Accuracy Question

[Trojan6]

Folks,

Seeking a little input from the collective wisdom of the group. I have a .308 rem 700 bdl with 20 inch heavy barrel and synthetic stock. Rifle has been bedded. I have been shooting it religiously over the last six months to build my data book. My typical range is only 100 meters, but I have been able on occassion to stretch it out to 300 meters to verify loads. This is my meat gun for deer and hogs, as well as occassional coyotes. My best (hunting) load in the rifle is 165 gr Sierra Game Kings over 44.5gr Reloader 15, CCI BR2 primers and Winchester brass. Rifle consistently shoots .5 MOA when I do my part, with some rare .25 MOA groups on occassion. Scope was recently upgraded to a Leupold MK 4 2.5-10 mil dot.
Here's the issue. My zero increases one MOA for every 8 degrees increase in temperature. Everything I've read makes this seem excessive. I've built the data book so that I can accurately adjust based on the conditions, and the rifle is very accurate as is. I am considering getting an Accuracy International stock for the benefit of the detachable magazine, but am concerned that I may lose some accuracy, and in turn end up starting all over again with the data book and possibly needing to get that stock bedded as well.
So, after all of the background, I reckon my question is what is your advice? Leave as is and adust for it, or go with the new stock? Any ideas what might be causing the shift (most reports I've seen recommend Reloader 15 as a non-temperature sensitive powder)?
Thanks in advance for the help.

LH3

 

[hoghunting]

Many of my loads also get larger when the ambient temperature reaches 90* or higher. When the temperature increases, you might try cooling your ammo in an ice chest and see if the cooler ammo decreases group size.

 

[taliv]

i was told by one of the sierra techs a long time ago that on average using double-based powders (of which R15 is one) will see a 2 fps increase for every degree F increase.

That may or may not be accurate (I haven't cared enough to verify) but assuming it's in the ballpark, I don't think 16 FPS would make any difference at all at 100 yrds.

In fact, I just ran some quick numbers and unless i've missed something, with a 100 yrd zero, with the 165g bullet, your POI at 200 yrds changes exactly 1 MOA when you go from a MV of 2500 to 3000. So that's 500 more FPS to get a 1 MOA change.

I can't imagine what ELSE is going on that would cause the change you're seeing, but I'm fairly sure it's not the weather

 

[Bitswap]

Now, you know why there's a change? Temperature and humidity effect how dense the air is. Add to that the bc of your bullet. Check it out with a ballistics calculator. Only way to be sure it's not the hardware is with a chrono.

I'll assume you know all about this, soooo....

I'd try loading with Varget powder. They say it is good for temp stabalization.

If your sure it's not the powder, I'd go to my gunsmith and check out how the barrel is bedded, or problems with the scope mounts. Can't imagine what else would cause this.

 

[Trojan6]

Thanks for the help. I'm beginning to lean most towards a bedding issue. It was done by my uncle who was a closet gunsmith and I inherited it from him after he passed away. He bedded all of his own weapons and although not the prettiest, the groups I'm shooting testify that he knew something about it. However, it is possible that something was not done completely correct. What makes me think that it may not be the ammo is that the group size isn't changing, just the point of impact rising vertically about an inch for every eight degree increase in temperature at 100 meters. I only recently acquired a handheld weather station to calculate other variables, but the best I had before was air and baro. Baro was remaining pretty much constant, elevation was not changing as it was always at the same range with the same bench and setup. Original zero was conducted at 60 degrees and as temperature has increased, the groups have accordingly increased in elevation. Thought maybe someone out there had a similar experience that could confirm/deny my assumptions. Thanks again.

LH3

 

[D&T]

Trojan6,

IMR-4064 is one of the most consistantly accurate powders for the 308 Winchester and is not temperarure sensitive...

If your rifle is properly pillar bedded and the torque setting is at 35 inch pounds, it should stay consistant in accuracy...

The same with restocking and going with a DBM, make sure it is pillar bedded and the screws are torqued to 35 inch pounds and you should have a consistantly accurate shooter....

 

[jlmurphy]

+1 on the Varget, I've stopped using Reliant powders because of the temperature sensitivity. I also agree on the bedding, install pillars.

 

[Davo]

Thats alot, but as has been said RL15 is known for that. I think one reason I got verticle groups with it was because after chambering a round for 20 seconds it heated enought to raise POI.
I believe it was this powder because it did not happen with Varget or any other projectiles.

 

[Trojan6]

Okay, might be a powder issue after all, which is splendid since I've got about seven pounds of it laying around and 200 rounds loaded with it... Oh well. I'm going to try it out once it gets to 100 degrees and see if the pattern continues, which I'm pretty sure it will. Thanks again for all of the help.

LH3

 

[Strongbad]

If you want to try some other powders (that you might not have on hand) let me know. Since I'm in the neighborhood, I can probably scrounge up some other stuff you can try and I've got plenty of friends who reload as well.

 

[Zak Smith]

I am considering getting an Accuracy International stock for the benefit of the detachable magazine, but am concerned that I may lose some accuracy, and in turn end up starting all over again with the data book and possibly needing to get that stock bedded as well.

The vast majority of AICS users have no accuracy issues from the stock and do not have to bed it.

 

[SaMx]

is the scope mount made of aluminum? This is a pretty crazy theory, but aluminum expands and contracts more with the temperature than most other metals. If only part of the scope mount is aluminum, it's possible that the temperature is causing it to deform, and pushing the scope out of zero.

It's not the most likely theory, but hey, just throwing an idea out there.

 

[Zak Smith]

Thoughts on the zero shift:

* You are unlikely to see velocity-based impact shift at 100 yards, which rules out the primary effect of powder/temp changes. It is possible the timing changes enough to change the barrel whip "node", but this is less likely.

* If the POI shift tracks the ambient temperature and not the barrel temperature, then the thing that is moving is probably something that does not heat up as more rounds are fired. This would seem to rule out the barrel and parts of the receiver that heat up.

* An experiment would be to try some factory match ammo and see if it tracks the same shift pattern. This would help to rule out any issues with your specific load.

 

[CDignition]

I think factory barrels are prone to POI shift due to heating. a Good Custom polished barrel usually wont see these problems, even if the barrel is hot.