Different bullet types - Very different results?

[PoFF]

For some time now I've been using Speers Hot-Cor Soft Point bullets, 150 grains in weight, flat base, to top off my 308 loads, with great results (good groups, and dead center with sights properly adjusted). I recently ran out of those bullets and switched to Hornady FMJBT 150 grains, using the same exact powder load, same brass, same primers, and a seating depth only 0.005" longer (due to the longer pointed tip).

Now, results aren't bad, groupings are still good, but bullets hit 4 inches low, 4 inches right, and it's not due to weather conditions (temp, humidity nor wind) as the range is indoors. Now, I did expect a little difference by switching bullets shape and construction, but never expected that much of a difference since they're the same weight and propelled by the same combination of powder/case/primer.

Since I'm curious by nature, I'd like to know what physical facts can explain such behavior?

 

['Boo]

Since I'm curious by nature, I'd like to know what physical facts can explain such behavior?

Different bullet construction like jacket "hardness" as well as different amounts of bearing surface can cause different types of bullets to produce different chamber pressures and different velocities. Your new loads may be a fair bit slower - or a lot faster - than your previous loading.

BTW, Just plunking a new bullet on top of an established load can be dangerous unless the powder charge used is close to starting levels for that bullet weight / powder combination.

 

[bearhunter]

There are a lot more issues with changeing bullets than there is room for here. Boo is correct, but basicly every time you change something, you get bbl harmonic fluctuations that will change POI. If the group is good and you're going to stay with that bullet, adjust your sights. If you're going to go back to your old loading, either trade off the new bullets or just use good old kentucky windage. bearhunter

 

[PoFF]

No problems on the pressure, my current load is 0.5 grains over starting load (accurate enough for the range I'm shooting at, enough to cycle the gun and not too hard on the brass).

I'm probably gotta stay with this load, those bullets are more accessible.

Kenticky Windage, that a slang term I'm not familiar with, what does it means?

 

[maynard]

If you are not using this for hunting, which I assume you are not by switching to FMJBT bullet, why not try some match bullets?
The pointed tip of the bullet means nothing when seating your bullets, you are seating off the ogive of the bullet. Even match bullets from the same lot and same box will vary in length.
FJMBT bullets are not known for great accuracy.

 

[canucklehead]

Kentucky Windage... my understanding is 'shoot, miss, watch the spot where the dust flew in the scope, put that 'spot' on the target, shoot, HIT!'

I think hold-over is Kentucky Windage in some circles too...

 

[Boomer]

We shouldn't get too carried away with the idea that dangerous pressures can occur simply by changing the style of bullet we seat above a safe established load. If that was the case the Lyman Manual and a few other manuals would be unsafe to follow because they give load data based on bullet weight rather than for a specific bullet or bullet style. There can be increases in pressure due to longer bearing surfaces and in some cases to changes in bullet material - but to suggest that a safe load will become unsafe by going to a different style bullet within the same weight is a bit of a stretch. If the loads were built right to the limit of that rifle's tolerance, they were not safe to start with. If you make a change, and accuracy becomes erratic, or there are pressure sights then you know that adjustments need to made, but the rifle will not fail in such a scenario.

 

[Splatter]

Boomer, ideed you are right. In fact the new Hornady manual groups all of their .30 cal, 150-to-155 together, all the 165-to-168 bullets together, 178-to-180 together, etc.