Can 3 Grains Make a Difference?

Yes, it's entirely possible that your problem lies with the different ammunition. It's not necessarily the 3 grains of bullet weight, though that would slightly affect point of impact... But like others have said, the weight of the bullet combined with the different powder and any other differences between the cartridges would be the reason you're seeing different points of impact.

If I were you, I'd stick to just one brand and load of ammunition for a while. For example, just buy some 150-gr milsurp 7.62x51mm NATO, such as M59, and see how just that one load works.

I spend decades doing military long distance competition.

Normally - I would say no. BUT - you mention 147 FMJ - OH HELL YEAH BABY!! LOL.

See - that 147gr ammo - most all was made for the M60 machine gun and other belt feeds that have followed - typically 4 of the 147's followed by a tracer. With a machine gun - the idea is not to be shooting excellent groups like a laser beam. Here we need to have some inaccuracy so the machine gun sends out a 'cone of fire'....... the bullets hit a bit left, right, up, down and work their way into a target. Needless to say - 147gr ammo - specifically the projectiles - are not made to exacting specifications!

When I was on a military national shooting team - new shooters would start by delinking M60 ammo and use those 147 at the 200yd line slow fire and 300yd rapid. You usually had better ammo for 600yds. Once your score got to a certain level - then you 'graduated' out of that crappy 147gr stuff.

Yea, you should take an accurate measurement on each, and tape some masking tape+a note on it to give yourself an accurate hold/adjustment.

I've had .308 point of impact shift a solid inch + and - with a grain of powder + and -, even though, in theory, that was 4x more than what velocities would have caused the impact to shift on their own.

That is because those slight differences cause harmonic changes in the barrel, and with any vibration, if it isn't tuned right, it isn't just the place where the end of the barrel is in 3-D space, but the angle of the barrel as well. And, those bullets are different in weight, profile, and other things that will make it tune differently from another bullet.

To keep it simple, and not delve deep into a fantasy life to prove my point:

Two different bullet weights and two different loads would more than likely have a different point of impact. With all other things being equal, a lighter bullet will be higher in velocity and have a shorter time of flight, so it will have a slightly higher point of impact. There are really too many variables to figure out exactly what the difference would be, but you would expect there to be a difference.

Okay, here is how know this is a make-believe trip to the range:

Mystery as to what sighting device is being used, but the two that are mentioned are two common video game sights

Firing a 308, worried about accuracy, but the two optics you talk about upgrading to are fast acquisition close range sights (or mid-range depending on which specific ACOG you are referring to) whose targeting reticle ^ or . is big enough that it covers up more than the click or two adjustment you suggest.

If you are talking a more traditional optic like a scope, a click is generally 1/4 MOA, so you are deviating 1/2 MOA from an earlier POI. At 100 yards that's a half-inch. You are barely going to be able to see that, it's basically the same size as the bullet hole you are punching. Plus, if you can't afford a $500 optic, you don't have a 1/4 MOA accurate rifle, and you aren't a 1/4 MOA shooter...and if you were, you'd not be talking about an EOTech or ACOG (or whatever other Trijicon product you are thinking of)

PLUS, if you were an accurate of enough shooter that you were able to detect a change in POI that was simply 2 clicks off, you'd know that using two different brands of 150 grain ammo is likely to cause a 2-3 inch shift in POI at 100 yards, which would be an 8 to 12 click adjustment, let alone going to a heavier bullet which may well change the POI more or less, but as ANY ammo change can cause a 2-3 inch shift, its absolutely not surprising that the 147 gr vs 150 gr alteration causes a POI shift, but it's actually suspicious that it is as small as you claim

PLUS if you were an accurate enough shooter that you were able to detect a change in POI that was simply 2 clicks off, you'd know that the very same ammo may have a shift in POI equivalent to 2 clicks due to variation in humidity on a given day, how warm the barrel is, and other very minor factors.

So, no, your sight isn't jiggling loose, your story is.

But to answer your question basically ANYTHING can potentially give you a '2 click' change in POI.

The bigger factor than the wind would be the different powders, the different barrel harmonics and different "dwell times" in the barrel and, ultimately, which direction your muzzle is pointing in it's harmonic cycle at the moment when the bullet leaves the barrel.

The short answer is YES, that could actually have a significant effect on your point of impact relative to point of aim. It's not going to be a very huge deal for "minute of boiler room", but if you get happy looking at tiny groups on a piece of paper then that could certainly put a hitch in your giddyup.

3 grains is not going to make that big of a differents. But the differents could be possible if the bullet is shaped different, if it is a hollow point or it is a hotter load. But then it could make that big of a differents if you are shooting 200+ yards and are trying to keep them grouped in an area the size of a dime. And I can't see that because from shell to shell factory loads are not that accurate to be able to do this because there is a % differents from load to load out of the same box of shells

i know for a fact that 3gr variation does have an effect. every box of bullets i buy for reloading is seperated into each wieght an tagged. i've recorded enough that i have adjusted loads for each 1/2 grain variation for hunting rifles. an every box of bullets has a variation of appox 3-1/2 -4+gr. some i've found just over 5gr like sierra 150s for 303br.

The weight difference itself would have little or no noticeable effect. But the REASON for the weight difference; bullet length, shape, center of gravity, and such will have an effect. One probably produces a little more drag than the other. It would be interesting to pull one of each of the bullets and compare them.

If you are shooting at 100 yds, 3 gr can easily make that much difference. I have seen as much as 1/2 MOA difference in the same ammo with just different lot numbers.