Tracking Velocity Changes in a New Barrel

[Jay Christopherson]

I've been tracking QL predictions vs. actual velocity since the first shot down my barrel. Basically, my idea was to track velocity until I got to the "plateau", where the difference between actual and prediction stops changing. At that point, I would be free to modify my QL burn rate and other factors to match my actual velocities and start playing with powder charges.

The only variable input is the # of bullets through the barrel and of course, temperatures. The QL data actually refers to powder temperature I believe and I try to let my ammo equalize as much as possible to ambient, so I use current outside temperature as my QL variable to change expected velocities. The temperatures I've been shooting at have been pretty consistent at 47º, 42º, 44º, 52º, and 47º.

What's interesting (although not unexpected given my previous experience), is that with no other changes, average velocity in my barrel has increased from 2923fps to 3026.3 as of yesterday. Over the course of 164 fired rounds, it has picked up 103.3 fps! I believe it is probably at or near plateau at this point; I wouldn't expect it to pick up much more velocity, if any at all. What's also strangely weird is that my S.D. in each of the 5 sessions has been very low, ranging from 6 to 12 over the course of an entire session. Individual strings *within* a session show some slight (3 - 5fps average) increase, which I simply attributed to heat.

Here's a graph from my 5 sessions with this barrel:

I figure that if my next session shows little to no increase, I can start safely playing around with powder charge variations and get actual, meaningful results from doing so.

I don't know how interesting this is, but with my first rifle, I ran into some serious overcharge issues in the early phases of testing because I wasn't aware of this. I know some (or maybe a lot) are already familiar with this because I asked about it a few years ago. Still, it was interesting to "know" something and have it play out empirically.

 

[WyleWD]

That is intersting for sure.

You can tell me it is none of my business if you want to.... but what is the caliber, bullets used, barrel maker, twist rate of the barrel and powder & charge?? :-\

The reason I'm asking is because I saw almost identical numbers in a barrel that I broke in a few years back. Thanks for the post and it is good information. WD

 

[Jay Christopherson]

This is a 6 Dasher in a 30" Bartlein 1:8 twist barrel. I'm using Berger 105 Hybrids in front of 33.5gn of RL-15 (standard caveats regarding powder charge apply).

 

[RDavies]

This 150 round kick is something Ive seen in quite a few barrels of mine and other shooters. Often, they seemed to increase all of a sudden at around the 150 round mark, not gradualy. I always use the first 150 rounds for basic fireforming, seating depth experiments and for letting new shooters at the club have a play. Once the velocity jump is out of the way, then its time for final load developement.

 

[Keith Glasscock]

I had the same thing happen with a new 30" Krieger in 30 cal.

I'll have to try to remember that every time I get a new barrel. For that matter, they are never as accurate during that first 150-200 rounds.

 

[deadlyswift]

I strongly believe this happens and more than once in a barrels life time. I have seen this in my last 2 Brux 30 cal 308win barrels. It's a long story but both times barrel 1 at around 200 rounds went crazy in the middle of a match. Barrel 2 done it in a practice match at just under 170 rounds. Glad it was practice. Both jump Vel from 75fps to over a 100fps. One relay the barrel was hammering then all of a sudden I was getting vertical that was unexplainable. I came back home and shot the same ammo left over from the match and not only did Vel pick up but groups opened up as well.

 

[deadlyswift]

I had the same thing happen with a new 30" Krieger in 30 cal.

I'll have to try to remember that every time I get a new barrel. For that matter, they are never as accurate during that first 150-200 rounds.

Busdriver, Have you went back and retuned your load? This is what I had to do both times but the barrels went back to hammering after I retuned my powder charge. With my first 308win barrel I had to retune my load about every 300 to 400 rounds after the first initial pick up in Vel. Sometimes it's just seating depth, and sometimes its powder charge, and then sometimes it's both. I honestly believe it's just a matter of staying on top of your load for the condition of the barrel. Then you just get to that point where nothing works and you chunk the barrel and start over with a new one. ;D

 

[Keith Glasscock]

I really didn't find the tune until after the pick-up. For that matter, I doubt I have it down to brass tacks at this point. Right now, I have an honest load capable of winning any match I enter, but I bet it isn't the best the barrel can do.

Once the weather warms up a bit, I'm going to screw it back on and give her a wringing just to see if I can get a feel for how close I am to getting this thing tuned.

 

[Jay Christopherson]

I had a very similar situation last year, although it happened during a match. RL-15 isn't the most temperature stable power in the first place, but when I got out for practice after the match, I was WAAAY higher than I had been at that level. And that was with a lot of rounds down the barrel.

Just for the heck of it, I'm going to keep this track going over the entire lifetime of the barrel, at least as much as I can since I can only track during practice over a chronograph.

I strongly believe this happens and more than once in a barrels life time. I have seen this in my last 2 Brux 30 cal 308win barrels. It's a long story but both times barrel 1 at around 200 rounds went crazy in the middle of a match. Barrel 2 done it in a practice match at just under 170 rounds. Glad it was practice. Both jump Vel from 75fps to over a 100fps. One relay the barrel was hammering then all of a sudden I was getting vertical that was unexplainable. I came back home and shot the same ammo left over from the match and not only did Vel pick up but groups opened up as well.

 

[Jay Christopherson]

Yes, my first barrel (Broughton 5C, 6Dasher), did the same thing. It jumped quite suddenly at around 150 rounds. However, if I knew then what I know now, I would have understood that it was shooting much slower than QL predictions and I would have known to wait until the jump happened before pushing the powder charge up. Instead, I was running what I thought was a good load until pressure was suddenly an issue.

This 150 round kick is something Ive seen in quite a few barrels of mine and other shooters. Often, they seemed to increase all of a sudden at around the 150 round mark, not gradualy. I always use the first 150 rounds for basic fireforming, seating depth experiments and for letting new shooters at the club have a play. Once the velocity jump is out of the way, then its time for final load developement.

 

[queen_stick]

Jay,

thanks for posting this... I was not aware of this situation at all! Very cool that you're keeping track of this through the barrel life

There has to be a pressure increase to jump velocity up that much... is it caused by 'build up' in the bore (carbon, copper, etc..)? I'm thinking that something is causing a restriction on the bullet passing through the bore, causing the pressure increase. If that's what's happening (and I could be very wrong here) then I would think you would run into overcharge issues LATER in the barrel's life, not in the beginning.

not criticizing in any way, just curious what your thoughts are on the root cause...

Walt

 

[Jay Christopherson]

Good question. I don't know the answer. My best guess might be the throat breaking in (slight carbon ring build up, wear, etc...), as inspection with a borescope shows some slight buildup there, despite my best efforts. I've never been able to completely remove the carbon ring, but I can keep it from building up too much. Maybe it just builds up to the point where my cleaning efforts are able to keep it from building any further and at that point, things even out? I clean the barrel after about every 40 rounds or so (which is to say, after a normal practice session). So I start every new practice session with a clean barrel. Guaranteed.

The overcharge issue is the false feeling of security you get EARLY in the barrel's life- QL is telling you that you should be getting Xfps. You run out for your first practice session and you are getting Y fps, which is FAR lower than you expect from QL. So, hey, it should be safe to bump up that powder charge and run it hot right off the bat. Works great for the first 100 rounds or so. Suddenly, you've got hard bolt lifts, flattened primers and all other signs of pressure and your previously "safe" load is seriously over pressure. Experienced guys probably know about this and know what to look for. New shooters may not know what to look for. They start out, everything looks good, and away they go.

So, yes, early on, you are probably safe enough. You probably aren't seeing any real pressure signs in the beginning, even with what QL would state is a "hot" load. It's that jump, if you aren't looking for it, that'll get you.

Jay,

thanks for posting this... I was not aware of this situation at all! Very cool that you're keeping track of this through the barrel life

There has to be a pressure increase to jump velocity up that much... is it caused by 'build up' in the bore (carbon, copper, etc..)? I'm thinking that something is causing a restriction on the bullet passing through the bore, causing the pressure increase. If that's what's happening (and I could be very wrong here) then I would think you would run into overcharge issues LATER in the barrel's life, not in the beginning.

not criticizing in any way, just curious what your thoughts are on the root cause...

Walt

 

[queen_stick]

Makes perfect sense, Jay. Thanks for the response.

 

[WyleWD]

Jay,

There has to be a pressure increase to jump velocity up that much... is it caused by 'build up' in the bore (carbon, copper, etc..)? I'm thinking that something is causing a restriction on the bullet passing through the bore, causing the pressure increase. If that's what's happening (and I could be very wrong here) then I would think you would run into overcharge issues LATER in the barrel's life, not in the beginning.
Walt

If this is true, it contradicts what I was told by a barrel maker & a trustworthy gunsmith regarding custom made stainless barrels. Was told that as the rifle was shot it would "smooth out" the small imperfections, rough spots, etc in the barrel from the rifling & drilling processes and speed would increase and there might be some DROP or leveling out in the pressure curve. Thus a reduced load used in break in may be able to be bumped up or a change in powder burn rate after break in.

I know it is debateable without data, but.... there is merit to either side of the debate IMHO. WD

 

[Jay Christopherson]

That could be too. Maybe it's a combination of both. I wish I knew how to design a test that could prove it one way or the other.

Jay,

There has to be a pressure increase to jump velocity up that much... is it caused by 'build up' in the bore (carbon, copper, etc..)? I'm thinking that something is causing a restriction on the bullet passing through the bore, causing the pressure increase. If that's what's happening (and I could be very wrong here) then I would think you would run into overcharge issues LATER in the barrel's life, not in the beginning.
Walt

If this is true, it contradicts what I was told by a barrel maker & a trustworthy gunsmith regarding custom made stainless barrels. Was told that as the rifle was shot it would "smooth out" the small imperfections, rough spots, etc in the barrel from the rifling & drilling processes and speed would increase and there might be some DROP or leveling out in the pressure curve. Thus a reduced load used in break in may be able to be bumped up or a change in powder burn rate after break in.

I know it is debateable without data, but.... there is merit to either side of the debate IMHO. WD

 

[queen_stick]

Jay,

There has to be a pressure increase to jump velocity up that much... is it caused by 'build up' in the bore (carbon, copper, etc..)? I'm thinking that something is causing a restriction on the bullet passing through the bore, causing the pressure increase. If that's what's happening (and I could be very wrong here) then I would think you would run into overcharge issues LATER in the barrel's life, not in the beginning.
Walt

If this is true, it contradicts what I was told by a barrel maker & a trustworthy gunsmith regarding custom made stainless barrels. Was told that as the rifle was shot it would "smooth out" the small imperfections, rough spots, etc in the barrel from the rifling & drilling processes and speed would increase and there might be some DROP or leveling out in the pressure curve. Thus a reduced load used in break in may be able to be bumped up or a change in powder burn rate after break in.

I know it is debateable without data, but.... there is merit to either side of the debate IMHO. WD

WyleWD,

I was thinking about that exact info as I typed my response. It's interesting that Jay's results seem to contradict that (along with a bunch of other people). I agree, it's one of those topics that could be argued either way, while it's very difficult to scientifically prove one vs the other.

I think it's important to recognize this issue, and know to watch for it. Maybe it's less drastic or almost non-existent in some barrels, which may also depend on what cartridge it's chamber for? But expecting a change like this would be a big help if you all the sudden lose your "sweet spot," as a couple guys mentioned above. Before hearing about this, I likely would have questioned my equipment, not the load, and certainly not the barrel reaching a specific round count (a round count that low anyway).

This issue also magnifies the importance of keeping records and notes on loads, velocity #'s, and etc. Also verifying those numbers by continuing to use you chronograph in practice sessions, not just the load development session. Something I need to start doing my self :

 

[Jay Christopherson]

This issue also magnifies the importance of keeping records and notes on loads, velocity #'s, and etc. Also verifying those numbers by continuing to use you chronograph in practice sessions, not just the load development session. Something I need to start doing my self :

I have found this statement to be absolutely true. I never shoot a practice round anymore that is not over a chronograph. I think it's really helped me immensely. Might not be for everybody, but I love me some data to crunch.

 

[6BRinNZ]

This is quite an interesting thread. I have been working on ironing it these odd ball vertical jumps as well over the last season.

I chrony every load development and practice session so I have a reasonable amount of data.

In my situation I have been able to characterise the vertical as having 3 characteristics;

1./ Environmental. I shoot varget in a 6Br, I initially developed my load in winter ~8 c at ~2880fps. In summer at ~25 c I am now running at ~2980fps. We have just moved into autumn so I am going to see if the fps starts to drop back.

2./ We shoot 2 x 12 shot strings and I found inconsistently that on the second string somewhere around shot 3 that I would get a jump in vertical, I would come down a click, a shot would go appropriately low and then it would stay consistent. This is tricky to resolve, I started off looking at the barrel but I am now going down the brass route.

3./ The group starts to print high on the bull (in the middle of a relay) less than 1 clicks worth (a real pain with .25 moa clicks) and then pop, 1 will go high, I bring it down 1 click and all is good. The nature of this one looks like some sort of pressure issue but a bore scope has shown no specific fouling issues.

Anyways - I have posted this as I am curious to see if you guys would characterise what you are seeing as being similar or if you are simply seeing a FPS jump that stays and is consistent at any time past that point once it has occurred.

Also I am curious how you define your ES SD with the FPS jump that you see - I always see low SD ES figures posted but I assume it is mostly over a small sample as to my mind the fps jump effectively blows ES until the load is re tuned.

TIA

 

[Jay Christopherson]

I don't really pay any attention to E.S., I'm all about Std. Deviation. I usually measure my S.D. at two points:

* over a string, which in practice is 10 shots
* over a session, which in practice is usually 40 shots

I generally don't see a real jump in velocity across a single session, although by shot 30, I've picked up a few fps, mostly due to heat I am guessing.

As for what you are seeing- I think what I'm describing in my original post is different from what you are seeing. I see some of the same behaviors you do (temp. sensitivity of powder, vertical dispersion) which are usually attributable in my case to not accounting for increase/decrease in temp (RL-15 is fairly sensitive), bad loading procedure, or poor consistency in my shooting technique. There are lots of variables for vertical stringing and I know there are lots of topics about it.

In the case of this topic, I'm seeing a FPS increase at the beginning of the barrel life to around 150 rounds, that has been consistently present on my last 4 barrels (Krieger, Bartlein, Broughton, Broughton), across 3 different chamberings. After 150 shots, it seems to even out (plateau) and then stay consistent for quite a while. I did notice a small jump in my first Dasher (Broughton) at around 900 - 1000 rounds, but I haven't shot any of my other barrels enough to verify that. Also, I wasn't very nice to my first barrel so it could have just been me not taking care of it well enough.

 

[6BRinNZ]

jaychris - yep all good, I realised I was slightly off but the fact that you and the others were seeing differences mid string made me post. In retrospect I should probably have simplified my post to this - are you simply seeing a FPS jump that stays and is consistent at any time past that point once it has occurred.


While I have clear problem definitions I am still looking at the most productive ways to solve them and some of the comments sparked my interest.
Definitely not looking for this thread to solve my problems

Thanks