Correlation between velocity, barrel length and powder burn speed

Ape_Factory · Feb 26, 2024

I worked up two loads, same bullet, same case, same case specs, different powders. Both grouped sub-MOA for 5 or more shots at just over 2700fps in a bolt gun.

One powder was IMR 4064 and it's my "established" load and the other, which I just tested two days ago, was Vihtavuori N550. Both powders are somewhat close in the burn chart rate but not the same, with VV being slower. Accuracy isn't as good if I use something like Varget.

Caliber is 308, 20" barrel. Neither shows any pressure signs although in really high temps, over 90 degrees F, the 4046 load caused the bolt to be a tad harder to lift. When I tested the VV load, the highest grain count was the winner, 46.5 grins. With 4046, the winner was 44.5 grains and while it's been a bit, I do remember that higher velocities didn't group nearly as well.

I'm honestly not sure what question to ask because sometimes you just don't know what you don't know. I'm rather surprised the accuracy for both powders were identical at the same velocity which peaked my curiosity. I don't understand why that's the case other than both powders (in different amounts) possibly create the same barrel resonance at that velocity.

Does that mean the barrel is essentially tapped out at that speed? If I wanted to eek out a bit more fps yet retain accuracy, would I look at a different powder and if so, how do I determine what direction to go in or is it all simply dark magic? I can pretty much get any powder I want but I really, really don't need more variety.

I do plan on trying to go a bit higher with the VV550 and see if accuracy falls off. It's already a compressed load though. Brass and primer looks absolutely perfect and I generally size .001 under and keep things clean.

I've been working on another load with Berger 77gr OTM's for my SBR with a 12.5" barrel. Accuracy seems to be best with TAC at 2475fps and nothing has come close to it in terms of velocity and accuracy. Not 8208, H335, Benchmark, CFE223 or Varget. I didn't find some intersecting velocity with those powders like I did with my 308 load. So maybe just coincidence?

TheOfficeT-Rex · Feb 27, 2024

Ape_Factory said:
Neither shows any pressure signs although in really high temps, over 90 degrees F, the 4046 load caused the bolt to be a tad harder to lift.

Bolt lift is a pressure sign, and you can either bump loads down in the summer( some guys do summer and winter recipes) or back it down full time.

Ape_Factory said:
Does that mean the barrel is essentially tapped out at that speed? If I wanted to eek out a bit more fps yet retain accuracy, would I look at a different powder and if so, how do I determine what direction to go in or is it all simply dark magic?

Not necessarily tapped out, but you likely won’t eke out too much more with those powders. If it needs to be faster, you’d need a change in powder so I’d look at load data from a couple sources - if they list 4046/550, you can compare other powders to see what may give you a bit more speed. Likely it will be double base ball powder so you’d be back to watching the summer temps if you’re planning to load it up near max.

Ape_Factory · Feb 27, 2024

Doesn't need to be faster really...just trying to learn a bit and see what I can apply to other situations, seeing if I'm missing something in terms of pushing velocity safely and not being on the ragged edge. I've never had two different powders perform identically at exactly the same velocity so was an eye opener.

Doom · Feb 27, 2024

It sounds like you are shooting something in the 168/175 gr range and in a 20" barrel 2700 fps is overpressure and your bolt lift is a positive indication of that. Most people find accuracy around 2500-2550 fps in that barrel length. Depending on the bullet it's not unusual for the best accuracy to be below maximum load/pressure. As for two powders giving the same accuracy it's possible that the powder isn't the limiting factor, it can the the shooter. What is the basis for your same accuracy?

If you are looking for velocity you have the wrong barrel. In 308 you can get more velocity with 2000MR but in a 20" barrel you'll get less than 90% burn and have a lot of muzzle flash.

Ape_Factory · Mar 2, 2024

I'm shooting 169 grain Sierra HPBT's. Loads are either under or at max (VV550) book value and again, I had very slight bolt lift once on a 97 degree day last summer at a range in Texas with the IMR 4046 load. I'm sure the sun helped heat the rounds up as well as it's not fully shaded. In any other conditions I've ever shot in, I've not had any increase in bolt lift. I usually get high 2400's to low 2500's in my 16" gas gun with mild load so I dunno. And I do agree, best accuracy has often been well below max load, particularly with my gas 308, less so with the bolt gun. My "plinker" load for the gas gun uses standard Hornady 150 grain FMJ's, no even a match bullet, and I can get half-MOA when shooting well but they're slow.

I went back and looked and my five shot group averaged 2690.2fps on my Garmin. With 4046, I see around 2700 to 2707. The five shots were 2692.7, 2681.4, 2690.5, 2701.5 and 2684.8.

Not necessarily "looking" for more velocity, just curious about any correlations between a barrel, velocity and finding a good node while being able to possibly target another powder scientifically without just randomly trying different powders. That shit adds up! But essentially I'm just trying to increase my knowledge as I've only been reloading since about 2018.

Here's a photo of the cases with the VV550 load. Zero signs of pressure, no cratering, primers aren't super flat, cases show no bulging, no lines, etc...and a shot of the grouping. It's marked "P4" and if you look at my load card, you'll see P4 designated there as well. I'm no sharpshooter and I was battling serious mirage which is particularly bad at this indoor range for some reason. Have a cover on the way which'll hopefully help. But someone better than me would have probably put all five through the same hole. Rifle is a Sig Cross with the Schnee's CarbonSix 20" barrel. I'm .002 of the jamb with the Sierras and I use MDT metal mags without the plate.

Doom · Mar 2, 2024

The two right most rows of cases show signs of either ejector or extractor marks. Not familiar with the action so I can't tell which.

Ape_Factory · Mar 2, 2024

I looked at them with a magnifying glass and there are no marks other than those caused by me unifying the primer pockets with an RCBS prep center. I'll go and look again and take a few closeup photos of some of them.

Here's what the Cross bolt face looks like.

ma smith · Mar 3, 2024

see column A, row2 at the 9:00 position.

Ape_Factory · Mar 3, 2024

I'll pull that one and photograph it. Realize the first row of five (vertically) is P1 with the lowest charge. I may have run some of these cases in my gas gun prior which always tears brass up. So first row on left, vertically, is P1 on my load card. P2 is the next row, then P3 and P4 on the far right.

I am using a suppressor but from preliminary testing, it didn't add any velocity. VV's load data says 2703fps with a 168 grain HPBT out of a 24" barrel with 46.5 grains of N550. I'm running less seating depth too, 2.865 to their 2.795.

Ape_Factory · Mar 3, 2024

Out of all of them, that's the only one exhibiting ejector marks. Wouldn't make any sense on the lowest charge. Pretty sure I fired that out of my gas gun initially as I found the X-Tac wildly inaccurate and I simply wanted to get through them and reuse the cases.

Had to try and rotate it, catching the light just right to even show it. Middle case in the first photo.

Here are two from the highest pressure load, 46.5 grains, again, within VV's book range.

CK1.0 · Mar 3, 2024

IDK where/when/what started guys down the path of thinking more speed is intrinsically linked to accuracy..? IMO running loads up until one sees pressure signs has a lot more to do with the old "That's the way we've always done it" mantra than anything else. Within reason, I don't think there's any correlation between speed and accuracy once one gets a load into a certain range.

Personally, I think it's psychosomatic, like a lot of reloading "lore", it seems guys have just heard it for so long that they just believe it without questioning/testing it.

After testing it a bunch for myself (while keeping my feelings out of it), I don't load anywhere near max anymore and my guns have never been more accurate. The only difference is I might need a couple more clicks on the turret and my barrels last a little longer, my groups are the same size whether I run my 6mms at 2750fps or 3000fps (unless it's windy duh lol).

IMHO, if one is seeing anything that even remotely looks like pressure signs, back it down. The size of one's group likely has more to do with how they slept the night before than how fast they're going and/or a grain or two of powder either way.

Emerson0311 · Mar 3, 2024

CK1.0 said:
IDK where/when/what started guys down the path of thinking more speed is intrinsically linked to accuracy..? IMO running loads up until one sees pressure signs has a lot more to do with the old "That's the way we've always done it" mantra than anything else. Within reason, I don't think there's any correlation between speed and accuracy once one gets a load into a certain range.

Personally, I think it's psychosomatic, like a lot of reloading "lore", it seems guys have just heard it for so long that they just believe it without questioning/testing it.

After testing it a bunch for myself (while keeping my feelings out of it), I don't load anywhere near max anymore and my guns have never been more accurate. The only difference is I might need a couple more clicks on the turret and my barrels last a little longer, my groups are the same size whether I run my 6mms at 2750fps or 3000fps (unless it's windy duh lol).

IMHO, if one is seeing anything that even remotely looks like pressure signs, back it down. The size of one's group likely has more to do with how they slept the night before than how fast they're going and/or a grain or two of powder either way.

I find brass lasts a lot better when slowing things down from max too.

Rio Precision Gunworks · Mar 3, 2024

I think most would be shocked how high the pressure is on a lot of their ammo.

Book max loads don't have a "built in lawyer." They are actually where the test barrels reached max or just below max saami pressure on average.

Obviously there are differences from rifle to rifle and barrel to barrel. But when you see people running things like 3gr over book max, its almost guaranteed they are running over max saami.

Ape_Factory · Mar 3, 2024

CK1.0 said:
IDK where/when/what started guys down the path of thinking more speed is intrinsically linked to accuracy..? IMO running loads up until one sees pressure signs has a lot more to do with the old "That's the way we've always done it" mantra than anything else. Within reason, I don't think there's any correlation between speed and accuracy once one gets a load into a certain range.

My question wasn't about "more speed for accuracy" but a particular velocity providing identical accuracy with two different powders that have different burn rates with everything else being equal.

I can say that almost invariably, I've never found a good load that was anywhere near max book values. In fact, my other load with the same bullet using 4046, is about smack dab in the middle of the recommended book values.

ma smith · Mar 3, 2024

Nosler's manual is arguably worth reading for this reaoson alone. In Nosler, the * indicates accuracy load. If you believe Nosler data, Accuracy is (within the safe range) quasi-independent of pressure. Note below, the most accurate load can vary from min to max, based on powder. In .223 REM, Varget is more accurate at slower speeds, H4895 likes to be pushed hard. RL-15 does best in the middle of its range, etc.

MHO good sense to pay attention to each powder/cartridge, individually.

Ape_Factory · Mar 3, 2024

Rio Precision Gunworks said:
I think most would be shocked how high the pressure is on a lot of their ammo.

Book max loads don't have a "built in lawyer." They are actually where the test barrels reached max or just below max saami pressure on average.

Obviously there are differences from rifle to rifle and barrel to barrel. But when you see people running things like 3gr over book max, its almost guaranteed they are running over max saami.

Neither one of the loads is above book max.

Rio Precision Gunworks · Mar 3, 2024

Ape_Factory said:
Neither one of the loads is above book max.

Never said it was.

Ape_Factory · Mar 3, 2024

ma smith said:
Nosler's manual is arguably worth reading for this reaoson alone. In Nosler, the * indicates accuracy load. If you believe Nosler data, Accuracy is (within the safe range) quasi-independent of pressure. Note below, the most accurate load can vary from min to max, based on powder. Varget is more accurate at slower speeds, H4895 likes to be pushed hard. RL-15 does best in the middle of its range, etc.

MHO good sense to pay attention to each powder/cartridge, individually.

I do look at Nosler's stuff and Sierra will often have similar suggestions for powder/accuracy. I'd think it'd be very barrel dependent though if harmonics, like we all assume, is a real thing. But it's a good way to compare best accuracy with each powder and see what velocity each powder gets at that load.

ma smith · Mar 3, 2024

Harmonics has nothing to do with pressure calculations, its a concept relating to inertial response.

Ape_Factory · Mar 3, 2024

I never referenced pressure calculations. In fact, I've tried repeatedly to leave pressure cacls out of it. Those were other people posting and not understanding my initial question or making assumptions about the load being over pressure. I'm simply giving a very small velocity range where everything seems to be working really well, regardless of pressure differences (if there are any). Obviously I don't want over pressure and that should be a given in a public discussion on reloading.

Concerning pressure, I do often look at the book values and will always choose the powder that has less pressure over a given load/velocity range that I think might work with the rifle I'm reloading for. But that's a separate discussion, IMO.

I think it comes down to both powders, despite their different burn rates, producing the same harmonics at a load that proves to be accurate with this particular bullet. The velocity being identical may be a complete coincidence. My hope was there WAS some correlation that could be applied to other loads, other rifles/barrels where one could make a more educated guess on powder choice/changes when accuracy with more velocity is desirable. Like with an SBR and heavy bullets. For me, powder choice has been very random, based mostly off of the experience of others and a lot of trials on my end. I think I've tried five or six different powders for my 77 grain OTM load.

I obviously need more powder with VV vs. 4046 so there's some powder efficiency to be had with 4046. I haven't looked to see which one is more temp stable yet.

ma smith · Mar 3, 2024

...

Ape_Factory said:
I'd think it'd be very barrel dependent though if harmonics, like we all assume, is a real thing.

Most comments here are saying fundamental accuracy has to do with consistent MV, which means consistent energy + consistent geometry, which is the "pounds" and the "square inches" part of PSI (pressure). Also, this is why published loads are tested in pressure-test barrels, or whatever.

While it maybe true that accurate loads vary slightly from 'barrel to barrel', that means simply, chamber to chamber (throat to throat, bore to bore, etc). The harmonics of the barrel are another animal, and the fundamental concepts and their relation to accuracy is wholly different.

Its not really not a consensus view to say internal ballistics and harmonics are on equal footing when it comes to development/evaluation of accurate ammo. Maybe other guys have different views, tho. I just think its like mixing up apples and bannanas.

Sorry for any confusion.

grayfox72 · Apr 9, 2024

OP, your original question is what I've been working with for several years now, and that is based on Chris Long's work of optimal barrel time. His study boils down to this, that in general, accuracy (node, if you will) is primarily based upon barrel length and MV. A 20" barrel will have a node at about 20" length and 2700 fps. As long as the powder is suited to the caliber and usage, mechanically the longitudinal pressure/sound wave (18000 fps in steel) rebounds back and forth in the barrel, so at certain MVs the bullet will exit the barrel at times that correspond to the muzzle tip being "quiet", ie, the long. wave is far from the tip. Cartridge OAL seems to have a small impact on it, mostly in the timing of bullet exit vs wave position (my way of explaining that). Also, internal friction and some chamber features probably impact the MV but, again, that's currently lumped into some equation constants, so no generic advice on that either (other than always use good barrels lol). I use an excel spreadsheet to track and estimate this, which guides me in loads and powders for a given rifle.
I've had 223, 6mmArc, 6.5 grendel, 6.5 creedmoor and 308 barrels of 20" and this has held true. And barrels of 16", 18" up to 24" and still holds together.
Now there are some variations and to me, the "node" is actually a range (example, in 6.5 CM. 2.800 OAL, one node is ~2705, 2680-2735), so somewhere in there a barrel ought to show a good accuracy, for me I look for 0.75"/100 yds or better (hunter more than a competitor) and repeatable. So it is not a surprise to me that 2 powders might have an accurate load at about the same MV, in your 20" barrel case 2692 and 2700-ish.

ClamHammerXpress · Sep 28, 2024

Ape_Factory said:
Out of all of them, that's the only one exhibiting ejector marks. Wouldn't make any sense on the lowest charge. Pretty sure I fired that out of my gas gun initially as I found the X-Tac wildly inaccurate and I simply wanted to get through them and reuse the cases.

Had to try and rotate it, catching the light just right to even show it. Middle case in the first photo.

Here are two from the highest pressure load, 46.5 grains, again, within VV's book range.

No pressure signs there that I can see; when they appear, they’ll be obvious…

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Correlation between velocity, barrel length and powder burn speed

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