6BR Bolt lift increases with each shot.

[iam1964]

I was running 29.4gr of VV N140 this morning and after each shot the bolt lift was getting stiffer and stiffer to open the bolt. The average velocity was around 2780 fps with a 105gr aeromatch bullet. I did not see any pressure signs on the casing or primer. In fact I ran this exact same load last week (75 rounds) and experienced no hard bolt lift.

Any ideas why this would be happening?

 

[JFrank]

Beyond the normal over pressure signs excess or traces lubrication on the rounds or in the chamber, another might be a bit of moisture ( dew drops) on the ammo.
Wipe down the ammo and run a bore mop in the chamber..

 

[SGT. Slaughter]

Is this new brass shot in this 6br or has the brass been shot in a different barrel also? How many firings has the brass had on it? Is this Lapua brass? Has the cases been annealed? Just somethings to ponder on.

 

[ELR LVR]

I was running 29.4gr of VV N140 this morning and after each shot the bolt lift was getting stiffer and stiffer to open the bolt. The average velocity was around 2780 fps with a 105gr aeromatch bullet. I did not see any pressure signs on the casing or primer. In fact I ran this exact same load last week (75 rounds) and experienced no hard bolt lift.

Any ideas why this would be happening?

I experienced this once on a cartridge I had spec'd the reamer to be a tight chamber
Even loads that showerd no pressure signs and FL sized had a "Tick" in the bolt lift
I measured the brass after firing and noticed it did not enlarge
But then again, it was not allowed to
(it seems it is better to allow the brass to expand even .001" )
The remedy I came up with was to enlarge the chamber where the head is
.001"
This allowed to brass to expand (balloon) under pressure and then retract
The bolt lift tick was then gone
------------------------------------------
Is yours perhaps a tight chamber?
And did you FL size, or even try small base size?
------------------------------------------
I'm not sure but I think I read someone say 45 ACP dies can squeeze the head down slightly.
My 6 BR Forster Die will size down the head area another .003" allowing brass to easily enter and chamber
While My Redding Dies will not and brass will grow to chamber dimensions not sizing down enough then requiring me to then FL size in the Forster die.

 

[iam1964]

Thanks for the responses. Some more info below:

The cases are Lapua
They have now been fired 5 times.
They have been annealed after each firing.
Using a Forster FL sizing die with .266 bushings.

After depriming, I measure the case bottom to shoulder with a comparator.. the cases that had a normal bolt lift measured 1.162, the cases that had the heavy bolt lift measured 1.164. For comparison, when I measure my go-gauge with the same comparator, it measures 1.161

One thing that I did not do with these cases was trim the neck which I have ALWAYS done in the past. I’m not sure if this could be the cause!

 

[ELR LVR]

Thanks for the responses. Some more info below:

The cases are Lapua
They have now been fired 5 times.
They have been annealed after each firing.
Using a Forster FL sizing die with .266 bushings.

After depriming, I measure the case bottom to shoulder with a comparator.. the cases that had a normal bolt lift measured 1.162, the cases that had the heavy bolt lift measured 1.164. For comparison, when I measure my go-gauge with the same comparator, it measures 1.161

One thing that I did not do with these cases was trim the neck which I have ALWAYS done in the past. I’m not sure if this could be the cause!

After 5 firings,
Yes I would check case length
Some chambers only have .030" allowable freebore in front of the neck
OR, whatever the reamer was spec'd for
You could be close to hitting the end of freeplay there.
This is bad if your necks start pinching bullets when chambered and will drive up pressures
---------------
Also measure your head dia. Just above the rim extractor groove
And compare that meaurement to a FL sized case
If you are in safe pressure areas these two measurements should be the same
if your head is expanding above FL head dia. drop your charge back until it no longer expands the head

 

[G Liles]

Have you checked to checked to see if you have a carbon ring?

 

[iam1964]

Have you checked to checked to see if you have a carbon ring?

Yes, there was a bit of a carbon ring so that may also be the culprit.

 

[Dolomite]

I went through the same thing, I narrowed it down to a donut forming in the cases. A pin gauge wouldn’t drop below the neck shoulder junction. The deeper I seat the bullet the heavier bolt lift. So im assuming thats the culprit.

I thought it was a carbon ring, nope chamber was clean to bare metal, though it was lube or solvents in the chamber, nope cleaned chamber with alcohol. I thought it could be the cases grew too much, nope trimmed the cases back to 1.550. Bumping shoulders too much or not enough, I now bump shoulders .002-.003 helped relieve some pressure.

Something else I was told if you partially size the neck and pressure ring of the bullet is below the sized area that could cause pressure issues. The pressure ring of the bullet when fired has to enter the sized portion of the neck which may cause a pressure spike.

Not sure if that is true maybe someone can confirm.

 

[Old Navy]

I was running 29.4gr of VV N140 this morning and after each shot the bolt lift was getting stiffer and stiffer to open the bolt. The average velocity was around 2780 fps with a 105gr aeromatch bullet. I did not see any pressure signs on the casing or primer. In fact I ran this exact same load last week (75 rounds) and experienced no hard bolt lift.

Any ideas why this would be happening?

What king of grease you using on the lugs?

 

[Dolomite]

More food for thought:

Did you clean the bore really good before the 50 shots? Try cleaning out all the carbon and see it if still happens.

Is the base of the bullet seated close to or below the base of the neck?

The headspace measurements you show is exactly what happened to me, the shoulder blew out almost .002 above my headspace measurement which means the pressure got too high expanding the case too much causing the heavy bolt lift. Trying backing down the load and see if it continues.

I switched to a lighter bullet from the 105 to 95 and the pressure issues went away using the same Lapua brass.

Till this day I can only blame the donut or the pressure ring of the 105 was seated below the sized portion of the neck. I’ll have to test more.

 

[iam1964]

What king of grease you using on the lugs?

A small dab of birchwood Casey universal gun grease.

 

[Mike Casselton]

Not just grease on the lugs, but you also need to grease the cocking ramp.

Take the rifle as is, and cock the action, close the bolt.
Now, lift the bolt back to the top.
Was it hard to lift?

Close the bolt again and dry fire it.
Lift the bolt handle.
Was it harder to lift?
If so, you might have some light galling on the cocking ramp.

 

[iam1964]

Not just grease on the lugs, but you also need to grease the cocking ramp.

Take the rifle as is, and cock the action, close the bolt.
Now, lift the bolt back to the top.
Was it hard to lift?

Close the bolt again and dry fire it.
Lift the bolt handle.
Was it harder to lift?
If so, you might have some light galling on the cocking ramp.

The bolt is normally very easy to lift and still is.

 

[Mike Casselton]

Even after dry firing?

 

[iam1964]

Even after dry firing?

It is not as easy but does not feel overly stiff. I also have to add it is a savage action so it’s not butter smooth at the best of times.

 

[urbanrifleman]

Thanks for the responses. Some more info below:

The cases are Lapua
They have now been fired 5 times.
They have been annealed after each firing.
Using a Forster FL sizing die with .266 bushings.

After depriming, I measure the case bottom to shoulder with a comparator.. the cases that had a normal bolt lift measured 1.162, the cases that had the heavy bolt lift measured 1.164. For comparison, when I measure my go-gauge with the same comparator, it measures 1.161

One thing that I did not do with these cases was trim the neck which I have ALWAYS done in the past. I’m not sure if this could be the cause!

Maybe wipe your cases making sure to get off all the lube.

Shot in the dark.

 

[MikeRu]

Thanks for the responses. Some more info below:

The cases are Lapua
They have now been fired 5 times.
They have been annealed after each firing.
Using a Forster FL sizing die with .266 bushings.

After depriming, I measure the case bottom to shoulder with a comparator.. the cases that had a normal bolt lift measured 1.162, the cases that had the heavy bolt lift measured 1.164. For comparison, when I measure my go-gauge with the same comparator, it measures 1.161

One thing that I did not do with these cases was trim the neck which I have ALWAYS done in the past. I’m not sure if this could be the cause!

Does a bullet drop into a fired case mouth? If not then maybe needs a trim.

I once got bolt lift or clickers by over annealing and softening the shoulder too much (30 cal), your base to shoulder datum was higher when there was bolt lift.

 

[Dolomite]

What is the case overall length?