Winter loads & summer loads

[gunner]

I have had these issues before years ago when using my worked up winter load in summer , now on closer inspection of my br brass look like its a tad hot , slight extractor marks on the head / base . My load with a 105 was Hodgdon 4895- 28.4 grains , so went on QL to re do the math and think reducing to 28.0 would be around right for this time of year . Id noticed my zero was a tad high 200 yards . And confirmed after re-zeroing at 200 then cheaked at 400 , my dope should of been 5½ moa so dialled in 5moa knowing there running hot but not dangerously . Low and behold that was bull's-eye . Soo just cheaked at 600 , normally 12½ moa needed so dialled 11¾ bingo ! so just gotta rework my `summer` pet load . Anyone else run two or more loads for a given rifle ? Atb ...

[mikeroz]

Yes I noticed my loads were a tad high at 200m when I shot on Sunday. It was a nice warm day. Must have been temp related. I am sure my rounds hadn't been in the sun too long. I didn't think about it at the time. Sadly the batteries in my Kestrel were dead so I was unable to take a record of the temperature on the day.

[gbal]

In general,as atmospheric /cartridge temperature increases,pressure and velocity increase (and barrel temperature-probably affects chamber/cartridge no-one seems to allow for this-see earlier post...).

 

How much velocity & presssure increase depends on the powder-some powders enjoy a good reputation because they are RELATIVELY insensitive to the kind of temperature variations shooters might encounter(these will tend to be less in UK,compared to USA).

So ball park figures can vary-powder dependent (as well as the cartridge,calibre and temperature range-it is not quite linear.) SO estimates vary:2-4 fps per 1C degree (or 1.8F degree) is suggested by some;+/-30 ffps giving +/_.5moa at 1000 is suggested by Long Range Hunting magazine.

 

Exterior Ballistics.com quote some detailed controlled testing by Sierra (acknowledging the variables above) which found for a 165 moderate (NOT hot load) in a 30-06 giving 2700fps at 85F,dropped to 2600fps when fired at 35F,so that is about 100 fps for 50 Farenheit degrees,and in the 30-06 means: about 1 inch more drop at 200y,2 inches at 300y,and 4inches at 400y in the winter temperature,compared to the summer temperatures tested (50F and 85F).

Sierra stress this applies to moderate reloads,and any reloads already near/above max may well increase considerably,perhaps unsafely,at higher temperatures....

(data for 243 gave reasonably similar conclusions).

 

Google "Rifle Velocity and Temperature' for the above studies,among others.

 

Altitude changes also affect temperature,but will be considerably less extreme in UK,and one inch more drop at 200 in the cold is probably manageably for stalking,more precision requirements might have to dial in correction .

Remember-it varies with powder sensitivity to temperature-so findings can vary because of this.

 

gbal

[simonl]

Is it all down to case/powder temp or is barrel temp also a factor? Appreciate a round will soon reach barrel temp

[gbal]

Barrel temperature can be a factor in itself (though it also heats/cools cartridges,over a period of ttime,once chambered-as briefly mentioned in post 3 above).

 

"Rifle temperature and velocity' googles some data-best are the very few studies such as follows:

 

"Temperature effects on Zero" Sam Pomerenke.Long Range Shooter Feb6 2009.

 

In this fairly well controlled study (eg scope temperature effects were controlled) two AI 308 rifles and a Blaser RG93 338`LM were tested (subsequent somewhat larger samples,including Remingtons,were also tested).In general terms,POI changed with rifle temperature,though there were rifle make differences.Shots were fired in the rifles at each temperature within 3 seconds of chambering to minimise cartridge temperature changes from the rifle(all ammo was kept at 18C (+/-1Cdegree)

The AIs changed POI by 1/4 moa per 6C degrees when barrels were in the range 26-45 C;and 1/4 moa per 3C degrees below barrel temperature 15C; the Blaser was more consistent at 1/3 moa change in POI every 10C degrees change in barrel temperature.

Those changes could mean getting on for an inch difference in POI at 100y between summer and winter with the same load,even in the UK (and broadly in line with gunner and mikeroz,though their data were not designed to address this effect exactly).

 

There is not enough research (on a variety of cartridges,powders and rifles) but there do seem to be POI changes due to atmospheric rifle barrel temperatures,though every rifle model does not behave identically,and this should be considered where optimal precision is needed (no ballistic solvers currently do it-remember too rifle model differences-each rifle might need testing for it's own behaviour-as with most shooting effects.

 

gbal

[gunner]

Yes I noticed my loads were a tad high at 200m when I shot on Sunday. It was a nice warm day. Must have been temp related. I am sure my rounds hadn't been in the sun too long. I didn't think about it at the time. Sadly the batteries in my Kestrel were dead so I was unable to take a record of the temperature on the day.

You didn't crono the ammo that day by any chance ? Im assuming the fps goes up slightly as its flatter trajectory , but also flatter trajectory because of warmer less denser air . I dont own a crono so cant cheak myself thanks for input as always mr g bal & Mike , taa

[VarmLR]

I've dropped my .308 loads by almost a grain after experiencing significant changes to pressure/velocity in hot weather (difficult bolt lift on extraction plus extractor marks on case heads). I can safely shoot the reduced load all year round without pressure signs and account for the changes to POI using Strelock + to account for temperature variation from the temp when the rifle was zero'd with the new load. Seems to work well for me. I'm using N140 which is fairly sensitive to rises in temperature. I take care to store the ammo out of direct sunlight where possible to prevent high temperature rises of the cases in hot weather.

[mikeroz]

Mr g, sorry I didn't chrono it. I took it along but because of time constraints in the end I didn't get it out. I have chrono'd the load before on a cooler day so have a reference point to work from.

Cheers

Mike

[activeviii]

TIMBO, load sherlok and put the data in for your loading with the temp at time, then store it. then enter 1000yrds and use current weather and calculate. scroll down to table and in that change the setting to yard increments you use. 25yard if I remember your drop chart. and then email it to yourself and print it off.

 

if not using the phones data yu can entre the temp and pressure. i have just done the data on the creed for 25c and then winter for 5c. just hope the temp doesnt get over 25 to often at my loading is rather warm with the 130 smk and even warmer with the 140eld. thank god Lapua are going to be running the creedmoor brass as i cant see mine lasting long