Why does velicity increase with # of shots?

[MTLIVIN]

So my buddy and I have both experienced a significant velocity gain with our benchmark & Krieger barreled custom rifles after finding the sweet spots. The gain on both of out rifles was significant, 100 - 150 fps with the EXACT SAME load, powder lot, brass, ect.... It required multiple fine tuning sessions to lower the load to the original velocity node, up to 2 full grains less than the original load to obtain the same velocity.

The loads are 7mm rem mag w 180 hybrid and H1000, and 28 nosler w 195 hybrid and RL33. Both guns shoot under .25 MOA in the right node, both were properly broken in barrels, load development started after 50 rounds. We both saw large enough velocity gains to move us out of the nodes at around 200 rounds and the velocity gains are still being seen in my 7mm after 450 rounds now. Barrels are thoroughly cleaned of copper and carbon about every 50 rounds.

How many of you guys see this? We both use the magnetospeed chrony and wouldn't have easily documented this velocity gain without one. These are our first full custom guns and I haven't paid enough attention to the details of other rifles to document this before (also didn't have a good chrony). What do you think causes the pressure & velocity gains and do you think it will even out and and stop increasing pressure/velocity eventually? (pressure definitely increased along with velocity, went from no pressure signs to ejector marks with same loads)

 

[J E Custom]

So my buddy and I have both experienced a significant velocity gain with our benchmark & Krieger barreled custom rifles after finding the sweet spots. The gain on both of out rifles was significant, 100 - 250 fps with the EXACT SAME load, powder lot, brass, ect.... It required multiple fine tuning sessions to lower the load to the original velocity node, up to 2 full grains less than the original load to obtain the same velocity.

The loads are 7mm rem mag w 180 hybrid and H1000, and 28 nosler w 195 hybrid and RL33. Both guns shoot under .25 MOA in the right node, both were properly broken in barrels, load development started after 50 rounds. We both saw large enough velocity gains to move us out of the nodes at around 200 rounds and the velocity gains are still being seen in my 7mm after 450 rounds now. Barrels are thoroughly cleaned of copper and carbon about every 50 rounds.

How many of you guys see this? We both use the magnetospeed chrony and wouldn't have easily documented this velocity gain without one. These are our first full custom guns and I haven't paid enough attention to the details of other rifles to document this before (also didn't have a good chrony). What do you think causes the pressure & velocity gains and do you think it will even out and and stop increasing pressure/velocity eventually? (pressure definitely increased along with velocity, went from no pressure signs to ejector marks with same loads)

It is not unusual for a new barrel to gain velocity as the barrel gets broke in. normally 25 to 50 ft/sec at most. Something else has changed to gain 100 to 250 ft/sec. could be a carbon build up or case neck tension after very many uses.

Very strange ***

J E CUSTOM

 

[Pdvdh]

A 250 fps increase is unreal. Check your reloading scales? You both using the same reloading scale?

That or your Magnetospeed has gone belly up... in a consistent manner.

 

[MTLIVIN]

A 250 fps increase is unreal. Check your reloading scales? You both using the same reloading scale?

That or your Magnetospeed has gone belly up... in a consistent manner.

My velocity gains were not that extreme, started at 2930 and went up to over 3000 fps (7mm rem mag), but I think they would have kept going up if I didn't lower the load. My buddy's 28 Nosler went from 2980 fps to 3125 fps, so not as high as I originally thought but very significant. The chronys are not to blame and the reloading practices are meticulous, only 2-3 shots on each case over this entire period too. I will edit my original post to say a 150 fps gain. Both use RCBS Chragemaster for loads, separate scales too.

 

[brentc]

I've seen barrels gain up to 70 fps during break in. I've also seen fire formed brass loads gain another 20-40 fps over virgin brass loads.

A combination of both could be what you're seeing. Belted mag brass expands significantly at the shoulder after they've been fired the first time.

 

[longgunshooter]

Marking

 

[Frank in the Laurels]

No, no, no.....when the chamber heats up it transfers that heat into the new cartridge while your sitting there readjusting everything... The new unfired round goes from air temp to hot very quickly which affects the velocity, the more you shoot the hotter it gets inside the case causing pressure increases which in turn causes velocity change and impact difference....what a thousand yard shooter in action, not only does he want as many as he can get in the same air but he doesn't give the new cartridge time to heat up...I've watched many that have two or three in the air at the same time!!! I've heard shooters say they've seen four but I've never seen that many...just another perspective to drive you nuts.....lightbulb

 

[MTLIVIN]

I've seen barrels gain up to 70 fps during break in. I've also seen fire formed brass loads gain another 20-40 fps over virgin brass loads.

A combination of both could be what you're seeing. Belted mag brass expands significantly at the shoulder after they've been fired the first time.

I think your probably right on here, I did see the increase in velocity from virgin brass to fire formed. The thing I can't figure out is why the velocity is still creeping up after 400 rounds, hopefully it levels out now cause I'm tired of changing the load to stay within pressure that won't ruin my brass!

 

[parshal]

...when the chamber heats up it transfers that heat into the new cartridge while your sitting there readjusting everything... The new unfired round goes from air temp to hot very quickly which affects the velocity, the more you shoot the hotter it gets inside the case causing pressure increases which in turn causes velocity change and impact difference....

I've definitely seen this. I shoot my 22 Creedmoor like a 22 rimfire and don't care about the barrel. I've not seen increased velocity over days, though, only when I shoot as stated above.

 

[Bravo 4]

A 250 fps increase is unreal. Check your reloading scales? You both using the same reloading scale?

That or your Magnetospeed has gone belly up... in a consistent manner.

Two totally different rifles in different calibers, different barrel manufacturers, using different powders and powder measures, shot on the same days(?) and both experiencing the same phenomenon....look for a common denominator.
That appears to be the magnetospeed, if that's the only chronograph you are both using. If that's the case then try another chony or send yours back. If not, then disregard this whole post.... and carry on!

 

[Pdvdh]

Bingo! Two rifles - what's the common link?

However, if all else is ruled out leaving the barrel as the suspect cause of increasing pressures and velocity, then I would run that rifle to the nearest friend or gunsmith in possession of a quality bore scope and have the cleaned bore examined. The only other example similar to your experience ever Posted on this forum, to my knowledge, was caused by premature bore degradation - which was visually apparent thru a bore scope. Perhaps bad steel in combination with a bore-burning cartridge - the 300 RUM? Here's a link to the Thread:

http://www.longrangehunting.com/forums/f19/300-rum-barrel-life-whole-documantation-40785/

 

[MTLIVIN]

Bingo! Two rifles - what's the common link?

However, if all else is ruled out leaving the barrel as the suspect cause of increasing pressures and velocity, then I would run that rifle to the nearest friend or gunsmith in possession of a quality bore scope and have the cleaned bore examined. The only other example similar to your experience ever Posted on this forum, to my knowledge, was caused by premature bore degradation - which was visually apparent thru a bore scope. Perhaps bad steel in combination with a bore-burning cartridge - the 300 RUM? Here's a link to the Thread:

http://www.longrangehunting.com/forums/f19/300-rum-barrel-life-whole-documantation-40785/

What I can tell you for sure is that there is zero correlation between the two rifles, we actually haven't even shot them side by side, just both experienced the problem of groups opening up and then confirming the velocity had increased enough to move out of the accuracy node (with 2 separate Magnetospeeds). I will say my buddies 28 Nosler seems to have moved up in speed and out of the node the worst of the two rifles.

The above link does show some good evidence that I agree with, if velocity goes up it is because pressure also went up and if the charge hasn't changed that leaves only the barrel to look at. I clean mine with Wipeout and Boretech, alternating every 50 shots or so. We both let the barrel cool between shots so it wouldn't be fun to find out the barrels are already heat cracked and eroding at 200 rounds for the 28 Nosler and 400 rounds for the 7mm mag. The smith that built the 28 nosler may have a borescope to look at them, but Im not even going there until rifle season is over in MT.

So when you guys are putting on new barrels, what do you typically see for velocity gains over 50, 100, 500 shots (same charge, lot, ect...)? Do you get the velocity to level out perfectly after 100 rounds and how long does that last???

 

[parshal]

I clean mine with Wipeout and Boretech, alternating every 50 shots or so.

Are you cleaning because you see accuracy degradation? I cleaned my 6 and 22 Creedmoors at 500 rounds and neither had seen a degradation in accuracy. I wouldn't clean unless the rifle tells you must be cleaned. I doubt this has any effect on your increasing velocity, though.

So when you guys are putting on new barrels, what do you typically see for velocity gains over 50, 100, 500 shots (same charge, lot, ect...)? Do you get the velocity to level out perfectly after 100 rounds and how long does that last???

I've never seen velocity gains on my custom barreled guns even after 500 rounds. If I do it's usually due to a different load lot and usually within a small margin. I'm real curious to see if you figure out a reason for the gains.

 

[lancetkenyon]

No, no, no.....when the chamber heats up it transfers that heat into the new cartridge while your sitting there readjusting everything... The new unfired round goes from air temp to hot very quickly which affects the velocity, the more you shoot the hotter it gets inside the case causing pressure increases which in turn causes velocity change and impact difference....what a thousand yard shooter in action, not only does he want as many as he can get in the same air but he doesn't give the new cartridge time to heat up...I've watched many that have two or three in the air at the same time!!! I've heard shooters say they've seen four but I've never seen that many...just another perspective to drive you nuts.....lightbulb

Just to clarify...

Are you saying that at 1000 yards, a single shooter can have two to four bullets in the air, between exiting the muzzle and hitting the target, at the same time?

You are talking fire, target re-acquisition, fire, target re-acquisition, fire (and maybe one more set of that)...in the span of 1.4-2.0 seconds TOF of the first bullet, depending on cartridge and bullet.