Velocity change question

[muleman69]

Friend of mine bought a new rifle and worked up an accurate load back around the first of the year, took it out this week and the velocity jumped up 100fps and groups went south . The barrel has 70 rounds through it now, could it have sped up that much during break in? Temps did change but not all that much and barrel is clean. He backed powder off to his original velocity and ended up with a pattern not a group. Just asking, thanks

 

[Poor Shot 65]

IMO performing load analysis on a new barrel is a waste of time. Wait until a barrel has 50+ rounds through it and then start. I always see an FPS increase when a barrel is broken in (the exact increase is different for every barrel).

Adam

 

[Texas10]

Unless I'm mistaken, barrel time, not velocity, and the two are not necessarily connected, is what drives precision. IMOP downloading to a previous muzzle velocity, especially when the barrel is still breaking in should not be expected to recapture lost precision since there is no way to predict changes in barrel time.

Instead, only once the muzzle velocity has stabilized with a particular load, I would begin to work a load up, and having some sort of confidence that it would remain accurate.

 

[muleman69]

Unless I'm mistaken, barrel time, not velocity, and the two are not necessarily connected, is what drives precision. IMOP downloading to a previous muzzle velocity, especially when the barrel is still breaking in should not be expected to recapture lost precision since there is no way to predict changes in barrel time.

Instead, only once the muzzle velocity has stabilized with a particular load, I would begin to work a load up, and having some sort of confidence that it would remain accurate.

We thought it had stabilized until the spike,the powder is suppose to be one of the least sensitive also?

 

[46and2]

Friend of mine bought a new rifle and worked up an accurate load back around the first of the year, took it out this week and the velocity jumped up 100fps and groups went south . The barrel has 70 rounds through it now, could it have sped up that much during break in? Temps did change but not all that much and barrel is clean. He backed powder off to his original velocity and ended up with a pattern not a group. Just asking, thanks

Has he cleaned it in those 70 rounds?

Also, if he is using a double based powder the velocity could increase because of the warmer temps.

 

[muleman69]

Has he cleaned it in those 70 rounds?

Also, if he is using a double based powder the velocity could increase because of the warmer temps.

cleaned yes, powder not double based

 

[Jager]

Statistical reversion-to-the-mean?

How many shots went into declaring that original accurate load?

 

[Ned Ludd]

A 100 fps increase in velocity is more than you should typically expect from a barrel speeding up in the first couple hundred rounds. Chances are good something else has changed, but the difficulty may be in finding out exactly what that "something" was. The obvious things to check are any change in components (i.e. new or different Lots of powder/primers/brass/bullets), temperature, and the inside of the barrel/chamber. Have you bore-scoped the barrel by any chance? It might be worth a look to find out whether you have some carbon buildup. Regardless, this is a situation where stepwise check and/or test of a single variable at a time is warranted to isolate the underlying cause. Alternatively, if the new higher velocity is holding steady, you can simply re-work the load after reducing charge weight to get it back to the original velocity window. Although we always want to know WHY something changed, sometimes we have to settle for re-working a load and getting it to shoot well.

 

[muleman69]

Statistical reversion-to-the-mean?

How many shots went into declaring that original accurate load?

Approximately,50

 

[muleman69]

A 100 fps increase in velocity is more than you should typically expect from a barrel speeding up in the first couple hundred rounds. Chances are good something else has changed, but the difficulty may be in finding out exactly what that "something" was. The obvious things to check are any change in components (i.e. new or different Lots of powder/primers/brass/bullets), temperature, and the inside of the barrel/chamber. Have you bore-scoped the barrel by any chance? It might be worth a look to find out whether you have some carbon buildup. Regardless, this is a situation where stepwise check and/or test of a single variable at a time is warranted to isolate the underlying cause. Alternatively, if the new higher velocity is holding steady, you can simply re-work the load after reducing charge weight to get it back to the original velocity window. Although we always want to know WHY something changed, sometimes we have to settle for re-working a load and getting it to shoot well.

Chamber and barrel has been scoped and looks great

 

[CharlieNC]

Via Google many gun builders report a velocity increase from 50-150fps within the first 200 rounds as not unusual, and they recommend doing load workup after it has stabilized. The potential reasons are not consistent, nor is the expected increase. I have not personally experienced this degree but I would shoot more until stability has been achieved, clean thoroughly, then work up the load. Did you observe an increasing trend within those initial 50 shots?

 

[JFrank]

I use a pro Chrono, one load over two days two different speed average s 2920 and 2987
Broke down the loads etc. All the same powder charges.
I can't explain it.

 

[Intheshop]

Bedding?

 

[muleman69]

Via Google many gun builders report a velocity increase from 50-150fps within the first 200 rounds as not unusual, and they recommend doing load workup after it has stabilized. The potential reasons are not consistent, nor is the expected increase. I have not personally experienced this degree but I would shoot more until stability has been achieved, clean thoroughly, then work up the load. Did you observe an increasing trend within those initial 50 shots?

No Sir

 

[muleman69]

Bedding?

No,has pro bed job

 

[MislMan]

could be related to the environment that existed when the loading of the cartridges was done. Differences in humidity during different loading cycles can affect burn rate of the powder (I believe single based powders are more susceptible to this). For instance, if you load ammo on Monday morning (1 humidity level) and then load again on Friday afternoon (different humidity level) you could see significant swings in muzzle velocity depending on powder used. If you happen to leave powder in the hopper between loading cycles there may be an even larger discrepancy in powder humidity level. Even different storage environments (inside house vs. in garage) could change humidity level within the powder itself.

 

[Texas10]

One other possibility is that being a new barrel, the leade (the start of the lands) is being worn down quickly with each successive round fired. I have measured this wear rate and it can be quite dramatic at first as the angle that is cut into the lands by the reamer is flattened out by the bullets.

This will measurably change the distance to the lands with possible changes in starting pressure, barrel time, jump or jam distance, all the things we play with to tune a load.

At 70+ rounds down the barrel, I would start with a seating depth study followed by charge weight ladder. Or you can do both in a 9 shot load workup. Start by identifying 3 jam/jump dimensions you want to try, then load three different charge weights for each seat depth. Shoot into targets arranged by seat depth and track each shot. This will identify what seat depth is not charge sensitive. If, for instance, on the .020 jump target shots 1&2 which differ by say, 1 grain fall into the same hole, with shot 3 slightly off, you've identified a forgiving seating depth.