What contributes to the "sweet spot" with a load?

[Snakum]

I finally nailed a load (that sounds weird

) that matches HSM 168gr BTHP in my weapon using IMR4895. I used 43.0gr for my commercial brass (Win, PMC, etc.) and 42.6gr for my Lake City and TAADS brass and it shoots right with the HSM to 400 yards so far, and the handloads group even better at longer ranges. Taking my time for a good setup, and in zero wind, today I shot a few 1/2 MOA groups with the reloads. My questions:

1.) The Win and PMC brass wasn't shooting so well at 42.5 despite meticulous care in case prep, weighing charges and loading. Then it tightened up at 42.7 and then started shooting lights out at 43.0 grains.

What contributes to, or causes, this sweet spot in the powder volume? I.e. why would one load shoot so-so then adding .5 grains make it shoot like a laser?

2.) The Lake City and TAADS cases seemed to shoot exceptionally well, better than the HSM and my handloaded commercial cases. Across three sessions using all three ammo types, each session including three-shot groups with each ammo at 100, 200, 300, and 400 yards, the milspec brass shot about 20 - 30% tighter than the other brass. Most of this brass is once fired, and there should be no differences except case volume and powder charge.

Would the brass itself contribute in some way to accuracy of the load, or this is strictly the powder charge finding a sweet spot in my weapon (as in number 1.)?

 

[Quackaddict]

Re: What contributes to the "sweet spot" with a load?

I personally think it has to do with getting to that sweet pressure spot in each barrel, if you can mimic that with each different bullet and brass choice you will get good accuracy. But I have no way to test this and will probably be told 1000 ways why this cant be.

 

[Short Round]

Re: What contributes to the "sweet spot" with a load?

Well I don't know if you can say what's happening inside to make it accurate.

We can say what details we control to make it accurate.

A lot is attributed to barrel vibration nodes. Your sweet spot rings the barrel so that the vibration is at the node at the time the bullet exits the muzzle. This avoids the bullet leaving at either end of barrel's whip action.

All I can say is pressure and timing and everything that affects them.

 

[DBohn]

Re: What contributes to the "sweet spot" with a load?

The right powder volume is going to make a difference as well as far as what amount gives the most consistent velocities. If you would have shot all those loads over a chronograph I bet the ones that shot the best groups would have the lowest SD.

 

[Unsichtbar]

Re: What contributes to the "sweet spot" with a load?

There are methods to developed loads; one is the OPTIMAL CHARGE WEIGHT method http://optimalchargeweight.embarqspace.com/ which IMO is the best. After you find the common POI point of impact and the optimum charge weight, you then will tune the OCW load by ajusting the seating depht.

Might seem a little confusing a first but it real is simple and fast. In about 20 rnds you will learn allot read and study the information from the link then do a search here, there are several threads. If you decide to use this method do it like it is written until you have a couple of years loading under you belt.

http://practicalrifler.6.forumer.com/ Is a web site that people who use this method post to to exchange information, you can post if you need help with results of you test. Its back to the old thing you have to do the work to understand the process. If you just select loads from other peoples work you will miss an oppertunity to learn a lot.

 

[DaveV]

Re: What contributes to the "sweet spot" with a load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Short Round</div><div class="ubbcode-body">A lot is attributed to barrel vibration nodes. Your sweet spot rings the barrel so that the vibration is at the node at the time the bullet exits the muzzle. This avoids the bullet leaving at either end of barrel's whip action.

All I can say is pressure and timing and everything that affects them.</div></div>

+1 Shock wave theory

 

[Snakum]

Re: What contributes to the "sweet spot" with a load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Unsichtbar</div><div class="ubbcode-body">There are methods to developed loads; one is the OPTIMAL CHARGE WEIGHT method http://optimalchargeweight.embarqspace.com/ which IMO is the best. After you find the common POI point of impact and the optimum charge weight, you then will tune the OCW load by ajusting the seating depht.

Might seem a little confusing a first but it real is simple and fast. In about 20 rnds you will learn allot read and study the information from the link then do a search here, there are several threads. If you decide to use this method do it like it is written until you have a couple of years loading under you belt.

http://practicalrifler.6.forumer.com/ Is a web site that people who use this method post to to exchange information, you can post if you need help with results of you test. Its back to the old thing you have to do the work to understand the process. If you just select loads from other peoples work you will miss an oppertunity to learn a lot.</div></div>

Wow, that was a pretty interesting read. I had to do things a little differently because I was trying to duplicate HSM Match, which I have a lot of and which shoots very well for me. Also had to duplicate it with commercial brass and milspec brass since I have a lot of both. At some point I'm sure I'll be using only handloaded ammo and will be developing one standard target load and one standard hunting load that works well in all my .308s. And the OCW concept seems it would work well for that.

Thanks for the info on barrel nodes/shockwave. That makes a lot of sense and actually explains some things I've seen while testing.

 

[Grump]

Re: What contributes to the "sweet spot" with a load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DBohn</div><div class="ubbcode-body">The right powder volume is going to make a difference as well as far as what amount gives the most consistent velocities. If you would have shot all those loads over a chronograph I bet the ones that shot the best groups would have the lowest SD. </div></div>
BWAWHAHAHAHAHAH!!!!!

RARELY the case for me.

My last sub-MOA group was with the crappy USGI Match 7.62 brass, ordinary and old CCI 200 primers, and the only case prep was deburring the flash holes...I think, gotta check. 0.6 something inch extreme spread in the group, 2591.8 fps, ES of 55.7, sd of 21.3.

Weighed charges of 4064.

Same load, in newer 2x-fired FC brass, flash holes deburred and case necks uniformed (most cut about 1/2-way around the circumference). Nice velocity ES of 16.1 fps, avg vel 2581.3 fps, and sd of 6.55.

The more velocity-uniform load from "better" brass shot to 1.1 inches.

Almost every time I test loads, the one with the lowest velocity ES and sd is usually middle of the road for accuracy. With 175s, that's at 22-inch barrel velocities from 2510 to 2630 fps.

Had one 155-gr A-Max load the same day that screamed out at 2969.3 fps, ES of 7.3, and an sd of 3.05 fps. Besides making the last primer blow a puff of gas (and loosening only that case's primer pocket), that load shot to an amazing 3.5 inches extreme spread between bullet impacts at 100 yards. Spherical powder, weighed charges too.

I have velocities and extreme velocity spread standards I aspire to attain with any load I use, but I just won't worship at the altar of the chronograph.

 

[wfjames22]

Re: What contributes to the "sweet spot" with a load?

+1000 for Grump's comments.

I like to strive for low sd's and es's, but more often than not my loads with the lowest numbers dont shoot the best groups at different distances.

 

[Bobcape]

Re: What contributes to the "sweet spot" with a load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: wfjames22</div><div class="ubbcode-body">+1000 for Grump's comments.

I like to strive for low sd's and es's, but more often than not my loads with the lowest numbers dont shoot the best groups at different distances. </div></div>

<span style="font-family: 'Comic Sans MS'"> <span style="font-size: 11pt">There was a very good article in <span style="text-decoration: underline">Precision Shooting</span> magazine (the benchrest magazine) about 2 years ago that went in to great detail about this. Basically it proved that ES and SD are not reliable indicators of accuracy. Always good to try for, but don't discard a load with mediocre SD that shoots well!

Bob</span> </span>

 

[Fuzzball]

Re: What contributes to the "sweet spot" with a load?

"What contributes to, or causes, this sweet spot in the powder volume? I.e. why would one load shoot so-so then adding .5 grains make it shoot like a laser?"

I've have an impression that it's probably some kind of magical forces. I don't care what kind so long as I can duplicate the load and the results stay the same. Or maybe it's barrel viberations and bullet exit timing, but magic seems more logical.

 

[DBohn]

Re: What contributes to the "sweet spot" with a load?

If you're shooting groups at 100 yards velocity differences won't matter much but at 600+ yards if your loads are +/- 50fps you won't be getting very good results. Back up and try your loads.

Read the optimal charge rate article that is posted on this thread it says loads with larger extreme spread can group tight <span style="font-weight: bold">AT CLOSE RANGES</span>.

 

[Fuzzball]

Re: What contributes to the "sweet spot" with a load?

"... loads with larger extreme spread can group tight AT CLOSE RANGES."

True. But that presupposes 300-400 yards is close range. That's really a long shot for most of us so "normal" ES means little or nothing to most of us.

 

[ChadTRG42]

Re: What contributes to the "sweet spot" with a load?

I've never had much luck with the OCW method. But I know 1 thing from testing lots of loads. I watch the chrono just as much as I watch the group size. There WILL be a sweet spot at or near the lowest ES/SD numbers. It might be the load with the lowest numbers, or the one just before of after it, but it will be there. One of these days I want to test a rifle with only using the chronograph.

 

[The Mechanic]

Re: What contributes to the "sweet spot" with a load?

You may also have some neck tension differences as well as neck thickness differences. Remember you can get into cumulative error. Take four variables (there are a lot more than four but this is just an example) neck tension, case volume, charge weight, seating depth. Now lets say we have a loose neck tension from annealing and turned necks, some thin brass (higher internal volume), some light charges (only a little but all light) and a lower seating depth. All this will lower pressure and velocity and give you a lower shot. Now you change brass and go all those in a different direction and you have a high shot and you say WTF? what happened?

 

[DBohn]

Re: What contributes to the "sweet spot" with a load?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fuzzball</div><div class="ubbcode-body">

True. But that presupposes 300-400 yards is close range. That's really a long shot for most of us so "normal" ES means little or nothing to most of us. </div></div>

IMO 400 yards is already too far to be shooting high ES and SD loads. Let's look at Grump's .6" load with 55.7 ES and 21.3 SD. I used the ES to run it out on a ballistics program instead of the SD because usually people don't shoot enough rounds to get an accurate SD. But anyway here is the vertical dispersion that can be expected with this ES at extended ranges.

400 yds - 1.83"
500 yds - 3.27"
600 yds - 5.29"
700 yds - 8.03"

This is only vertical dispersion from inconsistent velocities. Nothing else has been taken into account like wind, sight picture, trigger control etc. This would not be acceptable for me. Already .62 MOA at 500 yards and over 1 MOA at 700 yds. Definitely not good for long range. By the way I wasn't sure if he used 175 bergers or matchkings but they have a similar G7 BC so I went in the middle with .250. Bergers would perform a little better than listed and matchkings a little worse.

If long range isn't your game ES and SD aren't that important but for long range accuracy they are essential. Consistency is key.