Rising velocities in a warming barrel. Solutions? Remedies? Powder instability?

[cdherman]

OK, I would like some advice from some of you wiser fellas...

New Bartlein heavy barrel on a Savage SA, in 6.5 Creed. Shot sub MOA groups with the 30 break in rounds, cleaning crazy often as asked to by Bartlein. Next 24 rounds today, using IMR 4350 (have lots of it) loaded from start load to max in 1 gr increments, more or less. (Load data from Berger website).

Brass was random picked from a new batch of Lapua with CCI 450 small rifle primers, NOT FL resized, but necks were cleaned up using Sinclair mandrel, using graphite as lube. Used another dip in the graphite before seating bullets.

But how I loaded them has little to do with what I saw on the chrony.

I was shooting the six 4 shot groups round robin, and the first two shots had multiple velocities that were identical for a given charge. I was feeling the love building.

And then rounds 3 and 4 arrived. The heat had been rising. The barrel is a huge heavy M24 profile and it took a long time to heat up. I was shooting about a round every 3 minutes (yes).

Chrony velocities for rounds 3 and 4 just rose up and up.... In some cases 30- 50 fps over the first pair. Groups remained decent given the conditions, shooter etc..... But I have really bought into the idea that SD and ES need to be tight for long range stuff...

The cartridges were loaded in my basement at about 74 degrees and stored there till I headed to the range. So they were warming to range temp of say 88 at last in the shade. The bays are covered so no direct sun. I cannot really think that it was the IMR 4350. I chamber the cartridge only moments before I shoot, though it takes me 10 -15 seconds to get my rest dialed it and touch the load off...

Maybe the barrel was breaking in as I was shooting? I am probably try to develop loads too soon...... Or is it just crazy to develop loads in the heat. How do fellas in Texas deal with this.. I was being patient as heck, but that barrel just felt warmer and warmer. Never really hot. But it started at 75 and prolly reached 120 (guessing)....

 

[8andbait]

I think you are good to go, barrels continue to increase speeds sometimes up to 200 rounds down the tube. The funny thing is...most of the time the same load is accurate through out the process.

I had a 6.5 SAUM that started at 2980 fps and ended at 3100 fps after about 150 rounds and shot .5 moa or better the whole time with the same load.

My current 7 mag did the same thing, started about 2970 fps ended at 3040 fps after about 120 rounds with the same load and shot in the .2's and .3's the whole time.

Interestingly enough, One barrel was a krieger and the other was a bartlein.

With that being said, IMR powders are not known for being temperature stable so you will probably see some drift with temps anyway. One of the reasons I prefer Hodgdon Extreme powders if available.

 

[Bullet bumper]

My guess is don't close the bolt until you are ready to shoot .
The small sample of velocity variations could be many things including chrony issues .
Try swapping to H 4350 . Finnish the barrel break in first and then try and tune the loads using a three shot group regime . It may settle down after the break in is finished and cases are fire formed .

 

[gohring3006]

I was running at 3 mils for 550 yards last week when it was hot. As the day went on, the sun had come over the roof and started shining on the bench. The ammo box became hot. I noticed I was running at 2.5 mils to get the same impact.
I put the ammo in the shade for about a half hour and went back to 3 mils. This is with a well known temp stable powder.
(Imr 4451).

I bought one of those temp guns but never use it. Now I'm going to try and map the temp change/impact with several of my guns.

 

[Mikecr]

Once the barrel finally warmed up, were further groups better?

Group shooting, which is useless for hunting, needs a barrel at stable temps. Wanna see even worse? Try a 10min shot rate.. Or way different? Try 3hr shot rate.. This is where cold bore load development comes into play for hunting guns.

 

[Rich Coyle]

Once the barrel finally warmed up, were further groups better?

Group shooting, which is useless for hunting, needs a barrel at stable temps. Wanna see even worse? Try a 10min shot rate.. Or way different? Try 3hr shot rate.. This is where cold bore load development comes into play for hunting guns.

I did that ones with a Savage .25-06 I had reamed to .257 Weatherby and heavily fluted. Shot one shot every morning for six days. The group measured 1.040" at 100 yards and was centered around where I sighted it in. The bullets were Barnes TTSX 100 grainers.

 

[cdherman]

Once the barrel finally warmed up, were further groups better?

Group shooting, which is useless for hunting, needs a barrel at stable temps. Wanna see even worse? Try a 10min shot rate.. Or way different? Try 3hr shot rate.. This is where cold bore load development comes into play for hunting guns.

The groups and POI did not change enough at 100 yards to see the increasing velocity. I think the next time I shoot, I will pretty much rapid fire a 5 shot string to warm up the barrel to something approaching steady state. I actually have a infrared temp scanner, never thought to use it on a gun (I have for automotive uses).....

This is making more sense to me now. I started the round robin ( trying Optimal Charge Weight approach of late) and fired only one fowler, and was waiting at least 1:30 between shots and then the 4-5 min timeout every 15 min at the range.

I think for the first 20-25 min the barrel temp was slowly rising. I noted the changes in velocity only when the barrel got pretty warm to the touch. I've always used the approach of "if its getting close to hurting a little when you leave your hand on it for 15 seconds or so, then its too hot" Well, it took me 14 or so rounds over 25 min to get there, but only then were the velocities starting upwards....

Anyhow, I will keep shooting. For now, I have only one lb of H4350 and much more IMR. So I am saving the H4350 for when the barrel is far better broke in....

 

[Greyfox]

My 6.5 CM took about 200 rounds to stabilize velocities. Initial velocities were 2665, finally settling in at 2710/SD<6. I use 42gr of H4350. This powder has proven to be much more temperature stable compared to IMR 4350, maintaining accuracy and velocity(ES) shooting hot or cold in temperatures ranging from 50-85F. The only issue with H4350 is availability......black gold!!!

 

[Mikecr]

Shot one shot every morning for six days. The group measured 1.040" at 100 yards and was centered around where I sighted it in.

This is how you home in on your actual cold bore capabilities. And that 1.040 grouping could be 1/2moa cold bore accurate(which is very good). I would test this at a further range that is your least accurate.
For me, that's 200yds.
For some reason I shoot better in moa at 500 than 200, and 100 does not show external ballistic variances well enough..

 

[gohring3006]

I sorta do that at my local range. I set my steel up and I have a 8" plate that I put at 550 yards.
I grab a rifle out of the truck, dial, judge wind and shoot cold bore. I would like to say I hit it every time, but I don't. I do hit it more often than not.
I usually take 5 or so rifles to the range and my first shots are all at that plate.
That's the max range for this facility, I would love to try this at 800 or so...

 

[dsculley]

To the OP, there are 3 possible scenarios here. As has already been said, the increase in MV could simply be the new barrel settling in. If you are allowing the rounds to sit in the chamber longer, that could be the issue. Could be changes in ambient lighting causing your chronograph to be off.

If it were me, I would pick the node that I like from this range trip, then load up say 20 rounds or so for more testing. Compare your targets and chrongraph data to the original data. That should give you an idea what is going on. I started load development early with my 6.5x47 and pretty much saw what you are seeing. My MV kept increasing until well past 100 rds. During this time, I primarily relied on my targets to point to a good node rather than chronograph data and this worked for me.

FWIW, I will always believe my target over my chronograph. Chrony data is good, but the target is final for me. Also, when I am trying to prove a particular load, I will record chronograph data from multiple trips and combine it all in an Excel spreadsheet to calculate SD. I wind up with a large data set that soon becomes statistically significant (40+ rounds). The SD will probably be much larger than you are used to seeing from 3 or 5 round groups, but it is a better representation of what you can expect. I don't compute or consider ES as it simply shows the total velocity range for that group and has no predictive value.

 

[cdherman]

Chrony was in the shade. Ambient air temps rose about 10 deg, but ammo, shooting rest and gun was all in shade. Cartridges were in the chamber 10-15 seconds (I am a shaky, slow shooter and it takes me a few seconds to adjust my rest, a heavy unit with windage and elevation screw adjustments....)......

Thanks for all the inputs. I am just going to keep shooting for now....

 

[jrock]

I've used a lot of IMR 4350 in my .243 extensively. It gives great groups in my gun but I have never been able to get a good ES. Probably in the high 20's...don't remember all the data off the top of my head. PM me if you want to see some of my data. The temp sensitivity is about 1 fps per degree. When I do my load workups, I record starting and ending temperatures to use in corrections. That really helped correlate my data.

I have also used the inferred thermometer like you mentioned. I have found that 3 min between shots on days under 90 degrees works but needed 4 minutes for hotter days to keep a barrel at a reasonable temperature. I have done groupings both ways, shooting one shot every 3 min and waiting 15 min between 5 shot groups. For velocity data, the intermittent shot string works better. For grouping, both methods yielded similar results.

 

[Hired Gun]

One other aspect to consider when shooting a cold vs. warm barrel is the variation in barrel temperature changing the dimensions of the bore. It would be interesting to keep track of the size of the barrel with a micrometer with .0001" resolution. A caliper won't have the resolution to see minor changes. One would need to mark a spot, for example right in front of the fore end, where it would be easy to measure and check the size at the beginning and end of each string. A tapered barrel will be hard to measure with much precision but I still think you will eventually note a trend.

The proposed hypothesis:
Rather than velocity changing from powder sensitivity since it is stored in a stable spot until it is fired and it would take a while once chambered to actually warm the powder I suspect as the barrel gets warmer the bore diameter increases and creates less friction on the barrel causing bullets to go faster with the same given charge.