Accuracy=seating depth or tenths of powder

[Mikecr]

I agree with phorwath, and that graph supports his contention if nothing else.
The little 10fps 'average' variances about a 20thou change in seating depth -is insignificant.

It is of possible for seating depth changes to affect MV enough to miss-shape grouping or even to throw us out of a powder node. This could be a fully filled case of very fast powder(6PPC, 30BR, etc.), or donut issues mentioned.
So in that sense it's hard to lay down a universal rule. But it's not hard to counter common misconceptions about normal deeper seating adjustments causing pressure 'problems'.

 

[barefooter56]

Phil....

What works for me is those 3 x 5 'Post It Notes' with the sticky edge on the backside and Frankfort Arsenal ammo boxes (flip top type) for every caliber,,, I use 308 boxes, sometimes the rounds fit, sometimes they go in upside down, but they go ...

That way I can access the previous loads and compare jumps, propellant load, basically everything from one session to the next and everything is attached to the inside lid of the respective ammo box. I actually leave all the notes inside a small plastic zip bag that stays with the rifle so I have everything 'at hand' all the time.

I've also found (from trial and error) that there are no shortcuts to achieving accuracy and consistency. Everything from the initial case prep to the actual sizing and bumping of the shoulders to the seating and neck tension to the concentricity of the pill in the case neck, all impact grouping and accuracy and especially consistency....

From my hands on experience, being consistent and meticulous in the way you load and adhering to your (Berger) regimen, yields the best performance and best groups.

I built loads for 3 rifles this summer, 2 custom builds and one off the shelf and all are shooting sub moa at 200 yards very consistently,,, and none of the 3 rifles jump even close to each other (something I would have never found out without using the Berger regimen)... Heck, I'd still be fiddling around with less that desireable results.

IMO, chambers are like fingerprints, no 2 are the same.

SidecarFlip,
Good idea with the cards! Keep in mind though fellas that not all shooters are so experienced or technical in their reloading procedures. We should not overload them with information but give it to them as they ask/get more experience. IMHO new reloaders should be given a view of a series of hills to get over. They should not be looking at Mt. Everest. Also keep in mind that some of us look for accuracy on the reloading bench while others look for accuracy on the range (practice). Some do both. But what percentage of time is /should be spent at each? That is up to the individual and what makes him/her comfortable with their load so they can concentrate on the shot.
Just my 2 cents.

 

[SidecarFlip]

Phil..

I have to agree...

I know hunters that shoot noting but factory loads and do just fine (have their walls loaded with trophy mounts) so just because it works for 'me' or you don't mean it's the holy grail...

For me, handloading is a challenge in getting the most from a rifle / hangun in repetitive accuracy, but fror some, a trip to the local gun shop and a couple boxes of manufactured rounds is enough....

 

[Bart B]

Anyone using a single few-shot group (5 or less) to assess accuracy won't get groups the same size shooting that same load over and over again. They'll range from about 3 times as big to almost half its size. Shoot several of them with the same load if you don't believe me. Which is why benchrest rifles shooting 5-shot groups at 100 yards see them range from near 1/100th MOA to 3/10ths MOA. The several group aggregate record at 100 yards is about 2/10ths inch; the average of all group sizes. So, one or more groups was larger than 2/10ths inch. Statistical odds say the smallest one may well have been much smaller than 1/10th inch.

Humans holding onto recoiling rifles add more accuracy degrading things than the set of components will do. That's why benchresters shoot their rifles in free recoil; they know if they hold onto them, the groups will be a lot bigger.

A 100 fps spread in muzzle velocity for a given load in a given rifle as shot by several people is normal. Hand held rifles have a 2 to 4 times spread in muzzle velocity than fixed barreled actions; thanks to Newton's law and us humans not holding the rifle the same way for each shot.

A given load's accuracy in MOA will increase about 10 to 15 percent for each hundred yards of range past the first one. It's caused by a small spread in bullet BC, subtle cross winds and muzzle velocity spread.

I've never seen a significant accuracy change for a 3/10ths grain spread in charge weight through 600 yards. Such ammo's easily shot sub 1/2 MOA at 600 yards. Sierra Bullets' best match bullets have that much spread in charge weights testing them and they all shoot sub 1/2 MOA at 200 yards; typically around 1/4 MOA.

The throat erodes away .001" for every dozen or so rounds fired. About every 3 dozen rounds for a .308 Win, 1 to 2 dozen rounds for most magnums. Accuracy changes very little across a .070" increase in bullet jump to the rifling if all other parts of the ammo are good.

For bottleneck cases headspacing on their shoulder, it's the distance from the shoulder to the rifling contact point on the bullet that counts. All such rounds have a few thousandths spread in case headspace and their shoulder stops against the chamber shoulder; there's a spread in head clearance (bolt face to case head) for such ammo. So, don't fret all that much about a few thousandths spread in case head to bullet ogive measurements; that's not what is important in the first place.

 

[Pdvdh]

Nope. MV will decrease as you seat the bullet deeper in the case - farther off the lands. At least within the seating depth ranges almost anyone would ever employ.

Measuring MV vs Seating depth doesn't produce results which are "always the same" as you contend. Here is a chart of three 6mm BR bullets I tested not long ago. The muzzle velocity is the average of 10 identical rounds at each of the listed seating depths.

Your chart shows you tested very small changes in bullet seating depth, and you're measuring MVs with an instrument that has limits of precision. You've also tested MV with bullets seated very close to touching the lands - the tests you ran at 0.000" might as well be removed from your graph, since the slightest error in measurement or bullet construction will mean bullets touching versus not touching the lands. This is a bullet seating testing depth which needlessly adds error for the purpose of definitively and conclusively correcting me. No small variance in seating depth will change MV more so than a small change placing the bullets into contact with the lands versus a small change removing contact with the lands. You're testing changes in seating depth that could be statistically meaningless, because you don't have the instrumentation to accurately control the seating depths and record the results. In other words, you could run your same tests again and the background noise associated with your seating depth changes and MV measurements could produce a different authoritative "chart". Changes in powder charges, primer intensity, neck tension, internal case capacity, and the temperature of the cartridges and barrel/chamber are examples of additional factors that will add to the background noise limiting one's ability to draw meaningful conclusions - conclusions based on meaningful changes/trends in MV.

Lastly, you never stated your position on the question as to whether MV increases or decreases with bullet seating depth. Leaves one with the impression you were so focused on refuting my correct position, that you never shared anything useful, other than small changes in bullet seating depth of +/-0.005" may or may not result in a change in MV that can - statistically, reliably, repeatedly, and meaningfully - be detected and reported.

Which begs the question; do you have a position to share as to whether substantial changes in bullet seating depth - seating the bullet deeper into the casing (farther away from the lands) - increases or decreases MV? Answer that question in a meaningful way, and you'll have contributed something useful. Qualify your answer in a manner that restricts the changes in seating depths to minutia such that we lack the tools and instruments to measure reliable trends in MV - higher or lower - and you'll have accomplished only one thing. Nit picking.

 

[Bart B]

Regarding various distances bullet's jump to the rifling and pressure issues, check post #5 in:

https://www.shootersforum.com/balli...ullet-seating-depth-pressure-correlation.html

 

[Pdvdh]

Two charts illustrating chamber pressure measured while firing two different bullets at different seating depths in a 30-06, from researcher L.E. Brownell (1965). Powder charge was held constant. Only seating depth was altered. Within all reasonable bullet seating depths used by long range hunters, pressure decreased as the bullet was seated deeper into the cartridge casing (seated further away from the lands).

Pressure began to increase only with bullets seated WAY into the casing (1/4" to 1/2" off the lands in these two examples), due to the large volumetric reduction in case capacity. When the 150gr bullet was seated more than 1/4" off the lands, and the 220gr bullet was seated more than 1/2" off the lands, internal case volume was displaced by the bullet to the point that chamber pressures began to increase. But the pressures never rose as high as the pressures measured when the bullets were seated in contact with the rifling.

 

[Mozella]

Your chart shows you tested very small changes in bullet seating depth, and you're measuring MVs with an instrument that has limits of precision. You've also tested MV with bullets seated very close to touching the lands - the tests you ran at 0.000" might as well be removed from your graph, since the slightest error in measurement or bullet construction will mean bullets touching versus not touching the lands. This is a bullet seating testing depth which needlessly adds error for the purpose of definitively and conclusively correcting me. No small variance in seating depth will change MV more so than a small change placing the bullets into contact with the lands versus a small change removing contact with the lands. You're testing changes in seating depth that could be statistically meaningless, because you don't have the instrumentation to accurately control the seating depths and record the results. In other words, you could run your same tests again and the background noise associated with your seating depth changes and MV measurements could produce a different authoritative "chart". Changes in powder charges, primer intensity, neck tension, internal case capacity, and the temperature of the cartridges and barrel/chamber are examples of additional factors that will add to the background noise limiting one's ability to draw meaningful conclusions - conclusions based on meaningful changes/trends in MV.

Lastly, you never stated your position on the question as to whether MV increases or decreases with bullet seating depth. Leaves one with the impression you were so focused on refuting my correct position, that you never shared anything useful, other than small changes in bullet seating depth of +/-0.005" may or may not result in a change in MV that can - statistically, reliably, repeatedly, and meaningfully - be detected and reported.

Which begs the question; do you have a position to share as to whether substantial changes in bullet seating depth - seating the bullet deeper into the casing (farther away from the lands) - increases or decreases MV? Answer that question in a meaningful way, and you'll have contributed something useful. Qualify your answer in a manner that restricts the changes in seating depths to minutia such that we lack the tools and instruments to measure reliable trends in MV - higher or lower - and you'll have accomplished only one thing. Nit picking.

No, I bow to your superior knowledge.

 

[Pdvdh]

No, I bow to your superior knowledge.

Well then, for the record..., that's what causes me to chuckle.

 

[Bart B]

phorwath, thanks for the informative post on pressure vs bullet seating. Most interesting were the strain gauge PSI values; virtually unheard of back in 1965 when copper crusher gauge systems were the norm. In the .30-06, SAAMI's max average PSI spec is 60,000; for CUP, it's 50,000.

It's always interesting to me when someone has the wherewithall to measure stuff and get real numbers instead of guessing.

 

[NATE40]

Effects of COAL and CBTO | Part 1 | Berger Bullets Blog

This was not printed in 1965 its 2013 .today I shot my loads at my COAL without any problems. So going off what has been posted I took my load and only seating bullets deeper at 1/1000th deeper progressive to 5/1000th deeper stopped shooting at 3/1000th due to pressure. Using my chronograph mv did increase due to pressure I found. No matter what I would never do a drastic change to seating depth and suggest that nobody do so all testing should be done slowly.

 

[Mikecr]

You got pressure problems by .003" deeper seating depth change?
I seriously doubt it had anything to do with seating.

 

[Bart B]

I think the increase in muzzle velocity was caused by you holding the rifle tighter against your shoulder than before. An 80 to 90 fps spread in average muzzle velocity acrôss several folks shooting the same rifle and ammo easily demonstrates this.

A .003" spread in factory match ammo bullet seating depths is normal. Same spread in case head clearance to the bolt face across all rounds when they're fired. All of which means there is a .006" spread in bullet jump to the rifling. Yet the best of it shoots 1/4 MOA at short range.

If your gauge doesn't contact the bullet at the diameter it touches the rifling, it doesn't mean much.

 

[Pdvdh]

I doubt even a 0.005" change in bullet seating depth will cause a substantial enough change in pressure and MV with most cartridges, to be able to detect a reliable MV trend with any chronograph. With one possible exception. If that 0.005" change in bullet seating depth means that the bullet makes or breaks contact with the rifling, you might cause enough pressure/MV change to detect a MV increase/decrease with a chronograph.

When I've collected MVs at varying bullet seating depths, I changed bullet seating depths in minimum increments of 0.010" in the effort to identify best accuracy seating depth. I started with the bullet at or into the lands, and then continued to seat the bullets deeper in the casing until they were 0.060 - 0.070" off the lands, knowing that pressures would decrease. I could usually record lower MVs with each 0.010" increment, as I recall. At 0.020" increments, I think I virtually always received lower MVs.

My data would have been collected with 280 AI, 7mm RM, 300 WM, and 338 Edge capacity cartridges. Each bullet was fired over duplicate, or triplicate, chronographs, so I had a means of confirming I received valid MV data.