why the big diff (in load data) between SPBT and HPBT

A

AMMASHOOTA

Gary Gruber
I am working up a load for my new .308 and am using 150g Sierra HPBT and 150g Scirroco SPBT bullets.

i'm a bit partial to the Scirroco's but i have found that the Sierra's are working well with VV N150.

i've decided to compare both bullets side by side and was surprised that the VV load manual shows such a wide discrepancy in starting loads between the two bullets.

the Sierra load starts at 42.5g and the Scirroco starts at 44.4.

why is there such a huge discrepancy for the same weight bullet?
 
IMO it's mainly because no one has the inclination to go through all the reloading manuals and even everything out. If you have a chronograph you can try both bullets using say the 42.5 if you wanna' be safe, and you'll see very little difference.

Two items.....

#1- keep GOOD notes when checking this stuff and,
#2- WEIGH each load as you're working up. DO NOT trust your powder thrower.

I think that once you follow these simple rules it becomes apparent that simple physics is at work here. You will find that bullet weight dictates powder charge and very little else.

:)

opinionsby





al
 
thanks. that is what i have been doing. i ran my first ladder between 42.5 and 44.0g with the Sierra. It started to come together precisely at 44.0g. that's when i decided to try both bullets with the same powder, from 44.3 up to 45.8. That's when i noticed the diff in the manual.

i use an RCBS Chargemaster combo -- so all loads are precise as can be.
 
AMMASHOOTA said:
I am working up a load for my new .308 and am using 150g Sierra HPBT and 150g Scirroco SPBT bullets.

i'm a bit partial to the Scirroco's but i have found that the Sierra's are working well with VV N150.

i've decided to compare both bullets side by side and was surprised that the VV load manual shows such a wide discrepancy in starting loads between the two bullets.

the Sierra load starts at 42.5g and the Scirroco starts at 44.4.

why is there such a huge discrepancy for the same weight bullet?
Click to expand...

Try looking at the BEARING surface, betcha the Soroc is much shorter than the Sierra!
Clarence
 
So how did they compare over the chrono?

I've found bearing surface, bore condition, lubrication etc to be non-issues. As Vaughn and Rinker have repeatedly stated, total bore friction is so far down in the dirt that differences in bore friction are nearly immeasurable. Now the difference between a lead core bullet and the same weight metal bullet can be noticeable but IMO that can be attributed to the reduced powder reservoir caused by the length difference of the all metal bullet.

al
 
AMMASHOOTA said:
I am working up a load for my new .308 and am using 150g Sierra HPBT and 150g Scirroco SPBT bullets.

i'm a bit partial to the Scirroco's but i have found that the Sierra's are working well with VV N150.

i've decided to compare both bullets side by side and was surprised that the VV load manual shows such a wide discrepancy in starting loads between the two bullets.

the Sierra load starts at 42.5g and the Scirroco starts at 44.4.

why is there such a huge discrepancy for the same weight bullet?
Click to expand...

Different makes of bullets may have different composition of jacket or a different bearing surface and one may 'drag' harder in the bore increasing pressure. Different makers obviously use a different test rifle as well. Chamber and throat probably not cut the same. One maker being more cautious...
 
cdupuy said:
Try looking at the BEARING surface, betcha the Soroc is much shorter than the Sierra!
Clarence
Click to expand...

One other thing i will add: The Scirocco is 0.001 larger in diameter than the Sierra. I have to use a larger bushing in my Redding sizer to avoid extra neck tension.

and, as you have pointed out, the bearing surface is quite a bit shorter.
 
Every powder-barrel combo will show different pressures with different styles of bullets of the same weight., even if a lot of other things are equal.

Example. I can take a 6mm 68 grn OO Bruno Boatail in a Krieger barrel, and place enough 133 behind it to go 3400 fps. I can then take and load a Fowler or Watson 68 grn behind the exact same load, and it will probably top 3470.

Same weight bullet, so what's the difference. It's the Watson or Fowler is about .0003 larger in diameter, and has about 30 percent more bearing surface. This increases the initial pressure curve, hense more velocity.

Any time I work up loads, (such as with my 30-06 project), I always set up my Chronograph, and work up untill I hit a velocity that is an industry accepted figure for the bullet weight cartridge combo, and work from there.

For instance, with the 178 grn A-Max, I knew that 2650 fps was certainly well within the acceptable pressure range with the 27 inch barrel and 4831.. So, I loaded to there, and then stared to tune up, while watching for any pressure signs such as outside primer cup radius or any bolt lift that my instincts tell me is questionable. It turned out that out of the 27 inch barrel, the 178 hit 2780 with no problems with great accuracy.

Load development requires, above all else, common sense. For instance, if your 308 suddenly starts getting 30-06 velocities, that should tell you something real quick. And remember, Loading Manuals are nothing more than well informed suggestions tempered with practicle caution.
 
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Just a guide

To be honest I have rarely consulted a loading manual. I look at what they recommend as a maximum for that powder type and bullet weight. Start about 15% below that and work up until pressure signs show up. Back up from that taking the ambient temperature into account. Ie develop on a cool day near max will probably be stressing things on a hot day because of the higher pre ignition temperature of the powder.

Then fiddle with all the variables until it shoots. I don't ever sacrifice accuract for velocity.
Andy.
 
AMMASHOOTA said:
One other thing i will add: The Scirocco is 0.001 larger in diameter than the Sierra. I have to use a larger bushing in my Redding sizer to avoid extra neck tension.

and, as you have pointed out, the bearing surface is quite a bit shorter.
Click to expand...

I am now shure that the load difference is due to the Bearing surface AND the 309 bullet diameter.

Clarence
 
Without extensive experimentation it is hard to know precisely what causes something. When two things happen at the same time, does not necessarily prove that one caused the other.

I am one who only considers loading manuals as a source for a safe starting load, and a list of powders that may be worth investigating. I do my load workups at the range, and the first thing that I do is to run a one shot per load pressure series, shot over flags, on the same target, on a day when the wind is not too bad. It seems to me that shooters who go to a book, or the internet as a substitute for doing some work themselves are kidding themselves, and in some cases taking a real chance with their personal safety.
 
Down here in the So Cal desert, we tend to keep our loads a bit lighter due to the temperature extremes. it is not unusual to be shooting anywhere from 60-105 degrees here. I try to develop loads toward the end of summer when the temp is higher, and then shoot through the cooler months.
 
AMMASHOOTA said:
Down here in the So Cal desert, we tend to keep our loads a bit lighter due to the temperature extremes. it is not unusual to be shooting anywhere from 60-105 degrees here. I try to develop loads toward the end of summer when the temp is higher, and then shoot through the cooler months.
Click to expand...

Ohh, but if you choose to believe the "SuperFormance" hype that problem will soon be a thing of the past!

"free velocity and resistance to temperature sensitivity"

LOL

al
 
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