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

[gohring3006]

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.

That would be a great test, but I believe the velocity will decrease with the expansion of the diameter/friction. It would fall under the same principal of friction reduction bullet coatings and the decreased friction decreases pressure.

 

[jrock]

I haven't tested my loads at distances over 500 yards. I have gotten nice 2" groups concisely at 400 yards with my load. The error on my F1 Chorny is very large when looking at single digit ES values. I hope to do a 1000 yard grouping test and use a more rigid chrony stand. I'm hoping a more rigid setup will get more consistent values. I'm using an F1 Chrony.

The bore change due to heat got my wheels spinning. If I think of a barrel section as a ring of metal, when it is heated, it expands both the ID and OD. I have often used this property to do shrink fit connections. I ran a quick calc with a temperature swing of 60 degrees. Using an OD of a 1.25" blank, it gave that the diameter would only increase by 0.00008". Thus the bore diameter would probably increase much less. However, since my barrels are all sporters, and thus smaller diameters, I doubt I could get a reading with my 0.0001" mic. However, it sounds like a good test to try to back up the numbers.

 

[jrock]

Just noticed the old date on this thread. Thanks for the good discussion and ideas though.

 

[Mikecr]

Measuring a bore is a lot more difficult than mics or calipers(which cannot). Even air gauging is not direct/actual but comparative measure. Far as I know, the only barrel maker in the world who has the capability to directly/actually measure bores is Loather Walther. They are for sure the only barrel maker capable of making barrels with any and all known methods -to specification of the order. This, in lots of 100 minimum last I asked.

Whether a bore increases, decreases, or remains static dimension with temperature transients is tied to manufacturing method and contour. Button,, cut,, hammered.
Given this, there is more to consider than barrel taper(weight) for field accuracy.

 

[Hired Gun]

It was just a thought. Nothing ventured nothing gained. Once in awhile I find a nugget of info that is relevant. I'm always trying to learn something.

 

[bassrods]

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)....

I have found that IMR will not be as stable at higher temps, (most shots when hunting is in cooler temps) and the hotter the barrow is the more it can in cress the speed and pressers.. Try H 4350.

bassrods