense a level state under all conditions, is petty optimistic.

On a target range, maybe.

In the real world where you may be standing on terrain that angles one way and shooting toward a horizon that angles a different way with intervening terrain that also may or may not be level, and you're shooting up or down slope, it becomes a bit less clear what's "level" when your inner ear and your eyes are in disagreement.  Throw in some  physical exertion, add some rapid head movements, and take away visual indicators like trees, and finding what's "level" is suddenly much more difficult.

In contrast a scope level is inexpensive and doesn't add significant weight or bulk. Plus, if you have one you can set up with what you think is level and then just look up and check it.  That alone will eventually show you that from time to time you actually need it.

Consequently, there are very solid arguments for having one, and no valid arguments for not having one.

completely incorrect, because you are a stud.

People used a plumbob before the scope mounted level. I used an arrow head with a string and a fishing clip swivel. But the scope mounted level is pretty much the way to go, and it's hard to loose. My buddy had a bullet drilled for his. And you would attach them to the ring on the front of your bipod. Wind would play with them a bit, but you learned if it was that windy it was a hail marry.

It's the insidious slight slope in view that can bite me. The almost imperceptible ones.

How you get the lateral shift (and lower strike).  Your bore line pitches up to fight gravity.  Your sight line is level. Same as tossing a football up andDropping it in to your receiver.  Now envision the bore line and sinker gut line and make a Triangle out of it.  The low corner should hang dead vertical.  If you can't or roll the gun you in essence tip you whole triangle out of the vertical.  You offset your chamber to the side but not parallel to the sight line. (They were never parallel).  Now when you launch your bullet it has a horizontal change as well as a lessor vertical drop as you traded your elevation adjustment to the horizontal.   It's an angle like a drafter's triangle but no longer vertical.    If you sighted the "bore line" it would be off To the side now.

Easiest way to make a model is tie two dowels together at a single point about 1/3 from the end.   One represents the sight line, the other the bore line.   Now move the bore line  around and look at the Direction it points to.  That's how sightlines and borelines work.  All the iron sights, scopes, cannon sights all work this way.  They offset the effects of gravity and are designed to work in the vertical arrangement of our two lines: sightline over bore line where their convergence is down range.

do you replace the string often from shooting it?

Lol everyone is a stud shot on the internet right.

Okay, I just got one and I was testing myself and found I think I was always canting a bit. Every time I thought I was level, is check the bubble and it was off. Correcting it to the bubble seemed to make the reticle appear canted to me.  But apparently it was just me being half a bubble off.

To setup the bubble I set up the rifle on a table stable and got the scope reticle lined up with plumb Bob and then adjusted the level. I assume I did it right? Any tips?

Ya that will work fine. Half the time I end up lining mine up off my houses trim.

So far, so good, but the entire set up is a multi-step process.

At home:

1. Level the rifle.

2. Level the scope on the rifle using a vertical plumb line placed far enough way that you can see the line and ensure it is parallel to the vertical cross hair. I prefer a plumb line to walls, as I have seen new constriction walls that are over 1/2" out of plumb over 8 feet.  It's only about a half degree error, but why start with an error that you'll have to fix later, and that will confuse the results later in the process?

3.  Attach the level to the rifle and ensure it is level when;
a) the rifle is level; and
b) the cross hairs are aligned with the plumb bob.

4. You can verify the relationship of the rifle and the scope using a mirror.  With the rifle level, and sighting on your image in the mirror, the vertical crosshair should bisect the bore and the center line of the scope.

At the range:

5. Perform a tall target test:
a) staple 4-5 targets one above the other on the target frame/backer at 100 yards so that the lines match up, using a plumb line to ensure the target is vertical.   Using 4 to 5 12" square sight in targets that will give you 48-60 MOA of elevation adjustment to perform the test.
b) shoot a three shot group at the bottom of the lowest target using your basic 100 yard zero.  Add elevation and shoot a series of three shot shots in the middle and top of the target.  You want at least 3 groups to ensure linearity.

If the test is successful the middle group with 30 MOA (or 24 MOA) added will be exactly on centerline, as will the top group with 60 MOA of added elevation added.

If the middle and top groups are angling off to the left or the right, when the scope is level, (assuming you did steps 1-4 correctly), then the reticle adjustments in the scope are not parallel to the reticle itself.   Sadly, a 2 to 3 degree tolerance is normal for many scope makers, and that will show up on a tall target test.  60 MOA (62.82" at 100 yards) is handy as a 1 degree error in the reticle/adjustment alignment will show up as a 1" change in POI from the zero group,  a 2 degree error equals 2" etc.

If you encounter this kind of error, your options are to get a better scope, or rotate the scope. For example, if you find the POI is 2" right at the 60 MOA group, you can rotate the scope 2 degrees left to level the adjustments with the rifle and scope level.  Assuming the scope level you are using tilts 5 degree when it is a half bubble off plumb, you'd rotate the scope so that the cross hairs are vertical when the level is about 1/4 bubble off plumb.   Then re-shoot the tall target test. This 2 degree change may or may not be something you notice and/or can live with.

6. The last step is a box test.

a) Put up a 12" square target at 100 yards, or better yet staple 4 of them together to give you a 24"x24" target.

b) Shoot a group one the lower right square of the target;
c) Add 23 MOA of left windage to hit the lower left square on the target.  (23 MOA at 100 yards is actually 24.081" so use 23 MOA, not 24 MOA.)
d) Add 23 MOA of elevation to hit the upper left square on the target.
e) Add  23 MOA of right windage to hit the upper right square on the target.
f) Remove 23 MOA of elevation to shoot a group over the first group.

If the test is successful you'll have:
a) a box pattern with 90 degree angles between the sides;
b) accurate and consistent changes in windage and elevation;
c) nice tight groups in each corner, (if you see one or two shots lagging outside where the group should be, it means you have backlash in the adjustments, which you should not have in a high quality scope); and
d) the last group should overlay the first group, meaning the adjustments come back reliably to center.

Reading skills are worse than mine.
I attached mine to the ring on the bipod. I've seen people hang one on limb and I've seen the spotter hang one by hand so the shooter could eyeball it. I've seen some shooter's in poor countries us a medicine cup or a bottle cap and water. Anything to determine a level right angle.

Yeah,  you're right... but I've seen you shoot, and puke

some levels are very sensitive, the nicer ones are very touchy

Nope, sounds like you did good

But it still sounds as if you do not have confidence in either your equipment... or your use of it

So...
Have you shot to check tracking yet?  I mean deliberately, for the sole purpose of checking?

Here's a pretty easy method

The further you adjust, the more apparent the results

I recommend shooting this at the distance you zeroed the rifle, or the distance at which you plan zeroing

If you have a spirit level mounted, don't use it until after you check your reticle

Yeah I need to do all that. I just bought and mounted this scope on a recently rebarreled upper plan to try my hand at some 1000ish stuff  and some local casual prs stuff maybe.

Normally my shooting causes the puking. Have you seen the arisaka scope level? Kinda a cool little setup I need to test it out. What bubble are ya using my friend?