Outside Temp/Pressure

[txstars]

I am fairly new to the world of precision rifle/reloading. With the help of a knowledgeable friend we worked up a good load for my .308 bolt. 175 gr. SMK, Winchester brass, Fed 210, 45.1 R-15, .010 jump. Average muzzle velocity of 2630, SD: 4.9, ES: 10.8. This load produced a .25" 100 yd group take away the 1 flyer. We saw the slightest signs of excessive pressure on the case from the extractor with this load.

My question is this. Temperature at the time was 65. As summer rolls around and it starts to warm up when will the outside temp. start effecting the case pressure? +10 degrees, +20,.......?

 

[Chanonry]

Re: Outside Temp/Pressure

yes there is a practical effect. best way is to chrony it for yourself in varying conditions and note pressure effects.

 

[Clark]

Re: Outside Temp/Pressure

The chamber can be a much hotter place than the air you are breathing.

Leaving an unfired round in the hot chamber for a while raises the point of impact at long range. Bart Bobbitt was compensating for that in one of his posts.

 

[txstars]

Re: Outside Temp/Pressure

Can I safely assume 10 to 20 degrees will not drastically effect the pressure.

 

[Clark]

Re: Outside Temp/Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TX-Stars</div><div class="ubbcode-body">Can I safely assume 10 to 20 degrees will not drastically effect the pressure.
</div></div>

What are the safety margin calculations on your handloads?
Vernon Speer said 6% powder charge for writing a load book.
It looks like 4% of the powder charge safety budget could get used up by temp with Re17 but none with Varget.
To calculate that, I am seeing a 2 gr change on 308 190 gr to move from 2400 fps to 2500 fps, is change from 52kpsi to 60kpsi per Quickload.

Kombayotch's graph posted on Canadian Gun Nuts.
He had the ammo in an oven at the range.
What at guy!

 

[Chanonry]

Re: Outside Temp/Pressure

well no, if you have one really hot load then have a hotter day and a hot chamber then you may get an issue.

i was shooting on sunday and it was 24 deg C vs 12 deg, i had a hot rl17 load. my chamber was well used so HOT result was a hard bolt lift.

Don't sweat it too much,in reality the pressure signs are gradual. If you need a hammer to open the bolt on a cold day then you may need to back off a bit ?!!

 

[Fuzzball]

Re: Outside Temp/Pressure

Nice info, nicely done graph. But it just confirms what I alrady know about my ammo and temp effects. Thing is, I'm not going to develop a load below zero and then shoot it at 175 degrees! I do my load developing in comfortable conditions, typically in the range of 65-85 degrees so there won't be any massive changes due to the temps in which I hunt or compete.

 

[Greg Langelius *]

Re: Outside Temp/Pressure

It will start immediately. But slight pressure signs may not develop into critical ones. The only true way to find out is to find out. Whenever you approach the dragon, take extra care when contemplating twisting its tail. I think you will eventually find, as we all have, that the dragon is better left at a distance.

I note that you are placing significant reliance on numerical values. That's fine, but I'd counsel against total reliance. Numbers can suggest outcomes, but are less reliable at predicting them with any real certainty.

As you load more, you will begin to develop your intuitive grasp of the relationships between the important factors governing safe and effective hand loading. It takes time and there are no shortcuts. There are also no hidden secrets, or earthshaking insights that are hitherto unknown. Save yourself considerable time and effort and recognize that if an idea is new, it's new to you, but not necessarily to the rest of handloaders as a whole. The basic advice and warnings are valid, and should continue to be so for our foreseeable lifetimes.

Greg

 

[MALLARD]

Re: Outside Temp/Pressure

thanks for the info clark

in an oven at the range hehe

 

[txstars]

Re: Outside Temp/Pressure

Thanks for the info guys. I have a very healthy respect for the potential dangers involved with hand loading. I started with pistol rounds this time a year ago but somewhat feared venturing into the rifle rounds. Part of the deal for me buying a buddies custom built .308 was him showing me the proper way to work up a load. I am fully aware I have allot to learn at this point.

Clark, I am unaware of how to calculate safety margins on loads. Information on how to do so would be greatly appreciated.

Am I understanding correctly the first signs of excessive pressure will be the pancaking, and or cratering of the primer? Where does difficulty opening the bolt fall in line with these signs? Or should I become concerned with definite signs of extractor marks on the brass. Forgive me if my terminology is incorrect.

Ron

 

[MitchAlsup]

Re: Outside Temp/Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TX-Stars</div><div class="ubbcode-body">Am I understanding correctly the first signs of excessive pressure will be the pancaking, and or cratering of the primer? Where does difficulty opening the bolt fall in line with these signs?</div></div>

Primer pocket edges tell a lot about the pressure cycle. One comming out with the same radius as they went in are almost undoubtedly well within normal pressures. Ones that are flattened with a sharp edge are overpressure.

Primer cratering on many manufactured (i.e. factory) guns is standard due to the sloppy way the firing pin hole in the bolt is machined. So you can only get a good reading on this parameter with a properly machined bolt.

By the time the bolt starts to get hard to lift, you will have already been able to see flowing brass (ejector swipe, and extractor dents). Whne a bolt goes from easy to close before firing to hard to open after the firinig, it is CERTAIN that there was too much pressure. You see when the pressure gets high enoguh for brass to flow, the brass has become a high-viscosity liquid under pressure and has 'forgotten' its previous (sized) shape {that is it has no spring back when pressure drives it into flowing.}.

 

[txstars]

Re: Outside Temp/Pressure

Thank to all for the information. You have been helpful in moving me forward in a safe manner.

Ron


<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MitchAlsup</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TX-Stars</div><div class="ubbcode-body">Am I understanding correctly the first signs of excessive pressure will be the pancaking, and or cratering of the primer? Where does difficulty opening the bolt fall in line with these signs?</div></div>

Primer pocket edges tell a lot about the pressure cycle. One comming out with the same radius as they went in are almost undoubtedly well within normal pressures. Ones that are flattened with a sharp edge are overpressure.

Primer cratering on many manufactured (i.e. factory) guns is standard due to the sloppy way the firing pin hole in the bolt is machined. So you can only get a good reading on this parameter with a properly machined bolt.

By the time the bolt starts to get hard to lift, you will have already been able to see flowing brass (ejector swipe, and extractor dents). Whne a bolt goes from easy to close before firing to hard to open after the firinig, it is CERTAIN that there was too much pressure. You see when the pressure gets high enoguh for brass to flow, the brass has become a high-viscosity liquid under pressure and has 'forgotten' its previous (sized) shape {that is it has no spring back when pressure drives it into flowing.}. </div></div>

 

[Clark]

Re: Outside Temp/Pressure

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TX-Stars</div><div class="ubbcode-body">

Clark, I am unaware of how to calculate safety margins on loads. Information on how to do so would be greatly appreciated.

Am I understanding correctly the first signs of excessive pressure will be the pancaking, and or cratering of the primer? Where does difficulty opening the bolt fall in line with these signs? Or should I become concerned with definite signs of extractor marks on the brass. Forgive me if my terminology is incorrect.

Ron </div></div>

Usually the thing to back away from with a safety margin is the threshold of short brass life.
If the bolt is hard to lift or the primer pierces, that must be backed away from.

The safety margin is the percentage or powder reduction from the threshold of any real limit.

So you work up. Something goes wrong; short brass life, stiff bolt lift, pierced primer, or anything else.

Then the powder charge is reduced. Determining what percentage of powder to reduce for a safety margin involves many things to consider.
Variations in charges, variations in seating depth, temperature changes, variations in case volume, changing lot numbers of canister powder, variations in bullet weight, shot to shot variation in primer power, the likelihood that the ammo will fall in the dirt, the likelihood the ammo will vibrated thus breaking the powder particles down into smaller ones, hot chamber from rapid fire, how much chance to you want to take with getting short brass life, or anything that is a variable.

I have been getting away with 3% and 4%, but after I saw Komayotch's Re17 temperature variation data of 2%, I think I will step it up to 5% for Re17.

 

[txstars]

Re: Outside Temp/Pressure


<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Clark</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TX-Stars</div><div class="ubbcode-body">

Clark, I am unaware of how to calculate safety margins on loads. Information on how to do so would be greatly appreciated.

Am I understanding correctly the first signs of excessive pressure will be the pancaking, and or cratering of the primer? Where does difficulty opening the bolt fall in line with these signs? Or should I become concerned with definite signs of extractor marks on the brass. Forgive me if my terminology is incorrect.

Ron </div></div>

Usually the thing to back away from with a safety margin is the threshold of short brass life.
If the bolt is hard to lift or the primer pierces, that must be backed away from.

The safety margin is the percentage or powder reduction from the threshold of any real limit.

So you work up. Something goes wrong; short brass life, stiff bolt lift, pierced primer, or anything else.

Then the powder charge is reduced. Determining what percentage of powder to reduce for a safety margin involves many things to consider.
Variations in charges, variations in seating depth, temperature changes, variations in case volume, changing lot numbers of canister powder, variations in bullet weight, shot to shot variation in primer power, the likelihood that the ammo will fall in the dirt, the likelihood the ammo will vibrated thus breaking the powder particles down into smaller ones, hot chamber from rapid fire, how much chance to you want to take with getting short brass life, or anything that is a variable.

I have been getting away with 3% and 4%, but after I saw Komayotch's Re17 temperature variation data of 2%, I think I will step it up to 5% for Re17. </div></div>

Cool deal, understood. That's some very usefull information.

 

[notquiteright]

Re: Outside Temp/Pressure

I dont push the ragged edge of velocity. As mentioned above it saves brass life, somewhat on throat wear, recoil and the dreaded 'overtwist'

One more thing to consider about powder temp- direct sunlight can amp the powder temp up well above air temp. What is mellow for fall hunting is hot during summer practise and loosens primer pockets if left out in the summer sun during off season practise-

ask me how I know!