Accuracy lost on warmer weather

[Joe-boy]

Any thoughts from the more experiences ones? Executive summary: groups nicely in the cold and crappy in the summer heat.

2-3 years ago I developed a load for 53 grain Hornady v-max for my .223 chambered CZ 527 that has an 1:12" barrel. I developed the load in the wintertime around freezing point and was relatively happy with how it worked. I used it for raccoon dog hunting and plinking for a while, until last summer I went shooting on a very hot day (around 30 deg C or 86 deg F) and got crappy groups. Like real crappy, around 2-3 MOA at 100 meters. I just thought I had screwed up something the previous time when developing the load and hadn't tested it properly and didn't think about it that much. I had not much of those left so I shot them away and proceeded to develop a new one with different brass.

This time I again got a load that grouped nicely below freezing point. From my archive I was able to find e.g. a 8 shot group of 0,66 MOA that wasn't cherrypicked but was pretty consistent with how it grouped; also 8 shot group already proves a bit more than cherrypicked 3 shot groups. Forward 6 months; I went to range when the temperature was about 20 deg C = 68 deg F. Again really crappy grouping. My 13-shot group was 2.3 MOA and doesn't get much better by excluding a flyer or two; it was consistently crappy. The bullet holes are nonetheless nice and round.

The load is developed using a ladder test and is using Lapua Match brass and Vihtavuori N133 powder which shouldn't be too temperature sensitive.

I guess there are basically two alternatives: either the muzzle velocity or the muzzle time changes enough to somehow screw up the barrel harmonics, or then the stability is compromised as the 53gr vmax is quite a long one for a 1:12" twist. However, I'd assume the stability should be better in the summer with the thinner air and probably increased muzzle velocity.

 

[Panhandle Pete]

Any thoughts from the more experiences ones? Executive summary: groups nicely in the cold and crappy in the summer heat.

2-3 years ago I developed a load for 53 grain Hornady v-max for my .223 chambered CZ 527 that has an 1:12" barrel. I developed the load in the wintertime around freezing point and was relatively happy with how it worked. I used it for raccoon dog hunting and plinking for a while, until last summer I went shooting on a very hot day (around 30 deg C or 86 deg F) and got crappy groups. Like real crappy, around 2-3 MOA at 100 meters. I just thought I had screwed up something the previous time when developing the load and hadn't tested it properly and didn't think about it that much. I had not much of those left so I shot them away and proceeded to develop a new one with different brass.

This time I again got a load that grouped nicely below freezing point. From my archive I was able to find e.g. a 8 shot group of 0,66 MOA that wasn't cherrypicked but was pretty consistent with how it grouped; also 8 shot group already proves a bit more than cherrypicked 3 shot groups. Forward 6 months; I went to range when the temperature was about 20 deg C = 68 deg F. Again really crappy grouping. My 13-shot group was 2.3 MOA and doesn't get much better by excluding a flyer or two; it was consistently crappy. The bullet holes are nonetheless nice and round.

The load is developed using a ladder test and is using Lapua Match brass and Vihtavuori N133 powder which shouldn't be too temperature sensitive.

I guess there are basically two alternatives: either the muzzle velocity or the muzzle time changes enough to somehow screw up the barrel harmonics, or then the stability is compromised as the 53gr vmax is quite a long one for a 1:12" twist. However, I'd assume the stability should be better in the summer with the thinner air and probably increased muzzle velocity.

Try 50 grain, the 53 VMAX grain requires a faster twist if I remember correctly. EDIT-Sorry, I looked it up and they recommend 1x12 twist. I had oreded these bullets but my factory Remington guns have a 1x14 twist and will not work in them. You may be just on the border of stable with your 1x12 twist.

 

[Joe-boy]

I have a box of 50 grains waiting for the load development for the mentioned reason. However, I'd assume stability should be an issue when the MV drops and air is denser in the winter, not in the summertime.

I don't think the temperature difference should spread the groups to 2-3 times larger, so there's something fishy going on. I shot an 0,9 MOA 10 shot group with the same rifle on the same day so it's not just me having an extremely bad day.

 

[Tiny Tim]

Not certain about N133 temp sensitivity, but if your load is near max at freezing, it may pressure spike in hot weather. 85 degrees F seems to be a common threshold. Check with a chronograph. I'm sure someone much more knowledgeable will chime in soon. As others have said. Your bullet may be marginally stable, but heat should help, not hurt your scenario.

 

[M1A1ABRAMS]

I have a box of 50 grains waiting for the load development for the mentioned reason. However, I'd assume stability should be an issue when the MV drops and air is denser in the winter, not in the summertime.

I don't think the temperature difference should spread the groups to 2-3 times larger, so there's something fishy going on. I shot an 0,9 MOA 10 shot group with the same rifle on the same day so it's not just me having an extremely bad day.

This is possibly a dumb question. But what type of stock ? Wood , composite, etc. Is the barrel still free floating ? Is it possible the heat could be affecting it somehow ?

 

[Joe-boy]

This is possibly a dumb question. But what type of stock ? Wood , composite, etc. Is the barrel still free floating ? Is it possible the heat could be affecting it somehow ?

Good question. It's a DIY pillar bedding job in a Boyds laminate stock. The barrel is very free floating so I wouldn't expect that to be the issue, but this was a good question nonetheless!

 

[Joe-boy]

Maybe it's worth mentioning the load is close to the maximum stated by the Vihtavuori reloading manual, but the COAL is significantly longer and close to the lands. I didn't notice any pressure signs, but unfortunately didn't have my chronograph with me this time. It might be possible that the bullet is touching the lands which combined with the temperature causes the pressures to peak.

Another weird finding is that another load of mine that has been working very well earlier in another rifle, in another caliber, using another powder, also grouped surprisingly poorly and I really think it wasn't my shooting. However, this might be just poor luck or shooting as I have way less data about that load and rifle available.

 

[Eric H]

One of the things I try is developing loads with temp stable powders. I don't know of any powders that will hold loads over that wide of temps. I know Varget, H332 and Benchmark are supposedly temp stable.

 

[Rflshootr]

Back the load off .5 grain and reshoot it. I'm betting the extra pressure is throwing it out of tune. Also, make sure your BTO measurement is the same as it was in the colder temps. A different lot of bullets can make a difference as well as a different lot of powder can make a difference.

 

[Dean2]

Given the information you are p[roviding it is pretty hard to give good advice. You need velocity and ES numbers for cold weather and then the hot weather, at a bare minimum. That will tell you if it is the load. Then there is bedding, optics, scope base mounting etc.

Lots of stuff changes when you take a gun from -45 to +90. Living in Alberta we see it all the time. As a result, all hunting loads are developed with magnum rifle primers, using temperature stable powders from Hodgdon's line up. Load development is done a 70 F, then test shot again at -30. We never settle on a load unless it produces stable groups over a 1.5 grain variation in the powder load, and we use the mid point of that node. I value consistent reliable performance in a hunting load, over absolute max velocity. I have never had a group blow up in size like you describe but I have seen velocity drop enough that it needs to be accounted for on shots over 500 yards. I specifically don't use Viht because supply here is sporadic and it has a reputation for showing large velocity swings.

 

[mink111]

Try 50 grain, the 53 VMAX grain requires a faster twist if I remember correctly.

I'm curious. How many rounds were fired between these 2 shooting times ?

 

[Culpeper]

Already mentioned --Most likely just fundamentals of the rifle build to load development. Example, action screws, fouled barrel, MV vs seating depth, etc. It's a long list will little info to go on.

 

[Joe-boy]

One of the things I try is developing loads with temp stable powders. I don't know of any powders that will hold loads over that wide of temps. I know Varget, H332 and Benchmark are supposedly temp stable.

I'd assume there are plenty of people who have shot the same load when freezing and when sweating and would be interested to hear if someone has had similar experience. When Googling I found a comment that someone somewhere mentioned that 1 deg C corresponds to about 0,35 m/s increase in muzzle velocity. This would mean e.g. the 25 deg C change would mean 8,75 m/s which is about 29 fps. Does not sound like something that should double the group size.

 

[Culpeper]

My experience with VV powders is they max out on MV a little slower than similar burn rate powders. VV is very accurate but slower. Book max for that bullet and powder is less than 3200 MV.