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Normal Topic Heat vs Pressure vs Velocity - effects on barrel wear, consistency and accuracy part five (Read 1944 times)
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Heat vs Pressure vs Velocity - effects on barrel wear, consistency and accuracy part five
Feb 23rd, 2019 at 8:00pm
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What we found was relatively simple in theory but difficult to execute.

To obtain truly reliable accuracy without mechanically destroying the firearm the first consideration is to allow realistic barrel length to develop your required velocity. There is an ever-growing move towards shorter (and slightly fatter) barrels ostensibly to improve handling and weight characteristics. This has either overlooked or ignored some basic physics. If you want velocity you either increase barrel length or powder weight. The first works without accuracy or wear penalties. The second provides a short term result but with questionable accuracy and distinctly shortened barrel life.  I do not dispute the underlying principles that the shorter barrel argument are founded upon but this all needs to be tempered with reality. If you want good barrel life or long range accuracy (than 500 to 1000 rounds) you have to acknowledge and obey and the laws of physics. I do support the need to save weight in some types of hunting. I cannot see any need for weight saving on range based shooting or long range hunting.

The 14.5 mm Truvelo rifle I used in the latter part of my career was a true ¼ MOA at 500 metres set up in any reasonable conditions with a MV of 1004 ms. Chamber pressure was 59950 psi and temperature was 486c. It had a 1200mm barrel. I put over 3000 rounds down range with no loss of accuracy. I was deployed from sea level to 3700 metres and at temperatures from -30c to 57c. Just before I retired I did a range test with that rifle. It was still delivering ¼ MOA at 500 metres. At the same time, I was periodically using a 7.62x 57 (300Winmag) AI rifle with a 685mm barrel. Peak pressure was a touch under 63000psiand temperature was 563c. It started out as a 1/3 MOA at 250m and after about 700 rounds turned into a shotgun. This is not the fault of AI, rather it is a misguided requirement for reduced weight and length which caused this issue.


Chamber pressures were similar but heat and dimensional tolerance were quite different. The AI conformed to all NATO, SAAMI and CIP standards. Crucially it did not exceed them where it should have. The Truvelo had a much better dimensioned Chamber, Throat and Leade. It also held the round in a properly centred, not slightly drooping, alignment with the bore. It relied on barrel length not powder burn rate to build velocity.

I also used an AI AS50 which I recall having a 700mm barrel and a very tight chamber. It was a semi-auto and still held .75 MOA after 2500 rounds downrange. Peak chamber pressure with the Raufoss ammo was just over 52000 PSI giving 883 ms MV.

When you look at these examples and compare them with the current trend to hot loads (the short magnums and their ilk) being used in short, sporter profile barrels is it any wonder barrels wear out prematurely.

To really force home the importance of chamber design and barrel length when pursuing accuracy and barrel life consider this. We built a 300RUM with a super heavy 998mm Lother Walter barrel in a Mausingfield action. It was loaded to match 338 Lapua ballistics. We tested this against a Sako TRG in 338 Lapua with a 689 mm barrel and a MacMillan Tac2 388 with a 686 mm Barrel. Our rifle ran chamber pressures of 58000 psi vs the 61000 psi of the Lapua. All used the same 275gn projectile at 860ms MV. Peak temp was at 437c for the RUM and 572 for the Lapua. Accuracy was ¼ MOA for the 300RUM and ½ MOA for the Lapua. The 330 RUM broke the mould by retaining 1/3 moa at 3150 rounds. Both Lapua chambered rifles were returning 1MOA at 1500 rounds downrange. Everyone knows what a barrel burner 330Rum is meant to be…

In all the examples above we worked strenuously to ensure optimal case filling and to observe all the matters discussed in the previous posts on this topic.

Our current venture, a hypervelocity 12.7mm round launching an 1162 grain projectile at 4800 fps is achieving ¼ MOA at 500metres after 2500 rounds. Chamber pressure was 62500psi. Peak temp 489.5c. There is not yet any sign of accuracy degradation.

To me, this is pretty conclusive.  Keeping temperature down and ensuring the best possible round to bore alignment is the key to accuracy and consistency across a range of conditions as well as ensuring good barrel life. Sensible choices of barrel length are the key to reliable and long term velocity goals.
  

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Re: Heat vs Pressure vs Velocity - effects on barrel wear, consistency and accuracy part five
Reply #1 - Feb 24th, 2019 at 5:26pm
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Clarification
The 300Rum was using a 157 gn GS Custom projectile at 1087ms. Temp was 486c.
  

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Re: Heat vs Pressure vs Velocity - effects on barrel wear, consistency and accuracy part five
Reply #2 - Feb 24th, 2019 at 11:38pm
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The 300 Win Mag is 7.62x67 not 7.62x57 ,  Wink
  

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Proverbs 12:27
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Re: Heat vs Pressure vs Velocity - effects on barrel wear, consistency and accuracy part five
Reply #3 - Feb 26th, 2019 at 5:40pm
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Flintlock wrote on Feb 24th, 2019 at 11:38pm:
The 300 Win Mag is 7.62x67 not 7.62x57 ,

Fat fingers.... thanks for the correction. Roll Eyes
  

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Re: Heat vs Pressure vs Velocity - effects on barrel wear, consistency and accuracy part five
Reply #4 - Mar 4th, 2019 at 8:29am
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Have you any recommendations please when ordering reamers in respect of the freebore diameter - how much clearance around the bullet shank when chambered - to minimise the drop you refer to and improve the concentricity of the bullet with the bore while minimising the gap for gas/particles to escape ahead of the bullet ? I'm thinking here of the trade-off with safety and not wanting a situation where perhaps after a number of rounds (without cleaning) the available clearance is then reduced from firing residue build-up in an already tight freebore.

A second question please would be where you envisage the ideal position of the bullet to be in relation to the lands to minimise the effects you refer to: setting up the round for just touching, or perhaps with a certain amount of engagement into the rifling to attempt to seal the bore ahead of the bullet -  cut off the initial flow of gas between the ogive and leade that must occur to some extent with any jump?  Typically jump/engagement would be determined from accuracy tests, but let's say we were coming at this purely from wanting to maximise barrel life ?

Lastly these two are related in that assuming the CoL is set for engagement of the ogive with the leade, then this would in itself hold the bullet and round concentric without the need for a tight freebore.       
  
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Re: Heat vs Pressure vs Velocity - effects on barrel wear, consistency and accuracy part five
Reply #5 - Mar 24th, 2019 at 4:26pm
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Puffin your question will be answered in the next article.
  

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