Safety, what is more important?

[FLshooter]

I shortened my OAL to .010 w/my 6.5 Grendel AR15 Barlein .I had to hand feed w/a sled. But it really made no difference in accuracy.So,I went back to seating my bullets 2.250-2.260 inches. That’s within 1 mm jump. Fits in the mags and accurate.
As Klem said,bolts are a whole different animal.

[Cheapshot]

Impressive list of wildcats. I started reloading about 45 years ago, but a bad marital situation kept me away for a couple of decades. While Dad didn't mind reloading, he didn't like to work up loads. At the tender age of 14, that became"my job". Information was much harder to come by back then. Especially in a small Midwest town. My notes from back then are long gone. Really sucks because I had some great loads worked up for the guns I inherited. Back to square one.

[Lemonaid]

I found this in the remarks section of more bullet jump research from Precision rifle blog. If the quote below is true then my understanding that seating the bullet out farther (but not touching the lands) would result in less pressure is wrong, in fact opposite of what I thought!

"Maybe the only thing that seems like it might be fairly agreed upon is “seating the bullet in the case so that it contacts the rifling in the throat increases the peak chamber pressure, which is not desirable.” (From Dr. Vaughn in Rifle Accuracy Facts). Dr. Vaughn also said, “The pressure change caused by changing bullet seating depth in the case can also be deduced from Reference I for 0.308 caliber bullets. The peak chamber pressure will drop about 1000 psi for every 30 mils of additional distance (free run) between the bullet and contact with the lands in the throat. In other words, if you seat the bullet so that it has about 60 mils of free run before contacting the Lands, the peak chamber pressure will be reduced by about 2000 psi.” So it’s not just that when you are seated into the lands that peak chamber pressure is increased, but the further you get from the lands the more you reduce peak chamber pressure. But, Dr. Vaughn’s next sentence there is, “This means that the chamber pressure is not very sensitive to seating depth.” So, I guess in the grand scheme of things a 2000 psi reduction in peak pressure isn’t much."

[grayfox]

The key as I understand it is I think somewhat like he is saying but yo have to put all the pieces together. From a given coal (some 0.100's less than lands but still in spec) max pressure will tend to drop at some rate until you get lands contact at seating. This is from 2 things: first there is slightly more expansion volume initially due to seating farther out, causing slight drop in pmax; second there is some time for gasses to escape around the bullet before full-seating (obturation? if I get that term correctly, but full-seating in any case). However at some point when seating coal into the lands there is no gas escape during burn, so you get a large or sharp, pmax increase since there is no gas-volume loss during ignition. The final point -- and this is mostly my theory -- is that, for a given powder charge there is really only a given X amount of energy to impart, so any pmax increase (but not yet into the lands) will only follow with a decrease in the final pressures, so just about the same amount of work gets done on the bullet.

The MV, from imparted work/kinetic energy, comes from the integral of pressure vs distance, or the area under a pressure-distance curve. If the initial quantity of work available comes from the powder burn (and, assumed to be relatively constant for a powder charge) then that P*d integral will be basically constant and MV will be pretty much unchanged until you get either "very far" away from lands (more gas, ie, energy, escape) or very "into" the lands (no gas escape), these bounding conditions could show big discontinuities. footnote, I've read in another derivation that MV is more proportional to 25% of pmax, not pmax itself, thus a "2000 psi" increase might increase the proportional pressure by 500 psi... not a lot of KE difference and probably buried in other uncertainties.

I guess, as a final note, for at least some of our bullets you could be engaged in the lands before the caliber diameter fully exits the case neck... if that's true I'm not quite sure what that would mean for all of this.

[Lemonaid]

Two different sources say the opposite thing about seating depth and pressure changes. What say you?
A.
The Sierra Reloading Manual says that adjusting seating depth to match your rifle’s throat/freebore and maximize accuracy “is fine, but bear in mind that deeper seating reduces the capacity of the case, which in turn raises pressures. Going the other way, seating a bullet out to the point that it actually jams into the rifling will also raise pressures.”
from : https://precisionrifleblog.com/2020/04/1...challenge/

B.
"Maybe the only thing that seems like it might be fairly agreed upon is “seating the bullet in the case so that it contacts the rifling in the throat increases the peak chamber pressure, which is not desirable.” (From Dr. Vaughn in Rifle Accuracy Facts). Dr. Vaughn also said, “The pressure change caused by changing bullet seating depth in the case can also be deduced from Reference I for 0.308 caliber bullets. The peak chamber pressure will drop about 1000 psi for every 30 mils of additional distance (free run) between the bullet and contact with the lands in the throat. In other words, if you seat the bullet so that it has about 60 mils of free run before contacting the Lands, the peak chamber pressure will be reduced by about 2000 psi.” So it’s not just that when you are seated into the lands that peak chamber pressure is increased, but the further you get from the lands the more you reduce peak chamber pressure. But, Dr. Vaughn’s next sentence there is, “This means that the chamber pressure is not very sensitive to seating depth.” So, I guess in the grand scheme of things a 2000 psi reduction in peak pressure isn’t much."

[grayfox]

A. Sierra has pressure-measuring equipment; they are speaking to the entire range of bullet jumps, from one boundary - deep in the base, to the other boundary - touching lands. With some kind of convex curve in between.
B. Don't know if Vaughn has pressure-measuring equipment. He is speaking about 1/2 of the range - going from the lands back to about 0.060 jump. With a decrease going from lands back to 0.060.

Changing bullet jump also affects the timing of the arrival of the bullet at the muzzle, not just peak pressure; such a timing change can affect accuracy (OBT theory) even if exit MV is not affected.
Neither quote-source specifically mentions MV in their discussion, that I can see anyway.
Remember it's not the pmax per se, but the integral of pressure-over-distance, that gives you the exit MV.
There are probably a couple other variables at work in all of this, too, that we haven't thought of.
The only study I read that showed a difference was that seating the bullet further out sometimes allowed more powder to be added of a desired type (IIRC 0.1 or 0.2gr), which in turn would give him a bit more MV.

[Lemonaid]

grayfox, thanks for the thoughtful, informative replies. I'm still trying to grasp what happens. Explain what you think will happen in my real world example of loading a Barnes 120 match burner with 29.3 grains of RL 15 seated to 2.290 vs. seated to 2.200 as is in the barns load data.
Will seating deeper to 2.200 increase or decrease chamber pressure and/or muzzle velocity?
To add to confusion, take a look at the test #5 data from the bullet jump test at precision rifle blog https://precisionrifleblog.com/2020/03/2...velopment/
Would you say the trend in velocity is going down as the bullet jump increases?
Thanks for the help, I need it!

[LR1955]

grayfox, thanks for the thoughtful, informative replies. I'm still trying to grasp what happens. Explain what you think will happen in my real world example of loading a Barnes 120 match burner with 29.3 grains of RL 15 seated to 2.290 vs. seated to 2.200 as is in the barns load data.
Will seating deeper to 2.200 increase or decrease chamber pressure and/or muzzle velocity?
To add to confusion, take a look at the test #5 data from the bullet jump test at precision rifle blog https://precisionrifleblog.com/2020/03/2...velopment/
Would you say the trend in velocity is going down as the bullet jump increases?
Thanks for the help, I need it!

LA:

You are being paralyzed by technical minutia that has marginal meaning for most chambers and barrels. If you buy a barrel with a minimum spec bore diameter like some Palma barrels, then you need to pay attention to such minutia but those rifles have gigantic actions that can take the excessive pressures of overloads plus minimal diameter bores. Unless your Grendel barrel is really a 6 AR barrel with a Grendel chamber, you won't have a problem providing you follow the data offered in an up to date loading manual. Jam the bullet and you will spike pressures no matter your load. Overload with a Grendel and you invite shearing bolt lugs and or weakening of the receiver.

Look at all the load data on the Grendel and make a rational decision on your powder charge. Seat the bullet off the lands with a safe charge of powder and you will be fine. Although I do not overload the Grendel, it doesn't mean I won't go to a max charge as long as it is in a loading manual from a respected powder or ammunition company. I have a hard time believing anyone could jam enough RL-15 into a Grendel case for it to be dangerous no matter the bullet being used. Been there and RL-15 only gave me slow speeds compared to powders with more bulk density.

LR55

[Lemonaid]

LR55, this forum is like going to school. A place of learning new things, knowledge passed on from helpful friends. You are very likely correct that the difference in pressure/velocity from seating depth is of marginal importance but I enjoy trying to understand how things work.
I now have no idea if pressure goes up, goes down or goes up and down and up in a sine wave as seating depth increases and my lack of understanding bugs me a little bit!

[grayfox]

Lemon,
(edited for correct max load below)
First off I would echo the comments of '55 in that these are by far minute details whereas larger ones, with more impact on your MV and accuracy, deserve working through long before you get to this example.
However, even though minute, may I offer some words on your example.
I share your sentiments that we can all go to school as it were, and I for one look to learn some things as well as try to share.

1. Your proposed load of 29.3 gr exceeds the book max from Barnes so I would not comment at all one way or the other for obvious liability issues.
2. Any load is highly dependent on your particular barrel, chamber sizing, bore/inner frictions etc, so that's why everyone always says to work up slowly watching your cases and MV behavior.
3. Barnes says (edit to correct) 27.0 (max) is compressed at its length of 2.260, AND, as a mono, it is already long for caliber... I personally would not consider going down to 2.200 even at 27.0 grs of RL15.
4. From a "going to school" perspective you will get a lot of insight from getting someone who has quickload to run you some what-ifs -- but they are just models and Covid-19 has graphically showed us how good/bad models can be!!!
5. If I were trying to find a fast and accurate load I would use my OBT-like approach, shooting over my chrony. This method works for me and I continue to use it.

And finally, once you find, and I do mean ONCE YOU FIND, a safe load you like with powder/bullet/etc combo in YOUR gun, load 4-5 of that load for each of varying lengths within about 0.030-0.040 total range, like 2.250, 2.260, 2.270, 2.280 (4-5 each) (as long as the longest length does not put you at the lands). Then shoot them either for MV data or for accuracy (or both), and tell us what you found in the real world with your barrel. That would be a super contribution in info for all of us I'm certain!


ps, from a hunting-effectiveness point of view, I would suggest that the 120 Barnes is not your best choice, since its high min opening velocity limits its usable hunting distance in a Grendel. But that's a "my $0.02" worth of comment for ya.

[Lemonaid]

grayfox, this is a case of finding a great load for accuracy, SD and velocity and hating to give it up due to being "over the line."
I'll test it some more and see what the brass tells me over three to five re loads as was suggested.
You can see my initial good results posted here: http://www.65grendel.com/forum/showthrea...barnes+120
If I recall correctly the Barnes 120 Match burners are conventional cup and core bullets.
I appreciate all the helpful advise, thanks!

[grayfox]

I do vaguely recall that thread, thanks for bringing it back up.
As a match load in your bolt-action RAP, looks like you have found one, yes.
Just be aware that you are very likely over-Saami pressures, also keep in mind the temp sensitivities of RL-15 powder.
If you get a chance you might want to look at the 123 Eldm and some Ar comp or CFE... with my RAP I'm shooting 27.8 grs AR-comp (below the 28.0 max btw), 2558 ft/s, SD-9, and something like 0.5" groups/100 yds but I can't find the pictures.
Just saying that if or when you might want to reduce the pressures on your brass and etc, you have some good candidates available for you. Another one I've done a little with is the 130 gr Nosler RDF, in Federal factory (need to do some load developments) but 2430 ft/s, 7.0 SD and 0.75" IIRC. There are some other guys who have more run-time with this RDF on here, but this tells me enough that it could be a nice bullet for heavy-for-caliber loads. From my bolt-rifle anyway.
BC's for both of these bullets are if I recall, higher than the 120 MB, not knocking it, just saying.
Stay safe on your shootin'.