Velocity plateau/drop off with increased powder?

[Iron Sighted]

First, let me say that this is from a small sample size, and I understand that chronographs have some latent inaccuracy, but while working up a load for my 375 Ruger today with 270gr TSX bullets and Win 760 powder I experienced something I thought was a bit strange.

Working up from 78 grains in .5gr increments the velocities were climbing steadily until I hit 80 gr(also seemed hit a sweet accuracy node there as well). At 80.5 gr the chronograph actually displayed lower velocity than at 80. At 81 gr the velocity was again pretty much the same as at 80 gr(and also a super tight group of maybe .400", possibly a bit less). Then at 81.5 the velocity finally picked back up again by about 20-25fps over the 80 grain charge. That is where I left off as the next .5gr increment would have put me over the "unsafe, never use" load in Quickload. Groups were only 3 shots each, but I still never saw velocity increase in 6 shots from 80.5-81gr.

So is this just the chronograph being quirky as many tend to be, or is this a common phenomenon?

 

[Andy]

I've seen it many times, expect it in fact. Velocity = Pressure, and pressure does not increase linearly. As well, a 0.5 gr change is rather insignificant in a rifle cartridge of even moderate capacity much less the 375 Ruger. In a large magnum cartridge I've seen no increase over a span of 2.0 grs and then a steady increase.

As you suggested, there are many variables, and you shouldn't be surprised if you went out today and the chrony displayed a slightly different picture.

Nothing to worry about.

 

[Iron Sighted]

Thanks Andy, I didn't think it was a real concern as I saw no pressure signs at all, even on the max load, but it's always nice to have any doubts put to rest by people who have much more experience.

 

[thump_rrr]

The whole point of looking for an accuracy node is to find the plateau where a couple of 10ths of a grain more or less doesn't affect the velocity.

 

[Eagleye]

I've seen it many times, expect it in fact. Velocity = Pressure, and pressure does not increase linearly. As well, a 0.5 gr change is rather insignificant in a rifle cartridge of even moderate capacity much less the 375 Ruger. In a large magnum cartridge I've seen no increase over a span of 2.0 grs and then a steady increase.

As you suggested, there are many variables, and you shouldn't be surprised if you went out today and the chrony displayed a slightly different picture.

Nothing to worry about.

My experience, as well. In big cases, I move up in 1 grain increments, because I have seen where ½ grain will not make a discernible difference.

Most of my rifles shoot best at, or near, max working charges. One of my 308 Norma mags has an accuracy node at around 2850 fps, but that is
30-06 territory.
The next accuracy node with the same powder is at 3140 fps, and is about 2 grains less than where obvious overpressure signs appear.

You can guess which load I am using, lol. Dave.

 

[c-fbmi]

I have experienced this many times, and no, there is nothing wrong with your chronograph. Are you using hot magnum primers? Ball powder is difficult to light at warm temps and exponentially so as the temp drops and the quantity goes up. As stated there is the node situation where accuracy is good and more or less powder (a little bit) makes little difference, as mentioned. There is another situation called "hitting the wall" where velocity actually drops with more powder and as you increase beyond this level velocities and pressures will become erratic with ESs becoming ridiculous. I had this happen with RL 17 in my 9.3X300 WM, and several other large capacity cases I have played with.

 

[Ron AKA]

First, let me say that this is from a small sample size, and I understand that chronographs have some latent inaccuracy, but while working up a load for my 375 Ruger today with 270gr TSX bullets and Win 760 powder I experienced something I thought was a bit strange. Working up from 78 grains in .5gr increments the velocities were climbing steadily until I hit 80 gr(also seemed hit a sweet accuracy node there as well). At 80.5 gr the chronograph actually displayed lower velocity than at 80. At 81 gr the velocity was again pretty much the same as at 80 gr(and also a super tight group of maybe .400", possibly a bit less). Then at 81.5 the velocity finally picked back up again by about 20-25fps over the 80 grain charge. That is where I left off as the next .5gr increment would have put me over the "unsafe, never use" load in Quickload. Groups were only 3 shots each, but I still never saw velocity increase in 6 shots from 80.5-81gr. So is this just the chronograph being quirky as many tend to be, or is this a common phenomenon?

A few comments. First I do not consider putting more powder in and getting less velocity to be a good sign. That means something is not right in the combustion process, and efficiency has gone down. Yes, I have seen it stated that a velocity plateau with increased powder indicates an accuracy node. I have not seen it. An accuracy node is the result of the velocity, and bullet arrival time at the muzzle, interacting with the barrel vibration. The best way to look for a node is to plot vertical POI (ignore horizontal) vs velocity. More velocity with no increase in POI or even lowered POI is a good sign. See graph below. I consider 3420 to be a good velocity for this barrel. However this is not at all the same as more powder with no more velocity. In my view that is not good.

One possible issue if the velocity measurements are accurate is that you are at max load and not good things are happening. I see you have Quickload, but Hodgdon lists 80.5 as the max load for this powder and the pressure is getting up there.

Another is temperature sensitivity of the 760, which I believe is a ball powder and is quite sensitive to temperature. Many think temperature means outside temperature like winter vs summer. However the other issue is chamber temperature. If you are shooting at a range your chamber can turn into a little oven. If your chamber is too hot and you leave the round in the chamber a significant time, the powder heats up and you get more velocity/pressure. Is it possible your chamber time was not the same for each load? Some go as far as to leave their ammo in a cooler, carefully control their barrel temp, and the time it cooks before firing. I think a better solution is to use a Hodgdon Extreme powder like H4350. According to Hodgdon 83.5 of 4350 will give you the same velocity as 80.5 760 with about 1,000 psi less pressure. Might work better for you...

And last is chronograph accuracy. I used to be skeptical of chrony accuracy. However, one day I did a test with two chronographs set up in series so I could compare the results from shot to shot. What I found out was that if you set up close to the muzzle, results are erratic. However when you get the chrony out further, to the max length of the cord, they gave virtually identical results. So depending on how you are set up that may be an issue.

Hope that helps some,

 

[Iron Sighted]

Thanks guys! Great to get some feedback from you guys that have been at this a lot longer than me.

Both the 80gr and 81gr loads printed 3 shot groups at 100 yards that were in the .400" range and had an extreme spread of I think 9 fps and 21 fps and velocities of about 2630-2635 for 3 shot averages. I was slightly disappointed with the velocities, Quickload indicated almost 10fps more than what I was seeing, but that is no big deal, I'm really happy with the accuracy potential. Pretty much all 8 groups I shot had 2 of 3 touching, so I'm assuming the "fliers" were more than likely me pulling the shots.

Chronograph was set up about 15' in front of the muzzle if that matters. I'm thinking about loading up the 80gr charge for hunting now, as the accuracy was almost identical to the 81gr charge and the velocities were nearly identical as well, with the absolute smallest E.S. and S.D. coming with the 79.5gr charge. I figure that way I lower pressures slightly which may be good for shooting in warmer temps. and it is a closer charge to minimal E.S./S.D., does that sound right? Also, do you think 2630fps is decent from a 20" barrel shooting a 270gr TSX? I'm not fanatic about chasing velocities by any means(I wouldn't be shooting a 375 if I was, lol), I just want to make sure I'm in the average range for the cartridge given bullet weight and barrel length.

Again, thx for the comments fellas, it's nice to be able to learn from the experience of others, I find this stuff quite interesting.

Hopefully I can put this exercise to the test and hammer a goat in the next few days, I leave tomorrow morning and with the .308 giving me extraction issues it looks like the 375 is getting the call.

***Forgot to add that barrel/chamber were allowed to cool to what felt like ambient(cold to the touch) temps. between groups( 7-8 mins between groups at maybe 5 degrees ambient temp.). Should also maybe add that I did not see any pressure signs at the upper ends tested, bolt lift did not change, no primer flattening or cratering, no ejector marks.

 

[Ron AKA]

Also, do you think 2630fps is decent from a 20" barrel shooting a 270gr TSX? I'm not fanatic about chasing velocities by any means(I wouldn't be shooting a 375 if I was, lol), I just want to make sure I'm in the average range for the cartridge given bullet weight and barrel length.

Hodgdon shows 2757 with 80.5 grains out of a 24" barrel. Losing 25-30 fps per inch of barrel is probably about right. How far are you seating the TSX into the case? That is going to hurt capacity and velocity? I'm not a big fan of copper bullets because they tend to be long for their weight. Prefer a Nosler Partition for big game.

The only way I can rationalize more powder and less velocity is that the powder is blowing out the end of the barrel unburned... 80+ grains of powder and 20" barrel - could be happening...

 

[Iron Sighted]

That is a good point, the O.A.L. is 3.340", so that would mean the bullet is seated approx. .600" in to the case? Bullets are about 1.374", case(after firing, but I doubt it has elongated much after a single firing?) is approx. 2.577".

3.340-2.577=.763 1.374-.763=.611 So, estimating about .61" is how far the bullet is seated inwards. The O.A.L. is limited by mag length rather than chamber, however it seems that Barnes bullets likea generous jump to the lands so this isn't such a bad thing in this case.

I have a full box of 300gr Partitions that came with the rifle, but after looking on line and in brick and mortar stores, these things are in the $90-95 range for 50, that almost makes the barnes look cheap by comparison which is crazy. The Nosler 260gr AB's are tempting, as are the 300gr Sierra game kings(which apparently have a thicker jacket than the smaller caliber G.K.'s), and possibly the 250gr TTSX's.

Realistically, I figure the 270 TSX's are good to probably 300ish yards before I'd be pretty worried about them not expanding, and I prefer to keep my shots within that distance when hunting anyways(200 yards if I can). Zeroed at 200, my drop I think should be about 10" at 300 which I hope to verify later today.

 

[Iron Sighted]

Back from the range. I loaded more rounds at 80gr of powder and saw velocities at pretty much the same as yesterday(maybe 15fps faster today) so I am content with that. Shot 2 groups at 200 yards today and managed just under an inch and a half with the first grouping, the second I used to adjust P.O.I. to be dead on at 200 so it wasn't much of a group though I did manage to drop the last shot right in the Bullseye so I walked away pretty satisfied with having found a good load for this rifle.

 

[Iron Sighted]

***Duplicate post, sorry

 

[ykkid]

Another way to rationalize velocity drop with charge increase is that you are overpressure, changed the burn rate, so pressure peaks fast and short, and does not sustain long enough to accelerate the projectile to max velocity. Powder burn rates are not absolute, but are predictable within specified pressures. Exceed that pressure, change the burn rate.

 

[c-fbmi]

Another way to rationalize velocity drop with charge increase is that you are overpressure, changed the burn rate, so pressure peaks fast and short, and does not sustain long enough to accelerate the projectile to max velocity. Powder burn rates are not absolute, but are predictable within specified pressures. Exceed that pressure, change the burn rate.

Sir, although what you infer is possible I suppose, the OP is using Win 760 and with the quoted velocity, he is no where near being over pressure...........

 

[Iron Sighted]

It sure doesn't feel overpressured, quite comfortable to shoot even off the bench. Not one physical pressure sign that I've been able to detect so far, I only really brought it up because the(slight) velocity drop or perhaps more accurately described as a plateau was something I'd not seen before, though the largest case I'd previously loaded for was a 7mmRM so the .5 gr increments I typically use are a much smaller percentage of case capacity from the 7mm and .308 Win I usually load for. This rifle is proving to be a real accurate shooter, that more than compensates for middling velocities but of course it's always nice to have your cake and eat it too : ) Perhaps I may try some other powders or push this one a bit further to see if I can't get at least 2700 fps with a 270gr bullet. Douglas, I recall you saying in a different thread that you loaded for your son's 375R, what types of velocities did you manage, and with what powder, barrel length, and bullet if you don't mind my asking?

 

[c-fbmi]

It sure doesn't feel overpressured, quite comfortable to shoot even off the bench. Not one physical pressure sign that I've been able to detect so far, I only really brought it up because the(slight) velocity drop or perhaps more accurately described as a plateau was something I'd not seen before, though the largest case I'd previously loaded for was a 7mmRM so the .5 gr increments I typically use are a much smaller percentage of case capacity from the 7mm and .308 Win I usually load for. This rifle is proving to be a real accurate shooter, that more than compensates for middling velocities but of course it's always nice to have your cake and eat it too : ) Perhaps I may try some other powders or push this one a bit further to see if I can't get at least 2700 fps with a 270gr bullet. Douglas, I recall you saying in a different thread that you loaded for your son's 375R, what types of velocities did you manage, and with what powder, barrel length, and bullet if you don't mind my asking?

I never actually loaded for him and his 375 Ruger, I did the experiment where we blew out the 300 WM for him. I do know his original load for it was using a 260 Accubond and Win 760 powder with a CCI 250. His velocity, through my chronograph was 3200 fps out of his 24" barreled Ruger African. This is how I know you are no where near over pressured, you are at least 400 fps light of the capability of this cartridge and bullet with this powder. Now I know some are going to say that 3200 with the 260 isn't possible but we chronographed it several times and this load and velocity was safe IN HIS RIFLE. It was actually within my expectations of this cartridge and bullet, I am getting 2925 with a 270 TSX in my 24" H&H and have no pressure issues and have used this load in 55 degree heat without any issues at all. The load has proven itself to be extremely accurate and effective on game in temps NA will never see.....James has also used his 260 AB load in Africa with great success and no over pressure issues, no extraction issues, no issues at all. The thing shoots like a 7 Mag and hits like an express train, right out past 400 mtrs.
Please note these loads and velocities have been worked up to in a responsible method, as have all my loads, and they are safe in the rifles in which they have been used. In most cases I look for the best velocity and accuracy the rifle can produce safely and get at least 3 firings from a single case. All my loads start within the books recommended loads and go up from there in increases of approximately 1-1.5%.
Not all who reload agree with my methodology but in 45 years of loading, building rifles, designing and developing cartridges and wildcatting, have I never cracked an action, set lugs or damaged any of the 100s of rifles I have worked with. I have blown the odd primer from time to time, for various reasons but I certainly don't make a habit of it and I don't explore any load beyond the limitations of the brass. This statement has a great many variables in it though, not all brass cases are equal......not by a stretch. What is a safe load in W-W cases may well completely blow primers from FC or Horn cases. I have found military brass (308 based cases), Lapua and W-W to be a large step ahead of all other cases in head hardness and ability to digest loads that the other brass fails dismally with. Norma and R-P cases are in second place and still well ahead of FC and Horn but will not accept W-W loads and retain their primers.
It is my position, that any load which is contained by the brass case while leaving the case intact and reusable is a safe load, WITH THAT BRAND OF CASE IN THAT RIFLE !!!!

Anyway IS, you should easily be able to step up your load to at least 2800 fps with 270s in your rifle, using Win 760 powder. Every 375 I have owned whether H&H or my 1 Ruger has been capable of this velocity with 270 gn bullets, safely. If you wish to go to the max with your rifle I might suggest you make some cases from W-W 300 WM and work with them for your upper end test loads. My dear friend Ted (Why not?) has had several 375 Wbys that he says will get 3100 fps with 270s. Considering that the 375 Ruger has virtually the same case capacity, it would be logical to believe that the Ruger should do the same. Which is why I say that when my son's loads chronographed at 3200 fps with the 260 ABs it fell within my expectations for this cartridge and bullet. Your 20" barrel is going to give less of course, just how much is anybody's guess, but that is why I said you should easily be able to safely attain 2800 fps...........and actually in my opinion likely considerably more, if you wish to go there.

 

[Iron Sighted]

Thanks Douglas, much appreciated. I was definitely left with the impression that I could likely work up to another accuracy "node" before I would likely be facing pressure issues. I definitely am the cautious type so whatever I work up towards will be in small increments and I plan to keep a close eye on bolt stick and any pressure signs on the brass as I move up, but knowing how much faster you have pushed similar cartridges does leave me with the impression that I can squeeze a bit more out of it.