Re-zero after switching loads? 130 gr to 150gr.

[aggie1982]

I read somewhere that if I switch from shooting 130 gr in my 270 to a 150 gr load that I should re-zero my rifle and vice versa....is that correct? I'm sure it is but can someone help explain why. I just joined the forum today and am new to firearms.

[Fjold]

Every bullet (different weight, shape, velocity, etc.) will shoot to a different place. The difference may be as small as fractions of an inch at 100 yards or as much as inches. Every time that you change ammunition components it will effect the point of impact.

Basically different bullets fly to different paths through the air, lighter bullets are usually loaded to higher velocities so that in the first couple of hundred yards you will see less "drop" in their flight path. Heavier bullets will start out slower so they will "drop" more over the first few 100s of yards. The lighter bullets will lose velocity quicker than the heavier bullets because they have a lower ballistic coefficient (resistance to the change in velocity due to the air resistance) so that at longer ranges they will drop more than the heavier bullets.

Another factor is that your rifle barrel vibrates as the bullets travel down it and different weight bullets will cause the barrel to vibrate at different rates which will effect where the barrel is pointing as the bullet leaves the muzzle.

[ExtremeX]

Most of what you are asking is basic ballistics... and some of what Fjold is talking about goes into barrel harmonics.

Chances are there will be some some variation between the two loads. My recommendation is to test the second load with the existing zero and see how much correction is needed at various distances. You can effectively build two separate dope sheets based off the zero for one load.

I do this with one of my rifles when I switch between 55gr plinking ammo and 75gr match ammo.

[PrimaryArms]

Dial in your favorite round and zero turrets. Dial in other rounds and track adjustment on paper.

Example load
A=0
B= 2moa up 1 right
C= 1 moa up 1.5 left.

This will allow you to go from load to load with out adjusting 0

Dimitri

[hermosabeach]

If you know the velocity of the two rounds, you can enter the info into a ballistic calculator and the results will show you the flight path of the bullet.

http://m.hornady.com/ballistics-resource/ballistics-calculator

At 100 yards, many people will not notice a difference in impact when not shooting from a bench.

At longer ranges, the impact distance will be greater.

[HK Dave]

I've had different loads not only change vertical trajectory but also horizontal. When going from Federal Gold 168 Matchkings to Prvi 145, we experienced a 2 inch horizontal shift of impact at 100 yards... that's just huge. We re-verified with different shooters and everyone got the same results.

I assumed the change in impact would be purely vertical, but we were wrong.

Re-zero whenever switching loads.

[rdavenport]

The weight and metallurgy of the bullet and the twist rate of the barrel effect the trajectory of the bullet (all else being equal - temp, altitude, humidity, wind, etc.) differently. Therefore, all bullets - even the same load from the same manufacturer can perform differently depending on the lot number.

Bottom Line, ALWAYS re-zero.

[postal]

I'm there with HKDave....

A switch in ammo, could go anywhere on paper at 100yd... I've seen as much as 2 inch difference as well.

The only explanation for that large a shift is barrel harmonic.

[aggie1982]

Thanks so much for all the answers...I definitely keep all paper targets of each load for my records....it's hard to find the same load so this really helps. I appreciate all the info.
By the way, you guys know any good books or sites that help break down ballistics or sights. I'm new to shooting/hunting and would like to learn all I can.

[calidude]

Welcome to the forum! PM sent about some references.