Sighted in for 77 grain tmk and shooting under 1moa at 100, switch to 55 grain and shooting 6-10 inches left and close 6 inch low. Is this normal?

There's going to be a POI shift between loads unless you do a lot of tuning.

For that kind of shift, it sounds like you have a hot 77gr load, and a not so hot 55gr load?  Both handloads?

77 tmk load is just under max and 55 grain is federal 556. I'm around 2650 with 77's and 55's say 3165 on the box. After reading I'm learning how the 55 grain would be lower but why so far left?

The greater velocity might make the 55s lower, depending on your zero with the 77s, but not 6" @ 100yds.

Both zero at 100, shooter err?

When I zero for 52gr, and switch to 77gr they are a few inches to the left.

[shrug]

.

Totally normal and expected.  Look at youtube videos on "barrel whip" or "barrel harmonics" to see how much firearms flex when fired.

Different loads and bullet weights will have the bullet leave the barrel at different positions of that barrel whip, changing the POI.

The AK47  is one of the worst offenders: - this is one video, I've seen another with far worse flex from brake to buttplate:    https://www.youtube.com/watch?v=HGivoWD9OvQ

Your original post made sense to me . . . could be the faster bullets spend less time in the rising barrel. The 52s also going left (or low for that matter) could be a matter of considerably different harmonics as dryflash said.

But this post seems to say when you changed your zero to match the 52s, the 77s are now shooting to the left . . . when they should be to the right, correct?

I apologize if I got it wrong, but if not there's something else going on here. Using an optic?

Perfectly normal and most of the time repeatable.

I keep notes. I know where each load's POI is in relationship to my standard zero which is 1" high at 100 yards with my 50gr V-Max load.

So if I decide to shoot something else like 69gr BTHP or 55gr FMJ'S or whatever I just look at my notes and either compensate or dial it in. I prefer not to dial it in because then I'll have to check my zero but some loads are just too far off to simply compensate.

Motor

Elwood has it summed up very well.

You have a 55,000 psi shockwave that runs down the barrel to the muzzle and comes back and forth many times at the speed of sound whipping the barrel around. Ideally you want that wave at the chamber when the bullet exits. Apparently that's where it is with the 77's. With the 55's going out faster, who knows where that wave will be. Perfectly normal and expected results with different loads.

@Elwood_Blues : WOW ! what a video, never saw that before and didn't think they flexed that much. I thought it was going to bend in half at some point.

Looks at screen-names, I am not the OP ( original poster )

I was agreeing with him, but My zero was with lighter bullets.

nothing wrong with my optics - BURRIS XTR  LARUE SPR-E mount.  Solid.

Different bullets will have different zero.

I have not gotten around to testing my handloads, but I have 3 different bullet weights for the 308 and the 7mm. Is in not practical to expect to be able to use each of the bullet weights in the same rifle for different applications without having to change the zero on said rifle for each bullet? I was hoping to be able to tailor different bullet weights for different applications, but if the zero on each bullet weight varies by inches, then it may not be practical to have multiple bullet weights loaded.

I could add notes to the load, like one post above, but I was really hoping to not have to adjust so much between bullet weights. I assume that different rifles will act differently when shooting various types of bullet. Only testing will tell for sure, but I am skeptical that all of these bullets will be useful to me if only one bullet weight will shoot accurately from the rifles.

Bullets in question:
308 = 150gr, 165gr, 180gr
7mm = 140gr, 160gr, 175gr

What barrel? That's an excessive shift to me.

I guess so, mine might be the other way around, or the same.

Buried in my log books are the change in POI when going from a .22-250AI to a 6 mm Rem AI.

Using a panda action, a rest and rear sand bag.

It was very repeatable.

I could change the barrel back and instead of up and right it was down and left.

Every time.

Bullet changes, load changes, velocity changes, BC changes, etc.

It was good enough to set up with one caliber and pick off a few groundhogs.

Then change the barrel and pick off a few at longer range with just a scope change.

I was very satisfied.

I could go out a year later and everything repeated.

100 yard (short range) groups around 3/8 inch.

The 6 mm Rem AI was actually fewer MOA at 300 than at 100.

Probbaly not sleeping as well yet.