308 Velocity increase

[Redrocket]

So I am shooting normal Berger 155gr Hybrid Ta rget308 rounds out of my Rem 40X and last three shoots start giving me higher velocities. First one I deleted because I thought it abnormal but kept the last two to look at at home. Only thing I can figure is the day was hot and am wondering how fast can they heat up in the chamber with the bolt closed. These were the last rounds I fired but I bet I fired them within 30sec of locking bolt. You can see the first 6 rounds all pretty consistent but last ones spiked. All weighed powder charges.

 

[Straightshooter1]

So I am shooting normal Berger 155gr Hybrid Ta rget308 rounds out of my Rem 40X and last three shoots start giving me higher velocities. First one I deleted because I thought it abnormal but kept the last two to look at at home. Only thing I can figure is the day was hot and am wondering how fast can they heat up in the chamber with the bolt closed. These were the last rounds I fired but I bet I fired them within 30sec of locking bolt. You can see the first 6 rounds all pretty consistent but last ones spiked. All weighed powder charges.

Different things could be a play. Details about everything would help get a better idea, including like powder used cleaning results.

As far as heat . . . I was having an issue with that today on my .308 as it was 95°F this morning where I and all my stuff was sitting in the sun. I tried to shade my ammo at all time, handfeeding a cartridge out from the shade into the chamber, but I wouldn't close the chamber until I was well sighted in. So my cartridge was fired withing 10 seconds. My barrel got hotter than ever before, even though I tried to keep it shaded during cease fire.

My last few rounds were definitely faster than than those when I started the day, but surprisingly, it wasn't extreme and my barrel at the end at gotten hotter than I ever let it get before, like at 148°F.

 

[CF Junkie]

IMR4064 is temperature sensitive - about 155 fps over a 0 to 125 deg F range.
That turns amounts to 1.24 fps per deg F. for changes in temperature and it is pretty linear.
How much your chamber temperature rises depends upon the barrel (thin barrels heat up faster) and how hot your load was. Letting the round sit in the chamber for 30 seconds will cause the round to heat up pretty quickly.
I have a temp strip on all my target barrels (mostly bulls) and try to keep the temperature within 15 deg F of my starting load. A couple of sighters gets the barrel to about 85 degrees.
The strips are not calibrated and are probably underestimating the temperature in the chamber, but I find they are very consistent and I rely on them to know when it is getting hot enough to cause POI changes.
With my .308 loads, a 15 deg. change 'after sighter temp' occurs within 5 to 7 rounds in ambient temps around 70 degrees.
But, my loads are in the middle of the load table.
If I was pushing for velocity, the barrel would heat up quicker because the powder load would be larger.
If it is 90 degrees F. ambient temp, I would be looking carefully at the barrel temp around 4 rounds because
the barrel heat is not dissipating quickly to keep the temperature from rising more quickly.

 

[mgunderson]

More information please. More rounds before or after. We are all guessing until you tell if this was the first rounds out of the rifle. Lots of good National team shooters in Texas that should be able to help.

 

[CF Junkie]

I shoot 25 rounds with each load. I generally shoot two sighters and then 5-round groups.
When I maintain my discipline, I stop when the temperature approached 110 to 113 degrees to keep temperature dispersion to a minimum. I often start a new group with a sighter if the temp is around 85 degrees.
I measure the 5-round groups.
I use a battery powered air mattress pump with a plastic tube that fits into my chamber to cool the barrel back to about 90 degrees so I can start another 5 round group.

With the new Shilen barrel on my .223, 25.7% of my groups measure under 0.200 inches. When I don't adhere to my routine, the groups grow to about 0.215. Of course, there are also some 'Oh Darn' trigger pulls and some shots where I didn't complete my set up correctly, they tend to be around 0.240.

So far, for 241 groups with the new barrel with 15 different bullets, the overall 5-round group average is 0.218.
I have tried 61 different loads.

 

[charlie b]

I'd say it was temperature related. Letting a round sit in a hot barrel will cause the powder to react more. Some powders are more sensitive to this than others, but, they will all do it to some extent. If the barrel is really hot you can cook off the round, an exciting event that needs to be experienced to be understood. Usually happens with a belt fed gun and a LOT of firing. Mine was a .50cal M85.

 

[Redrocket]

IMR4064 is temperature sensitive - about 155 fps over a 0 to 125 deg F range.
That turns amounts to 1.24 fps per deg F. for changes in temperature and it is pretty linear.
How much your chamber temperature rises depends upon the barrel (thin barrels heat up faster) and how hot your load was. Letting the round sit in the chamber for 30 seconds will cause the round to heat up pretty quickly.
I have a temp strip on all my target barrels (mostly bulls) and try to keep the temperature within 15 deg F of my starting load. A couple of sighters gets the barrel to about 85 degrees.
The strips are not calibrated and are probably underestimating the temperature in the chamber, but I find they are very consistent and I rely on them to know when it is getting hot enough to cause POI changes.
With my .308 loads, a 15 deg. change 'after sighter temp' occurs within 5 to 7 rounds in ambient temps around 70 degrees.
But, my loads are in the middle of the load table.
If I was pushing for velocity, the barrel would heat up quicker because the powder load would be larger.
If it is 90 degrees F. ambient temp, I would be looking carefully at the barrel temp around 4 rounds because
the barrel heat is not dissipating quickly to keep the temperature from rising more quickly.

I think you've answered alot of my problem. I too was using temp strips and a barrel cooler in between groups. It was 95F and firing last group I bet barrel strip was 122F which means it was hotter in chamber. Using a heavy palma contour. I think after I use up my 4064 I'll have to pick a more temperature stable powder. thanks for your replies

 

[Redrocket]

More information please. More rounds before or after. We are all guessing until you tell if this was the first rounds out of the rifle. Lots of good National team shooters in Texas that should be able to help.

It was the last 10 rds out of 40 on a hot Texas afternoon. But I don't think they were in chamber 30 sec, just long enough to lock bolt and aim. Goal for the day was really too learn and use and get experience on SEB Neo-X rest.

 

[222Jim]

How much jump? If you are close to jamming, say less than 3 thou of jump, you could have inadvertently jammed the bullets. That causes a pressure spike.

The sudden velocity increase you are seeing is significant, so I'm not convinced it's temperature related.

How did the bullets group relative to the rest? Is you velocity measurement right? Just questioning things to gain a fuller understanding.

 

[Redrocket]

How much jump? If you are close to jamming, say less than 3 thou of jump, you could have inadvertently jammed the bullets. That causes a pressure spike.

The sudden velocity increase you are seeing is significant, so I'm not convinced it's temperature related.

How did the bullets group relative to the rest? Is you velocity measurement right? Just questioning things to gain a fuller understanding.

Bullet jump is .020 off lands, bullet impact seemed a little higher. Was using match aperture sights. Great fun

 

[222Jim]

Bullet jump is .020 off lands, bullet impact seemed a little higher. Was using match aperture sights. Great fun

I went through ~600 rounds of .308 I've got data for (Varget using Berger 168 VLD and Sierra 168 Match King). Maximum velocity spread is 72 FPS, and the normal range is ~30 FPS. These are based on 10 and 20 shot groups. Different bullets and powder, but still, your ES appears to be ~175 FPS from eye-balling the chart, and that requires ~2.5 grains of powder.

I'm still skeptical it's temperature related, but have no idea what it could be given jump looks unlikely. Please let me (us) know what it is if you ever come to a conclusion. And given "Velocity = Pressure", I'd want to know before moving up to near or max charges!

 

[Redrocket]

I went through ~600 rounds of .308 I've got data for (Varget using Berger 168 VLD and Sierra 168 Match King). Maximum velocity spread is 72 FPS, and the normal range is ~30 FPS. These are based on 10 and 20 shot groups. Different bullets and powder, but still, your ES appears to be ~175 FPS from eye-balling the chart, and that requires ~2.5 grains of powder.

I'm still skeptical it's temperature related, but have no idea what it could be given jump looks unlikely. Please let me (us) know what it is if you ever come to a conclusion. And given "Velocity = Pressure", I'd want to know before moving up to near or max charges!

I can't but help think it was temperature. Last 12 of 40 shots ambient air 95F. Temp strip on bbl up to 122F. Would use bbl cooler down to 104F and restart. Shot 50rds with my PPC and no problems w/VV N-133. Just when 43.5grs going off instead of 29.3grs make a big difference.

 

[ptf18]

O.P. I'd give your setup (load,rifle,etc) another go at the range and see what occurs.

I've recently got 40X in .308 (repeater). The gun is OEM condition and have been working on load for it. Have not had an issue as what the O.P. has. I'm in Texas and the recent temps that I've shot in were in the 90's. I'm not shooting rapidly and I'm not getting the barrel overly "hot" BUT I'm not cooling the barrel down either.

 

[mgunderson]

Temperature is likely part of the delta. But we would normally see a more gradual increase in Temperature. A powder change might be a logical change. N140, varget, n150, are the typical in the 155 palma bullets.

Still smells like something else is contributing. Best of luck.

 

[Redrocket]

Temperature is likely part of the delta. But we would normally see a more gradual increase in Temperature. A powder change might be a logical change. N140, varget, n150, are the typical in the 155 palma bullets.

Still smells like something else is contributing. Best of luck.

It's a new bartlin 1:10(?) bbl on it. > 80rds, I load for accuracy not velocity so I don't chase for the hottest load.

 

[Doom]

I kept hoping you would give more information but I doubt that this is a temperature problem with the rounds or the heating from the barrel based on your description. But if these last two rounds were sitting in the sun for long then that might be an issue. I shoot a lot of 4064 and have never seen this issue when shooting from the 30s to 95 degrees ambient. However if the rounds were left in the sun or in a container left in the sun all bets are probably off.

 

[Redrocket]

I kept hoping you would give more information but I doubt that this is a temperature problem with the rounds or the heating from the barrel based on your description. But if these last two rounds were sitting in the sun for long then that might be an issue. I shoot a lot of 4064 and have never seen this issue when shooting from the 30s to 95 degrees ambient. However if the rounds were left in the sun or in a container left in the sun all bets are probably off.

What other info would be helpful. Glad to see you're a 4064 user also. These are all weighed charges. 44.5grs rem 9 1/2 norma cases with 155gr berger hybrid target bullets in a newish bartlien bbl. Not close to making max charge by what I see in some manuals. I work up from a lower velocity to normal range looking for accuracy. Wish I didn't have 2 1/2 jugs of this powder now. So now maybe thinking when I was putting chrono up one of the sky screens diffuser popped of to the side, sun was to the side also. Think that's an issue?

 

[kevinbritvec]

So I am shooting normal Berger 155gr Hybrid Ta rget308 rounds out of my Rem 40X and last three shoots start giving me higher velocities. First one I deleted because I thought it abnormal but kept the last two to look at at home. Only thing I can figure is the day was hot and am wondering how fast can they heat up in the chamber with the bolt closed. These were the last rounds I fired but I bet I fired them within 30sec of locking bolt. You can see the first 6 rounds all pretty consistent but last ones spiked. All weighed powder charges.

What other info would be helpful. Glad to see you're a 4064 user also. These are all weighed charges. 44.5grs rem 9 1/2 norma cases with 155gr berger hybrid target bullets in a newish bartlien bbl. Not close to making max charge by what I see in some manuals. I work up from a lower velocity to normal range looking for accuracy. Wish I didn't have 2 1/2 jugs of this powder now. So now maybe thinking when I was putting chrono up one of the sky screens diffuser popped of to the side, sun was to the side also. Think that's an issue?

Do you have a ring of hard carbon forming in the rifle's throat? That will spike pressure and velocity.

 

[Redrocket]

No I pay attention to that with my bore scope. Like I said it's a fairly low rd count. That was only 3rd time I cleaned it a was going to put some iosso on a patch for last step to treat neck but it was good. I use that sharp shooters carbon out everytime with plenty of residence time in there.

 

[paulT]

How many rounds through new barrel
Velocity increase could be due to barrel break in process.