Powder MV Change with Temp

[brown dog]

Here's a thought:

 

We all talk about 'temp sensitive' and 'temp insensitive' powders.

 

But they all do change MV; here's an example I have squirreled away from empirical data:

 

A 308 sized load of Vihtavuori N150 will change MV by about 3.6fps per Deg C temp change.

 

So zero at 5 deg C at (say) 2800fps, then shoot the same load at 25 deg C and your MV will be about 2872fps -and you'll miss stuff if you don't account for the new MV.

 

 

So.....

 

Wondering if we could try to build a library of powder MV change with Temp

 

 

To count; everything must be the same from your data collection EVERYTHING rifle, bullet, charge weight, same chrono, same everything - except temp!

 

 

If people can add to this it would be neat to have the empirical data and then a summary calculation eg

 

N150 in 308

 

MV 5 Deg C = 2800fps MV 25 Deg C = 2872fps

Calculated MV change:

 

N150 in 308 = 3.6fps per Deg C

 

Thoughts?

Offerings?

[MJR]

Matt, a question please. I have that data for my .308 but the initial data is with a new barrel. After 400 rounds or so the barrel shoots faster but it was also a much warmer day. Would the increase in mv be down to temp alone or the barrel 'bedding in'also?

Happy to conduct further test though.

[Deserttech Europe]

I am currently chronoing several loads at different temps.

 

I am happy to share the data when I am completed but am looking forwards to seeing what others are getting.

 

regards

 

DTA Europe

[brown dog]

Matt, a question please. I have that data for my .308 but the initial data is with a new barrel. After 400 rounds or so the barrel shoots faster but it was also a much warmer day. Would the increase in mv be down to temp alone or the barrel 'bedding in'also?

Happy to conduct further test though.

 

Mike, as you know, my background is 'big bullets' where the relative interactions of 'scale' maybe different. On big bullets, barrel wear reduces MV (because shot start etc becomes easier, so pressure drops, so MV reduces).

I can't think of a reason why that would be different for small bullets - unless you started with a really rough unfinished bore and it needed to be shot lapped smooth to reduce friction.

 

My instinct would be that an MV increase would be about charge temp ( and charge temp is always factored into the calculations with big bullets) - but it depends on how big a temp change you're talking about (and how big the mv change, and same chrono? etc - ie is the change 'within tolerance' of experimental and instrumental error and natural variance, or is it 'real')

 

I know anecdotally, people tend to believe rifle barrels change MV over the first few 100 rds as they're shot in; I've personally not seen that with my last 2 barrels (both Lothar) and I think you'd have to have a pretty shite bore surface finish for that to happen over anything but the first few rounds (as they condition the bore surface with copper and powder residue)....but then again that conditioning is removed everytime by anyone who does 'back to bare metal' cleaning and that's why people who do that always have to shoot a few foulers before their barrel is back 'on'.

[brown dog]

I am currently chronoing several loads at different temps.

 

I am happy to share the data when I am completed but am looking forwards to seeing what others are getting.

 

regards

 

DTA Europe

 

Thanks Ewen

 

Yup, it could be interesting.

 

If people can add to this it would be neat to have the empirical data and then a summary calculation eg

 

N150 in 308

 

MV 5 Deg C = 2800fps MV 25 Deg C = 2872fps

Calculated MV change:

 

N150 in 308 = 3.6fps per Deg C

[Ratwhiskers]

Alright, how do you then account for the following.

 

It's one lot of data taken from one lot of kit. The number of variables from each item within that list can be extraordinary.

 

To coin Captn Barbosa "They're not so much rules, more sort of guidelines". How many times have we seen someone use data as a "Given" only to have it fail for one reason or another.

 

Don't get me wrong I do like and use collected data, but for what it is..... Guidelines. That's why the Yanks "Lawyer Up" whenever they release anything into the public eye.

 

Mutterings from the back of the loading room over.

[brown dog]

Ratty,

 

I don't understand your point; this isn't load data.

 

This is attempting to get a feel for MV change fps per degC over a range of powder brands -where someone may be able to show the delta if they happen to be sitting on empirical data where everything (less the temp) happens to be the same.

 

The same load fired in the same rifle over the same chrono; one measured in (say) Feb and one measured in (say) August.

 

Not very controversial(?)

 

I'm interested in seeing whether we can get a feel for the view that 'temp sensitive' and 'temp insensitive' for powder brand 'X' is fact or fiction.

 

And also a feel for 'scale' of effect - this isn't readily available anywhere - and that's why people ignore it.

[Ratwhiskers]

Matt.

 

Sorry if I sounded argumentative or disparaging (heaven forbid contraversial), this was in no way intended. As you say, some people ignore such data and others don't know where to find it and need help, but some miss informed or ill educated take it as pure truth and this is what I'm seeing so many times it makes my head spin when they come back whining that what they were given didn't work for them.

 

I'm all for what you are proposing, but with caviats for those still learning

[Davy]

.308 N540 155 Scenar moly coated variance from 0C to 25C for me is around 60fps with a 24" Border barrel when it had around 5,000 rounds through it. I always work on a gain of around 30fps/inch at 18C which will change marginally at different temperatures so it may be worth asking people to include barrel length.

[Ratwhiskers]

And round count.

[brown dog]

.308 N540 155 Scenar moly coated variance from 0C to 25C for me is around 60fps with a 24" Border barrel when it had around 5,000 rounds through it. I always work on a gain of around 30fps/inch at 18C which will change marginally at different temperatures so it may be worth asking people to include barrel length.

 

Davy, I'm confused (easily done! ) I'm only after dV/dT - not sure how barrel length plays a part in that (as long as it remains constant between the 2 readings!

 

Surely roundcount will only be of significance ( and render the readings unuseable) if there's a huge delta between the 2 readings? eg 0 Deg C mv measured at 200rds; 25Degc C data measured 6,000 rounds later.

 

My takeaway from your data is that with all things constant (and, incidentally, a barrel that's done circa 5,000rds)you saw a temp effect of:

 

N540 in 308

 

MV 0 Deg to 25 Deg C = delta 60 fps

Calculated MV change:

 

N540 in 308 = 2.4 fps per Deg C

[brown dog]

Which gives a summary so far of

 

 

Temp Sensitivity

 

N150 in 308

N150 in 308 = 3.6fps per Deg C

 

N540 in 308

N540 in 308 = 2.4fps per Deg C

[brown dog]

I can add:

 

Vihtavuori N570 in 338 dV/dT = 2.95fps per Deg C

 

Which gives a summary so far of

 

 

Temp Sensitivity

 

N150 in 308

N150 in 308 = 3.6fps per Deg C

 

N540 in 308

N540 in 308 = 2.4fps per Deg C

 

N570 in 338

N570 in 338 = 3fps per Deg C

 

All very similar (within tolerance, for practical purposes, the same?) so far - not surprising really considering they're all made of the same chemicals!

Rule of thumb developing so far: Viht Powders = 3fps per Deg C

[Davy]

Browndog, yes you could be right in that barrel length may have no impact on the final numbers however it would be equally interesting to compare two identical loads at 0C and then 25C or similar in rifles with say a 20" and 32" barrel. The longer barrel is always going to give higher MV's so will the drop in MV/degree remain the same or would you see a percentage increase based on the different barrel lengths?

 

My answer is no idea so grab the data now and crunch the numbers but keep barrel length in the back of your mind. You might find that shorter barrels see less drop per degree than longer ones.

[TC]

Which gives a summary so far of

 

 

Temp Sensitivity

 

N150 in 308

N150 in 308 = 3.6fps per Deg C

 

N540 in 308

N540 in 308 = 2.4fps per Deg C

 

Matt,

 

Have you cross checked these with predictions from the temp function in Quickload?

 

I played with it a while ago (primarily looking at pressure increases in hot loads) but have not compared velocity prediction to real world to check validity.

 

My assumption was that the calc was based on powder manufacturer data but you know what they say about assumption....

 

Tony

[brown dog]

Matt,

 

Have you cross checked these with predictions from the temp function in Quickload?

 

I played with it a while ago (primarily looking at pressure increases in hot loads) but have not compared velocity prediction to real world to check validity.

 

My assumption was that the calc was based on powder manufacturer data but you know what they say about assumption....

 

Tony

 

Good idea No idea how they're done there. Be interesting to see how the viht QL data compares to the empirical stuff above ....I'll wager it'll come in around 3fps per deg C

[brown dog]

Good idea No idea how they're done there. Be interesting to see how the viht QL data compares to the empirical stuff above ....I'll wager it'll come in around 3fps per deg C

 

 

Just had a look at QL (I'm on v3.3) I can't spot a charge temp function. Where is it?

[Davy]

In the change powder window is a small icon low left from memory that allows you to set the temperature.

[brown dog]

In the change powder window is a small icon low left from memory that allows you to set the temperature.

 

Thanks!

 

Here are some egs (all run in 308 - 45gr charge weight)

N140

 

45gr

 

0 degC 2702fps

25 degC 2761fps

 

delta: 59

 

= 2.36fps per degC

 

Varget

 

45gr

 

0 deg c 2783fps

25 deg C 2843fps

 

delta: 60

 

= 2.4fps per degC

 

N150

 

45gr

 

0 degC 2652fps

25 degC 2715fps

 

delta: 63

 

= 2.52fps per degC

 

RL15

 

45gr

 

0 degC 2789fps

25 degC 2855fps

 

delta: 66

 

= 2.64fps per degC

 

 

Well...... according to however this database is calculating all this (but knowing it's literally written using work by rocket scientists ), the rule of thumb in 308 seems to be 2.5 fps per degC - close enough to empirical for government work.

[Davy]

I will have a check tonight and see how it looks against my N540 load and chrono work.

[brown dog]

This is how it theorises for 6.5x47 (picked charge weights that I know people here are using to get c2900fps):

 

N140

38.3gr

0 degC 2774fps

25 degC 2821fps

delta: 47

= 1.88fps per degC

 

Varget

38gr

0 deg c 2835fps

25 deg C 2886fps

delta: 51

= 2 fps per degC

 

RL15

37.5gr

0 degC 2820fps

25 degC 2876fps

delta: 56

= 2.24fps per degC

 

So -rule of thumb for 6.5x47 = 2fps per degC

 

And from "temp insensitive" to "temp sensitive" theoretically only shows a 10fps difference between powders for a wopping 25degC change.

 

I'm finding this interesting even if no one else is!

[brown dog]

Thus;

 

Rules of thumb summary so far(from QL data):

 

Viht, Varget, RL15

 

308 varies c2.5fps per DegC

 

6.5x47 varies c2fps per DegC

[rabbit fingers]

I'm finding this interesting even if no one else is!

I don't have any data to add( yet) but watching with interest.