Thread: ballistic calculator's can be wrong

I have been working up loads for my Churchill .308 with a 1:12 twist (iron sights 18 inch barrel non suppressed) according to the Berger online calculator 150grn spitzer pills are marginally stable while the 150grn boat tail were better, so loaded up some cases with 150grn Winchester PowerPoints and nosler 150grn ballistic tips both loaded at book minimum and out to the range i went expecting the nosler to be the pick, but at @ 50yrds the PowerPoints grouped 2inches and the nosler were all over the place best i could get was a 4 inch grouping, Anyone else had the same experience?

Yeah but not due to rotational stability issues - always due to a mechanical issue. Shoots 150gr factory OK? I have used 165gr boat tail in 1-13" twist so the twist shouldn't be the issue?

Originally Posted by No.3

Yeah but not due to rotational stability issues - always due to a mechanical issue. Shoots 150gr factory OK? I have used 165gr boat tail in 1-13" twist so the twist shouldn't be the issue?

haven't tried factory, its only intended for bush huntin so 50yrds would be a long shot

They're probably going too slow. Ramp them up a bit.

Originally Posted by Shearer

They're probably going too slow. Ramp them up a bit.

will load some more and try it. i only started low as i didnt want to strain the 80 year old mauser action.

The length of the bullet its speed and the rate of twist determine whether the projectile will stableise or not. This has been known since the 1860s and is encompassed In the Greenhill formula. Speeding up a marginally stable bullet makes it more stable till the velocity drops. You can get good groups at 100 m but fail to find a hole in an a4 at 200 I know this from experience. Conventional cup and core bullets like your winchesters generally give less trouble than hybrids designed for long range or improved ballistic coefficent as they are more compact.
One other thing if the throat of the rifle is a bit worn you generally find boat tail bullets preform worse than flat based ones. Your rifle will probably be best with the normal fodder the likes of Sierra pro hunter or gamekings especially if it's prime purpose is hunting or targetry out to 500 or so metres.
That's my 2c

T12 should be fine... @Marty Henry as above probably not enough speed....chronograph some factory and if groups better load to that speed. Berger barrel twist calculator is not a ballistic calculator.

I have not found a ballistics calculator that is not correct....if any error in trajectory is found it mostly is because the velocity input is not correct...the old story with computers is BS in, BS out.

If you want a mild pressure load for short range, load 30-30 projectiles at 308 book minimum. They will generally be doing a bit more than 30-30 speeds and have no problem stabilising.

@Marty Henry I knew someone would have a explanation, the throat will be worn(its a old Rifle),When i ran the calculator i used the same muzzle velocity for both pills and would'nt have factored in the condition of throat etc hence the different results compared to the calc, going to stick with the powerpoints as most of the time ill only be shooting 25/30 yrds max, so i have some 150grn nosler spare if some one want em:

never heard of a .308 that wont stabalize a 150grn projectile!!!!! before now....... most will happily digest 180s and will do ok with a 200 round nose too..... try a faster load.... middle of road book data wise and see what happens... speer flat based pills SHOULD be better still if bore is worn/tired as they in theory fill the grooves in bore (some fancy word like obliturate??/) better than other designs. of seen as though you are reloading and not wanting a hot load...try some of shootersnz great cast projectiles....

Mathematical calculations are pure. It is unlikely that the problem is with a calculator.

Greetings @Hbwanderer,
If it is a bush gun and 50 metres is a long shot ditch the spitzer and definitely the boat tail projectiles, Try 150 grain round nose projectiles. My experience with 150 grain boat tails is they often don't shoot that well if the throat is worn or long, You don't mention the powder but I have used from 38 to 45 grains of AR2206H with decent results. Stay away from ball powders with reduced loads. Post some more data on the components you have and I can provide some suggestions.
Regards Grandpamac,

Another thought. Is the rifle new to you and if so have you checked all the screws and mounts and the bedding? .308 rifles, even well worn ones, must be the easiest on the planet to get to shoot well.

@grandpamac, the load was 44grns of Varget pushing 150grn Winchester powerpoints,Used those because i have heaps of them, Dont get me wrong i was happy with the grouping (2inches @50yrds) 50 yrds will be a long shot for the bush i hunt.
My whole point for the thread was online calculators and real world results sometimes can be complete opposites, and yes the rifle is new to me but was in a sad state when i came across it,it had had a hard life,Made circa 1960, the throat is more than likely worn as i had to cut 2 inches of the barrel (muzzle end) before i found any decernable rifling,

Originally Posted by Hbwanderer

@grandpamac, the load was 44grns of Varget pushing 150grn Winchester powerpoints,Used those because i have heaps of them, Dont get me wrong i was happy with the grouping (2inches @50yrds) 50 yrds will be a long shot for the bush i hunt.
My whole point for the thread was online calculators and real world results sometimes can be complete opposites, and yes the rifle is new to me but was in a sad state when i came across it,it had had a hard life,Made circa 1960, the throat is more than likely worn as i had to cut 2 inches of the barrel (muzzle end) before i found any decernable rifling,

Ballistic calculators make a lot of assumptions and can be well wide of the mark. I prefer to use tested data together with a chronograph for load development. One thing that I have noticed with data for the .303 is that it varies widely with pressure tested data giving considerably higher than that from Lee Enfields. I often wondered if the difference came from throat erosion. Measuring the crush length for 174 grain RN was around 6mm longer on a used but in good condition barrel than two near new barrels. I still have to chronograph the same load to see how much difference this makes.
GPM.

At 50yds with a .308 shooting mid-weight bullets with a sensible load of 2208, they should be shooting way sub-inch. Is the barrel/crown very tired or possibly pitted? Has someone looked down it with a borescope- could well be badly fouled? Is the bedding sus? Ballistic considerations would be waaay down the list for me.