Sako 75 Finnlight - Headspace problem? Sticking Bolt

[David_S]

My apologies for the length of this post but I wanted to give all the relevant facts.

I recently purchased a Sako 75 Finnlight in 270 WSM. The rifle appeared in excellent condition. with only one previous owner. When opening the bolt I did notice a slight stickiness at about the ¾ open position (see picture) which surprised me given Sako's reputation for silky-smooth bolts. A bit of oil improved matters but not entirely.

I fired a box of Winchester 150gr Powerpoint to get a feel for the rifle and to get on paper after mounting a scope and then began working up a load of 130gr Hornady SSTs.

My son then asked a military armourer for his comments on the bolt but the armourer was more concerned at possible headspace problems than slightly sticky bolts. He pointed out that the fired cases were sealing on the shoulder not the neck and there appeared to be excessive stretching of the cases at the head. See photo.

He did not have the appropriate gauges to check the headspace but recommended that we get it checked by a gunsmith who did. He explained that the failure to neck seal indicated a loose chamber which could have been the result of stretching under high pressure such as might be caused if reloading above the max. recommended. Apparently he had experienced this with one of his own hunting rifles.

The previous owner assured me that he had only used factory ammo 140gr Federals and "not a lot". The load I was developing was 130gr Hornady SSTs and ADI AR2209 powder working in 0.5gr increments from 57.0gr up to the max recommended load of 60.5gr and then one increment of 0.25gr over that max, i.e. 60.75gr, as I have found with other calibres that the optimum load is often close to the max recommended. There were no signs of excessive pressure except that the velocity started to flatten off above 59.5gr at 3,100fps but this was what I anticipated. The bullet was seated 0.6mm (0.024") off lands. Cases were new Winchester and primers Winchester WLRM.

The ADI manual lists a pressure of 63,500 psi for the maximum recommended load of 60.5 gr. The max SAAMI pressure for the 270 WSM is 65,000psi.
I have measured the distance from the head to a point on the shoulder of fixed diameter for a new unfired case and several fired cases using the calipers and collar shown in the photo. The difference is 0.10mm (0.004")

My questions are
Headspace problem

1. Is it unusual for the case to seal on the shoulder not the neck?
2. Is the measured case stretching of 0.1mm excessive?
3. Would loading 0.25gr above recommended max cause such an increase in pressure as to "stretch" a chamber?
4. Is there a problem?
5. If I necksized rather than full length sized the cases would this help?

Bolt Sticking

1. I don't see how the slightly "sticky" bolt can be connected with the sealing/headspace problem but has anyone else with a Sako 75 experienced this?
2. Does anyone have any thoughts on the possible cause of the "stickiness' and how it could be remedied? (I don't really fancy using grinding paste on a Sako)

Thanks for any comments or suggestions. I would like to hear these before I try a gunsmith.

David

 

[Scorch]

Is it unusual for the case to seal on the shoulder not the neck?

If the cases were sealing at the shoulder and not the neck, I would be extremely surprised. Brass at the shoulder is thicker than the brass at the neck. Besides, if the neck were not sealing, you would have dark carbon deposits on the neck and shoulder area. You don't.

Is the measured case stretching of 0.1mm excessive?

No.

Would loading 0.25gr above recommended max cause such an increase in pressure as to "stretch" a chamber?

Not typically.

Is there a problem?

Not that I can see.

If I necksized rather than full length sized the cases would this help?

Neck sizing may reduce your case stretching, but there will be other issues associated with neck sizing, like cases being harder to chamber after a few resizings. I would not recommend it for a hunting rifle.
Bolt Sticking

I don't see how the slightly "sticky" bolt can be connected with the sealing/headspace problem but has anyone else with a Sako 75 experienced this?

Does anyone have any thoughts on the possible cause of the "stickiness' and how it could be remedied? (I don't really fancy using grinding paste on a Sako)

Try loosening the rear action screw and see if this alleviates the issue. If so, you are bending the action ever so slightly when you tighten the action screws.

My opinion: just shoot it.

 

[David_S]

Thank you Scorch for taking the time for your comprehensive reply which is very helpful. However the armourer thought that the cases were sealing on the shoulder rather than the neck as there was discolouration on the neck and halfway up the shoulder. My photos do not show this clearly but here are some more

It appears as if the case is stretching at the head until the shoulder contacts and then seals against the chamber. I was wondering whether this was a characteristic of WSM cartridges. I have not used them before.

I was surprised that you recommend against neck sizing for hunting rounds. I regularly necksize my other calibres (7x57, 243, 6.5x55 bolt action) without problems.

Your suggestion about loosening the action screw was a good one but unfortunately had no effect

I will try and get a gunsmith to look at the fired cases.

Thanks again for your input
David

 

[David_S]

Update

Well I found a gunsmith who has just set up locally. He did not have 270 WSM gauges but he reckoned it was unlikely that there was a problem with headspace and 0.1mm case stretching was pretty normal. And some of his Ackley Improved cases (can’t remember what calibre), which also have a sharp shoulder, had similar powder marks to my Sako on the case shoulders. So he did not think sealing on the shoulder was a big deal and was probably normal for this type of case, though he admitted he was not that familiar with the WSMs. He thought neck-sizing would be a good idea.

He also had a look at the sticking bolt and improved it by taking a sliver off the guide rail (I think it was that but can’t see for certain). It is much improved but still not quite Sako-perfect.

At the same time he fixed the safety on my old 7x57 Winchester Model 70 which had seized years ago so that I could no longer disassemble and clean the bolt, and recrowned the barrel. I took both rifles to him on Saturday afternoon and he dropped them off at my home 7am Monday.

Total cost was $NZ95 ($US72) of which $NZ50 was for the crown job, so I was pretty pleased.

David

 

[HiBC]

One small comment.You stated velocity "flattened out" at 59.5 gr.I assume you mean a diminishing return on velocity per increase in powder charge.For example,if 1 gr was gaining 70 fps at 57,58,59 gr,but going to 60 gr only incresed velocity 40 fps.

Years ago I learned from a Sierra Bullet newsletter that this point of diminishing returns indicates a good place to stop.

Note,this applies to modern strong actions and high pressure bottleneck cartridges.

So,You have gone approx 1 gr over uniform velocity increase.Another way to look at it,you are 1gr into spiking pressure.That may not work out well with your next pound of powder,a hot day,etc.

Realize the firing pin strike drives the cartridge forward,against the headspace feature;in this case,the shoulder.Upon ignition,pressure jumps as the bullet is driven into the rifling.The forward,thinner part of the case expands and grips the chamber wall.It stays forward.

Later,the case head is blown back against the bolt face,but the forward portion of the brass stays forward.The case stretch occurs pretty much at the rear of the chamber,where the case is not supported by the chamber so much.This is where the stretch ring thinning occurs.Look up the "paper clip test".

While a case stretch of .1 mm,approx .004 in,is not alarming,it is fine,you will do well to set your dies to only bump the shoulder back .002 in a bolt gun.

You might check cartridge case overall length.If the necks are long,they will be trapped by the chamber and cannot release the bullet.

I do not have a Sako,but you might study where the primary extraction cam on the bolt engages.This will likely be a ramp on the root of the bolt handle.

Study the cocking cam for any roughness or galling,or anything interfering with the striker being drawn back by the cocking cam.

Has the bolt been disassembled and cleaned?If anyone ever blew a primer,the little disc is inside the bolt.Sometimes,in manufacture,a steel chip gets stuck in a hole and makes it out the door.Could be some contamination in the bolt.

Does the extractor rotate with the bolt?If so,before extraction,the extractor is rotating on the cartridge case in the extractor groove.If the extractor has a sharp edge,or burr,it may be digging into the brass.

Good luck.

 

[David_S]

Velocity increase with charge

HIBC, thanks for that detailed reply There is a lot for me to digest.

To clarify the velocity flattening here is the relevant chart.

I do check the cartridge case length before reloading but have not had to trim them yet - only fired them twice. On my other calibres I have been neck-sizing only but don't have a collet die for the 270 WSM so have been FL sizing. I have recently learnt the importance of the shoulder position (as you mention) so made myself a collar which fits on my calipers ( as seen in my first post) so I can measure the position accurately and set up my FL die accordingly.

I will examine the bolt and extractor carefully as you suggest when I next have the rifle out. I have never dismantled the bolt and I doubt the previous owner did.

Thanks again, David