Reloading pressure and velocity
This may be opening pandora's box but what is the relationship between powder burn rates and the pressures developed.
It is my understanding that higher pressures lead to higher velocities.
Furthermore accuracy should be a function of the standard error associated with velocity/pressure but it should also have some impact from the ballistic coefficient(BC) of the bullet used. I have noticed that published BCs drop dramatically for many bullet designs when velocities over 2800 fps are used.
When I made up the loads for my 270 accuracy went out the window when I raised the powder charge above the one that produced 2900fps on Zeisloft's chronograph. I did not see any visible signs of incomplete powder burn nor did I see problems with overly high pressures. I wish I had a chrono to tell if the velocities on these loads were consistent. I am left with two hypothesis:
1. Higher powder charge not consistently burning completely. This would be verified by erratic muzzle velocities resulting in poor accuracy.
2. Powder is burning fine but velocity is too high for selected bullet. Muzzle velocities would be consistent but accuracy remains poor.
Solution:
1. Change powder to a faster burner?
2. Change bullet?
It is my understanding that higher pressures lead to higher velocities.
Furthermore accuracy should be a function of the standard error associated with velocity/pressure but it should also have some impact from the ballistic coefficient(BC) of the bullet used. I have noticed that published BCs drop dramatically for many bullet designs when velocities over 2800 fps are used.
When I made up the loads for my 270 accuracy went out the window when I raised the powder charge above the one that produced 2900fps on Zeisloft's chronograph. I did not see any visible signs of incomplete powder burn nor did I see problems with overly high pressures. I wish I had a chrono to tell if the velocities on these loads were consistent. I am left with two hypothesis:
1. Higher powder charge not consistently burning completely. This would be verified by erratic muzzle velocities resulting in poor accuracy.
2. Powder is burning fine but velocity is too high for selected bullet. Muzzle velocities would be consistent but accuracy remains poor.
Solution:
1. Change powder to a faster burner?
2. Change bullet?
posted by steven-hoffman | 2:49 PM
8 Comments:
I have a request for more info. Powder, charge, bullet (weight, type, and manufacturer). Also, have you tinkered with seating depth?
For the most part you are correct…except you forgot one crucial word. Sustained. Higher sustained pressure causes higher velocity. High pressure while it may be high can still leave a bullet stuck in the barrel. And accuracy is a function of the standard error associated with velocity/pressure (V/P), not exactly. In my experience, V/P is secondary to harmonics which is easy to describe but a bit harder to explain. I’m sure you know but imagine your garden hose stretched tight, give er a whip, same thing happens when you fire your rifle. However, approx .014" of movement at the muzzle translates to 1 inch at 100 yds. Combine the movement associated with barrel harmonics and shooter error (think heartbeat, trigger control, etc.) with environmental conditions: It is amazing we can shoot a 1" group in the first place.
The only way I have found to tame harmonics is through seating depth. I seat my “precision” ammo out as far as I can (touching the lands of the barrel).
I’m not sure what you mean by published BCs drop dramatically for many bullet designs when velocities over 2800 fps are used. I find just the opposite is true. Below is published BC for Bullet Name Diameter inches Weight grains Sectional Density Ballistic Coefficients
.277 dia. 135 gr. HPBT MatchKing .277 135 .251 .488 @ 2800 fps and above
.482 between 2800 and 2000 fps
.440 between 2000 and 1500 fps
.390 @ 1500 fps and below
The closer the # gets to 1, the better two bullets with the same BC launched at the same velocity will have the same flight characteristics REGARDLESS OF WEIGHT. The published ballistic coef is a function of diameter, sectional density, and bullet shape. A calculated BC based your data should include the above plus: bearing surface and twist rate (based on the Greenhill Formula), ACTUAL CHRONOGRAPH VALUES, and observed data (ie amount of drop per distance). Inputting published BCs on a ballistic calculator will give a very close approximation to flight characteristics. By inputting your observed data to the program allows you to “back door” your way into the ACTUAL BC. This is the value to be used for accurate drop chart development. Trends in published BCs are true, but exact values may vary.
As for your two hypothesis,
1. Higher powder charge not consistently burning completely. This would be verified by erratic muzzle velocities resulting in poor accuracy.
Possible, approximately what percent of the case capacity are you using? Again what powder? Have you considered changing primers? Seating depth? How are you measuring seating depth, OAL or a measurement from the ogive to the base? The latter is preferred due to anomalies in the bullet tip. By measuring from the ogive, a round nosed bullet may be seated to the same depth (relative to the throat of the rifle) as a FMJ or ballistic tip. Once you find this “proper” depth, it will be the same (+/- .01 or so) for all bullets used. This goes hand in glove with the “taming” of harmonics. I can not stress this enough. Another option may be inefficient burn rate which may be checked by firing at a target at about 5 feet. Look for powder on the target. Consider changing the primer. However, many very accurate charges do not burn the powder completely and still run just fine, others don’t. I still have plenty of research to do on that topic. If you are very precise and consistent in measuring powder and seating bullets to the same depth in cases with a very similar capacity (measured in grains of water), you will have a low SD in velocities. However, if this does not shoot well, there is obviously another problem. This is why I disagree that accuracy is a function of the standard error associated with velocity/pressure. The other problem is harmonics. If the bullet leaves the barrel when it (the barrel) is in a different position you can not expect accuracy.
2. Powder is burning fine but velocity is too high for selected bullet. Muzzle velocities would be consistent but accuracy remains poor.
This sounds logical in my experience. I can scream a 40 gn bullet out of my .22-250 at 4,250fps. But if I back it down to around 4,025, my groups shrink by 3/4". No undue pressure signs. Muzzle velocities would be consistent but accuracy remains poor. Again HARMONICS. This is why most of my rifles are heavy barreled. Minimize the resonance.
The bullets I was looking at that don't like over 2900 fps. I looked at two bullets, the ones I was loading and the one closed to the 130 grain bullets I was loading, dumb luck I guess I didn't pick very good bullets for going fast.
.277 caliber
Sierra 90gr JHP
.195 @ 2800 fps and above
.203 between 2800 and 2200 fps
.219 between 2200 and 1800 fps
.240 @ 1800 fps and below
Sierra 130 gr SPT
.370 @ 2700 fps and above
.379 between 2700 and 2200 fps
.383 @ 2200 fps and below
Maybe there is a reason people use these those Matchkings in shooting matches.
Sierra 130 gr SBT does better faster also.
Hornady doesn't break their BCs down by velocity for the ballistic tip bullets I was considering purchasing:
.370 110 gr V-Max
.460 130 gr SST
58 gr of H 4831 drives a sierra
90 gr JHP at about 2900fps with Winchester primers, shoots into 1.5inches. CCI primers gave same results but the silver primer is less aesthetically pleasing in the brass case.
Same 58 grains of H4831 was the best load of H4831 I could find for the 130gr ballistic tip bullets but I wasn't satisfied with 2+ inches at 100yrds.
58 gr of H 4831 in a .270 cartidge is a supposedly a compressed load but if you aren't seating really deep bullets you can actually squeeze in 60 gr but it don't shoot well.
I followed your previous advice on adjusting seating depth. The same setting leaves hollow points, soft points and ballistic tip bullets a hair back of the lands.
The whole point was I want to change powders and wanted to know if I should move to a little faster powder of a little slower
I would try both. I believe 4831 is rated as the "ideal" powder for the .270, created to the specs of Jack O'Connor, (the Elmer Keith of the .270), however I dont recall if it was H 4831 or IMR 4831(slightly faster). Once I get down there, I'll bring you over some powders to try before you buy. However, If I was ever to make a recommendation for a "general purpose powder" with out a doubt, IMR3031. Great stuff! I buy it by the 10# jug. While suggesting powders, my pistols consume nearly exclusively Unique.
As to the HPBT MatchKings, I love them. I shoot them for about everything but varmints. They hold together well enough to penetrate, and frag well enough decimate. You can buy a slightly better bullet from Berger, but they cost A LOT more. Berger does, however have a higher ACTUAL BC than most everyone else. A lot if the long range crowd grooves on them pretty well. For varmints I either use Sierras Blitz line of bullets or a ballistic tip. Those seem to husk them pretty well, and misses frag in the dirt so minimal worry about ricochets.
Once I get settled in, you may learn a thing or two. I have the technology, we can rebuild them, bigger….faster…stronger. Nananananan Nananananana (sound effect from The $6x10^6 Man)
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