IMR 4895 and 308

[boltman223]

I was wondering if IMR 4885 is known to be a temp sensitive powder. Is it common for a load developed at 70 degrees to go from a .4 inch five shot group to a 1 inch group in 30 degree temps. Would increasing the powder charge be a cure for this? Thanks, Andy

 

[Nodak7mm]

Andy,

With a temp change like that, you are pretty much guaranteed a velocity & POI change? Did you chrony any of your shooting? That would tell the story....

Rod

 

[mac86951]

+1 to above statement. Slower velocity will certainly change grouping, and a 40°F temp change will do it, but by how much you'll need to chrono, or do load testing at both temps.

-Mac

 

[boltman223]

Hey rod,

No didn't, I'm guessing it's going to be on the colder side next week so I was checking to see how my load is grouping in the colder weather before i load up a bunch if them.

How about you, are you going to make it out to do some load testing, I think you said you need to.

 

[seanpmc69]

In medium cases, if you're partial to IMR powders, I've always gotten the best consistency with 4064. 6mm Rem., .308, .243, 7mm-08, etc. Never had the same consistency with 4895.

 

[bigedp51]

IMR-4064 is now used in military 7.62 Special Long Range ball, MK 316 MOD 0 using a 175 grain bullet instead of RL-15, because it is a single base powder and less temp sensitive. IMR-4895 was used in 7.62 MATCH, M852 ammunition with the lighter 168 grain bullets.

 

[amlevin]

Sometimes groups open up when it's cold because of "shooter comfort" 8)

I use the Hodgdon H-4895, not the IMR and don't find it to be all that "temp sensitive".

Where I live we seem to see three temperature ranges. Winter rarely goes below 30 degrees. Summer (from the 5th of July to Labor Day) is rarely over 100. The other 9 months of the year hover between 50 and 70 so I don't see much temperature sensitivity in any powder I use 8)

I'm sure glad that powder isn't "rain sensitive" when shooting. Some days at the range I swear I can see "rooster tails" from the bullet traveling out to the target (that is if I can even see the target).

 

[wapiti25]

I feel that 30 degree change may be enough to consider raising the charge or dropping it. The best would be to chrono at low, mid and high temps for the ranges you shoot at, then shoot groups to see how they track to keep you in the node.

 

[boltman223]

Thanks guys For the help, great info. now i Know where to start. I think I need to spend more time with my chrony too.

 

[K22]

Air density changes with changes in temperature, I believe colder air being more dense. These changes would affect ballistics in addition to powder temperature sensitivity. How much I'm not sure. Perhaps some others may chime in on this issue since I'm not expert on the topic. If fact, I'm not an expert on anything except groundhog behavior.

I do know this from experience - I've used the so called temperature resistance powders like Varget and Benchmark extensively in the dead of winter and in the middle of summer and the point of impact changes enough that I need to make scope adjustments. Whether this is due to changes in air density or powder sensitivity I cannot say but it does change even with these so called temperature resistant powders.

 

[Laurie]

Air density changes with changes in temperature, I believe colder air being more dense. These changes would affect ballistics in addition to powder temperature sensitivity. How much I'm not sure. Perhaps some others may chime in on this issue since I'm not expert on the topic. If fact, I'm not an expert on anything except groundhog behavior. [K22]

That would affect the point of impact (that is make it lower in cold conditions) especially at longer ranges and is a result of the combination of factors that determine air density (temperature + atmospheric pressure + humidity where low+high+low slows the bullet most). It shouldn't affect group size which as the OP asks is most likely due to powder temperature sensitivity moving pressures and velocities out of a sweet spot.

Or ... it might be a different powder lot that needs the load fine-tuned, or several other alternative and/or contributing causes.

Living in the UK, we never worried over much about this factor, a typical winter day on the range at ~ 4 to 5-deg C / 40 F and a hot summers day at ~20-deg C / 70 F, only rarely hitting the 80s, but I have seen this effect on occasions. Years back I had a very accurate Swiss 1911 long rifle in 7.5X55mm that shot terribly high even at 300yd on a 6 o' clock hold. It needed a taller foresight blade, but I managed to produce an accurate but really low velocity heavy-bullet load that used a non canister ball powder and put it right on the target at 300yd with the minimum rearsight setting. It worked a treat until a cold day (just above freezing) on the range where I found that I needed far more elevation on the rearsight, with a progressively greater discrepancy as distances increased from 300 to 600 yards. It didn't shoot nearly as well either. The Hodgdon 'Extreme' powders reduce temperature effects, don't remove them entirely although the recently introduced IMR-8208 XBR is reputedly another step ahead of the earlier 'extreme' models such as H4895 and VarGet. IMR-4895 is not claimed to be in the 'Extreme' camp, so will presumably be a bit more affected than the those that are and have the special treatments.