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food for thought

2K views 29 replies 11 participants last post by  Beladran 
#1 ·
As i laid in bed sick last night (i think i was tripping out on to much tylenol pm and sinus meds) i couldnt sleep so i flipped the light on and grabbed my reloading manual and starting thinking of a load for my new 308. I have a springfield armory scope im going to put on it but its bullet compensated for 168gr match loads making 2550 fps.
well i dont have a cronny any more so i figure there has to be a relationship between amount of powder to the ammount of fps 1 gr makes. so i look at 168grbthp in the lapua reloading section go over to N550 and get the suggested start load and divide fps by grains(57.18). then i got the max load and did the same thing(57.81). there was only a .63 fps per gr difference between the start and max load. So i need about 44.5 grains to get me my 2550 fps. But thats not the point of this post. I began looking at other Vihta powders i used and started to compare their gr to speeds. I found that all the NN500's have the least gap between start and max, while the NN100's had the greatest gap (3fps per gr). also noted that the 100's made more fps with less powder and the more powder you add the lower the fps per gr went. With the 500's it was the exact opposit. :thinking: :scratch: maybe if you look at other powders this trend will show up to. I bet i could graph this out an show a point of diminishing returns on some of these powders and show what a "optimum" powder charge would be... lol this is gonna sound stupid but i think the heavier the bullet the more effecient the powder burns :scratch:
 
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#4 ·
I believe the BC has more to do with it than weight. You should load a similar powder charge behind two different shape but same weight bullets. For example, I load some 220 gr. Sierra Matchkings for my .30-06. I would like to know the result of the same powder charge behind a 220 gr roundnose.
 
#7 ·
thats like comparing a dart to a brick.. i looked at the 06 bullets 167,185,190 hpbt and the heavier the bullet the lower the fps per grain goes which is the opposite of a 308 case? idk now im confused and my head hurts lol 308 is more effcient with heavier bullets than 06 is??
 
#10 ·
ok i did this on a scratch pad going by the numbers on lapua website for both N550 for 06 and 308 using 150gr sierra hpbt to make 2700fps out of each

308 --47.42
30-06 -- 54.46

308 makes 56.94 fps per gr of N550
30-06 makes 49.58 fps per gr of N550
granted this math was done in my head so add a fudge factor of 10% lol
 
#13 ·
jbpmidas said:
.308 is much more efficient for that weight and powder! How about some other comparisons?
True, but a .30-06 with imr 4350 and that bullet just shines! Keep it out of an M1 garand though, it will bend the op rod. Ultimately as proven out in the national matches, a .308 is more accurate than a .30-06, they had to shrink the bullseyes down because everyone was cleaning the targets.

As to the 220 spire vs 220 round nose, you really need to look at how much of the bullet bears on the lands.

Hope you feel better soon!
 
#18 ·
ftsibley said:
I think i may go dose myself with a heavy shot of Nyquil and re-read this post, bet i will figure it all out once the Nyqil kicks in.
:lol:
 
#20 ·
jbpmidas said:
I wonder if the comparison should be % of case capacity instead of grains. The '06 is at a disadvantage when being compared to the .308 because of its increased case capacity.
ok after re reading my ballistics book this is how you calculate cartridge effeciency...
general rule of thumb if you dont know what the energy per grain is of a powder use 1gr of powder =200 ft/lbs of energy
your standard 30 cal M-2 ball ammo has 50gr of powder and has a muzzle speed of 2,800fps and a muzzle energy of 2,643 ft/lbs. its powder produces 178ft/lbs per gr of powder so multiply that times powder used(178x50=8,900ft/lbs) then find percent 2,643/8,900x100=29.7% that is how effecient that is
 
#21 ·
You had me at "Hello!"

I think I understand what you are saying. So, how much of the same powder does it take to push a 150 gr. .308 to 2800 fps? If it only took 45 grains, that would be 90%. 29.7% divided by 90% is 33%. So, is the same 150 gr. bullet out of a .308 using 45 gr. of the same powder and travelling the same 2800 fps, 33% efficient?
 
#23 ·
na na na ft/lbs is what you need to look at not fps. im gonna pull a load from speer reloading manual and do this by the numbers in the book 168gr SMK with 46gr of imr 4064 making 2724 fps= 2768ft/lbs... now we do the math again(46gr x 200=9,200 ft/lbs) now take what it makes at the muzzle 2768 / 9200 x 100 =30%

now lets flip a few pages to the 30-06 using a 168 SMK and 49gr of IMR 4064. that load is going to make 2549 fps and 2424ft/lbs 49gr x 200 = 9800 divided 2424ft/lbs by the possible 9800 ft/lbs x 100 = 24%

fyi meds are wore off but gentlemen jack is my nurse now ;)
 
#24 ·
jbpmidas said:
You had me at "Hello!"

I think I understand what you are saying. So, how much of the same powder does it take to push a 150 gr. .308 to 2800 fps? If it only took 45 grains, that would be 90%. 29.7% divided by 90% is 33%. So, is the same 150 gr. bullet out of a .308 using 45 gr. of the same powder and travelling the same 2800 fps, 33% efficient?
ok the M2 ball has 50 gr of powder making 2800ft/lbs if you reduce it by 5 gr to 45gr and maintain the same ft/lbs then yes that is more effecient than having the 50gr. like always there comes a point were your just pissing away powder if a load is going to be its best at X then why push it higher to Y?

lol lets look at a big boomer the 300 ultra mag
168gr SMK with 101gr of retumbo making 3281 fps and 4016ft/lbs.. 101gr x 200 = 20200ft/lbs divide 4016/20200x100=19% lol talk about pee'ing in the wind
 
#26 ·
This would be a great problem for my physical chemistry class (my undergrad degree is chemistry), or chemistry + calculus + physics to figure how a reaction proceeds when you change one or more variables. There isn't going to be a standard 1:1 correlation between gr. weight and velocity; there are many factors going on over a "curve": case thickness and therefore capacity; temperature, density of charge vs. total internal case capacity(ie how much "air" left in case), burn rate at different concentration of powder/air ratio; primer, and then ballistic coefficients, rifling, steel composition of barrel (flex, resistance, etc.). Confused yet?
 
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