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Old April 17, 2019, 02:46 AM   #1
jonnefudge
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Sticky bolt??

After the barrel heats up I get a sticky bolt and shaved ejector mark on this load and setup:

Tikka t3x varmint, 6.5x55, 24”

Vv n150 38.5gr. Cci 200, norma gt 130gr. Norma brass.

Velocity 822 ms. ES 8, SD 2.8.

This seems very weird to me since the powder is very temp stable, I’m not close to the pressure limit, I don’t let the barrel get super hot and don’t the cartridge rest I the chamber for long before firing.

Some of my brass have been through hotter load earlier. Could it be that ejector marks from earlier firings are causing this?

Any input is greatly appreciated!!

Last edited by jonnefudge; April 17, 2019 at 08:09 AM.
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Old April 17, 2019, 07:58 AM   #2
F. Guffey
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jonnefudge, I can never remember getting into the situation you describe. I have local reloaders I have shot with, it takes a little getting used to. As soon as I fire a round they are telling me to open the bolt because they do not want the receiver to absorb heat from the case.

And I have fired rifles that by design were not built to 'get rid of heat'. One of those rifles was an 'all day project' when determining if it was going to be 'the sum of the parts' or a very accurate rifle. It took 7 hours to fire 120 rounds, I understand that sounds boring but at the time I never took less than 8 rifles to the range. After the 120 rounds if 12 different loads with 10 rounds in each group I decided there was nothing I could do to improve on the accuracy, the gun was ugly.

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Old April 17, 2019, 02:23 PM   #3
Unclenick
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Jonnefudge,

According to VV's load data, 40.1 grains of N150 produced 808 m/s from a 670 mm (26.4") barrel with your bullet. You are getting higher velocity with 38.5 grains of N150 and in a shorter barrel. The only way that can happen is if your pressure is significantly higher. Why that is happening is the problem to solve. The sticky bolt lift and extraction are due to the higher pressure.
  • One possibility is you are seating the bullet out to touch the throat or close to it. That can raise peak pressure by about 20%. Be sure there is a gap of a least 1 mm between bullet and throat to be able to match the published data.
  • Another possibility is you have accidentally used a faster powder number. Also, if your N150 is too old it could raise pressure. Powder that has aged to the point it has consumed its stabilizer can start attacking its deterrent coatings, raising its burn rate and making it burn less progressively.
  • Another possibility is you have accidentally used a bullet that is different from theirs; say, a copper solid.
  • Another possibility, is your chamber and bore are tighter than on the VV test gun, but you would notice pressure signs with commercial ammunition then, so I am not expecting that to be the case.

From the published data, 38.5 grains should fire that bullet at just over 770 m/s in the VV test gun. With your shorter barrel, you should have around 750 m/s. Instead, you have 72 m/s (236 fps) more than that. Usually about half the bullet acceleration in a barrel has occurred by the time it passes the peak pressure and little change occurs in muzzle pressure if peak pressure is changed by start pressure or heat or other factors, so we can estimate that roughly 10% additional velocity all came from a higher peak value, so you have peak pressure about 20% higher than VV would have had in their load development at that load level.

For those readers not familiar with the metric system, just divide m/s by 0.3048 to get fps. This is an exact conversion with no leftover decimal places.
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Old April 19, 2019, 03:13 PM   #4
Fnusa
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What is your crimp like? There could be a possibility of your OAL changing when your bullets are in the magazine. Seen that before, caused a blown primer.
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