Range Report Rounds Slowing Considerably Through Hot Barrel?

[BCT]

Hi guys,
I was shooting my Savage 12 LRP in 6.5 Creedmoor at Peacemaker training center in WV last weekend for their Long Range Challenge. Its a friendly type of match were everyone helps out everyone else. All 25 target ranges are given beforehand as well so there is no estimation element. My loads are Berger VLD 140 Target with 42.2 Grains of H-4350. I've found these to always be under 1 MOA and often in the 1/2 to 1/3 MOA range. I chronoed them beforehand, as well as verified with drop at 805 yards, to be in the 2830 fps range. Using that velocity, I developed a sheet to compensate for each target between 460 and 900 yards using Strelok+. Here's were my problem starts. On the 800+ yard targets, my dope was right on and I scored first round hits at 820 (5.5 Mil), 855 (5.9 Mil), 818, 845, 814, and 828. However as the match moved on, it got significantly warmer and of course more rounds were put down the barrel. I found that I had to add .1 Mil and then .2 Mil on all subsequent targets. My final target was at 512 yards and required 2.8 Mil elevation as opposed to the 2.6 I had originally calculated. That equates to a velocity drop from 2830 fps to 2721 fps after the temperature rose from roughly 68 degrees at the morning start time to 90 degrees by lunch time. Why would this occur? I generally expect, and have seen with this gun, rounds come out faster as temperature and barrels heat up, not drop 109 fps. In speaking to some other competitors, they too were uncertain. No one else I talked to had this problem so I'm kind of ruling out atmospheric conditions.

I'm open to any suggestions. The gun has about 900 rounds through it. I haven't run any copper solvent through it in at least 250 rounds. I have the gun zeroed at .1 Mil high at 100 but this was taken into account with my dope. The gun did get quite hot from shooting during the event. Anywhere between 5 and 10 rounds fired per stage fairly quickly, but then probably a 20 minute break before the next stage. I am planning on re-verifying my zero as well as check it with my chronograph later this week. Very puzzled though. Any help would be welcome. It's probably something simple that I'm just failing to see. I'm kinda new here, so if there is a previous post covering this type of issue please let me know.

Thanks

 

[260284]

How much did the humidity change throughout the day? Did you check your DA with a Kestrel?

 

[supercorndogs]

Looks like the first things I would do. Reconfirm 100y zero and chronograph again are already on your agenda.

 

[lash]

After you reconfirm zero and chrono, verify that your scope is still tracking correctly. Even a previously good scope with good turrets can sometimes just up and die on you without notice. A tall target test does not take many rounds and will also help you heat up the barrel to see if you can duplicate what happened. While we're on the subject of scopes, double check everything for tightness, like your scope rail and mounts. It also wouldn't hurt to check and make sure that your stock isn't contacting the barrel anywhere and that your action screws are still torqued correctly.

I've had all of those things happen to me while in the middle of a match and/or LR/ELR shooting session. It doesn't take long to check them and could possibly save you from going crazy chasing causes.

 

[CavReconScout]

How was the mirage?

 

[lash]

^^^That's a darn good question.^^^

 

[supercorndogs]

Wouldn't that just effect where the target appeared to be and not dope to make a hit?

 

[Lowlight]

It's possible that Savage barrel was wandering in the heat, as well it could be any number of issues from scope not returning to zero correctly, etc.

You need to test those numbers adminstratively, and under some controlled conditions. In other words verify it.

Humidity is meaningless... Higher Humidity is actually better for less dope, not more, and in general the difference between 100 % humidity and 0% humidity on a 1000 yard target is .4" at the worst ... adjustments wise, it's less than your scope can actually dial. It's like .08 Mils between 0 and 100. You can't dial that.

Mirage could be a problem, not sure it would mean adding dope, but you never know until you check.

Honestly you have to systematically check youre data at distance and then move to the next element of weakness like the scope

 

[sirhrmechanic]

If your barrel has some internal stresses from the manufacturing/forging, it is possible that heat is causing it to warp slightly.

Consider having it Cryo-treated and see if it stabilizes.

You are right that heat should cause in impact to move upwards. Further, if the barrel was expanding because of heat, it will expand in all directions... making a slightly smaller bore... which should raise pressures slightly and create higher, not lower, velocities.

However, if there is a stress in the barrel, heat can cause it to shift unpredictably. Cryo treating a barrel can remove stresses and might alleviate the problem.

Agree with LL, though. Before looking for an arcane possible problem, check mirage or optical distortion.

One way to narrow it down may be to pick a cold day and shoot enough to heat the barrel up. The atmospheric conditions of cold weather and the reduced mirage/optical challenges will disappear in colder weather. While your barrel heating will probably not be affected too much by cold air. It will heat up regardless of a few degrees of ambient temp.

Interesting problem and I'll be fascinated by what you find out. Thanks for posting!

Cheers,

Sirhr

 

[Ledzep]

Actually hotter barrel would expand radially, making a larger bore diameter. But... We're talking small numbers here, it would have to be on the brink of having problems initially for you to notice as it heated up 100deg F or whatever the temp difference from ambient to however hot it gets. That said, it could cause a pressure drop. You might even see the difference on your primers (rounder vs flatter), but I'd think accuracy would fall off along with it if that was the problem.

.2 mils isn't a ton of deviation all things considered. It's not good, but I wouldn't immediately jump to a 100fps velocity drop. I agree with the above to re-test at 100yd to confirm zero has stayed put. Run it over a chrono then shoot 10-20 fairly quick and run it over a chrono. See if you get POI shift with heat.

 

[BCT]

Thanks for all of the tips! A number of the suggestions were things I had been considering as well. I will post as soon as I can do some of the verification suggested above. Mainly re-checking my zero as well as verifying scope tracking. The first time I shot in this competition with this gun I did very well and my dope held true for all stages 600-1100 yds. That dope was calculated before the match without taking into account rising temperatures through the day. However, since doing well means less rounds fired, I don't think the barrel heated up as much so I did not experience any of these problems. Mirage for both events was zero. Will keep you posted.

 

[Greg Langelius *]

As bore drag increases, the bore pressure increases with it. This raises velocity. The opposite is equally true.

As stated above, bore diameter increases with barrel heating, but also, additional fouling accumulates.

Powder fouling consists primarily of unburned kernel coatings which tend to consist of graphite, which is an ideal dry bore lubricant. Between the decreased bore drag from bore expansion, and the decreased bore drag from the increased dry bore lubrication, pressures can drop, reducing the resulting velocity.

As an aside, the addition of moly dry bullet lube has a well known and accepted consequence of decreasing velocity, generally requiring increased powder charges to attain the same velocities.

Graphite has been the standard propellant kernel coating for probably over a century, added primarily for its electrical conductive properties which prevent the accumulation of static electric charges from vibration/powder kernel friction during handling and transportation. Without the presence of this graphite kernel coating, handling and transporting propellants would be too hazardous. The dry lubricant feature is just a freebie bonus.

I have resisted the trend toward moly coating projectiles, mainly because there is already a reliable dry bore lube present in the propellant itself, in the form of graphite kernel coatings. The difference being that without the moly, I probably burn less propellant in the bore. I have always equated bore life as a product of how many pounds of powder get burned in a bore overall.

I have occasionally pretreated my clean bores with a homebrew graphite/alcohol mixture to reduce the need for some of the 'fouling shots'. The alcohol evaporates, leaving a light coat of graphite lining the bore. I prefer graphite coating the bore post-cleaning to oil, since the graphite will not retain any dust or grit that might be present.

I don't really see a lot of difference, but I just sorta tend to cringe at the idea of sending naked copper down a naked bore. Maybe it reduces initial copper fouling before the powder fouling accumulates.

A lot of the last is pure conjecture on my part, but the presence and lubricating effects of graphite in powder fouling are not unknown.

Greg