Gunsmithing Sticky Primary Extraction

[turbo54]

I've been having some sticky PRIMARY extraction with my FN SPR (CRF Win 70 action). Chambered in 284 Win, with a PTG "284 Win Match - no turn" reamer. More on the reamer later.

Load is WW-Super brass, 54.0gr H4831sc, 175smk, Tula primer, 2.950" COAL (tip to tip), goes ~2825fps from a 28" bbl.

What it's doing:

After firing a cartridge, the bolt lift is light and easy, until the primary extraction cam engages, at which point the bolt is tough to lift.

Nuances to the situation:

The reamer was intended for Lapua brass. I understand there is a slight difference in body diameters (down by the web) between Lapua and Winchester brass.

I bought some once fired brass that required small base FLSing to fit my chamber. When I fired these SB FLS'd cartridges the first time, I did NOT have the stick primary extraction issue. I did, however, have it the second time I fired them. I FLS every time with a Forster FLS die, but only bump the shoulder back ~.002-.003"

If I chamber a spent case that extracted stiffly, it will extract stiff again.

I sharpie'd up a case that exhibited the problem, and I can see the "rub" is *right* at the web.

My casehead diameter of spent cases measures between .4960" and .4970", obviously measured with a mic.

This is not a high pressure load, so let's not have this thread become a "hot load" discussion. My experience with this particular rifle has taught me it begins to show pressure via a cratered primer. Higher pressure makes the crater ridge sharper and sharper. Higher pressure yet will cause the casehead to begin to flow into ejector slot in the boltface. The 54gr load I am running doesn't even begin to crater the primer.

Any ideas? I haven't really tried much Lapua brass in it. I've fired a few, but don't remember if it had the sticky extraction then. I don't really want to run Lapua.

 

[LRI]

Re: Sticky Primary Extraction

Here's what I'd do if you were to walk through my front door:

1. Check the chamber headspace. A chamber cut too deep, even by a small amount can behave this way.

2. Assuming the HS is within proper limits, the next thing I'd do is load up a half dozen or so of virgin cases and fire them through the gun. This sets a baseline with new brass that hasn't been put through a "pasta press" of who knows what. (meaning buying once fired brass from lord only knows chamber conditions)

3. According to the reamer prints I have (published in the PTG master reamer print book) the 284 Winchester has a diameter of .501" across the web of the case. this being the portion of cartridge +.200" above the head of the case that purchases against the bolt face. Now I don't know what a "match" 284 uses for a web OD, but I'm willing to make an assumption based on the 6.5-284 match chambers that are numerous and well documented.

The chamber calls for a .4995" web OD. -.0025' from the plain vanilla cartridge.

Assuming this chamber dimension emulates what your rifle has and your mic is correct, you've got a post fired case clearance between +.0025" to .0035".

-Yet you still have a stubborn bolt.

I'd next take a look at what your cases are before you fire them. Next thing I'd do is cut a case length wise to see what the web looks like. If the nice, linear radius and case wall taper is still present after a few (3x) firings then I'd say your loading practices are sound when it comes to resizing. If the case wall taper looks like a piece of taffy pulled tight (hour glassing/"hips") then it kinda points towards the HS being a touch deep and/or your banging the shoulder back further than you may think you are.

None of this really addresses the problem, but it's eliminating things that aren't the problem, which is how this stuff often goes.

Next is make sure you don't have an extractor putting excessive pressure on the case body. Make sure it's not somehow locking up on the extractor cut of the breech.

A properly tuned CRF extractor should be able to hold onto a case rim with the bolt out of the gun and just hanging there. It should have a good pinch, but not be so excessive that you need to get heavy handed with it to peel if off the bolt face.

If you case has "hips" (what I call muffin top) after firing I'd first measure your brass after sizing it. Winchester brass is notorious for being very undersize. Lapua is bigger and closer to actual chamber dimensions. The hips are from the case bulging out to fill the chamber. It doesn't mean the chamber was cut wrong-especially with your fired cases coming out undersize anyway.

I've thrown a lot at your plate. Give it a go and see what you come up with.

Good luck.

C.

 

[dieselten]

Re: Sticky Primary Extraction

I say your chamber diameter is tighter than your sizing die. You say it is gtg when you use the small base die so there is your answer.

 

[turbo54]

Re: Sticky Primary Extraction

Chad, thanks for the ideas. I'll check them out. I can say for sure chamber headspace is dead nuts.

Diesel: when I FLS my brass and set the shoulders back ~.002", the cartridges will drop right in/out of the chamber so I'm not sure I understand how the SB die is the solution...though you're right - those cases didn't have any issue...

 

[300sniper]

Re: Sticky Primary Extraction

i small base size all my winchester brass in my 284 chamber. i will eventually have a new reamer ground with a web diameter slightly larger but it has been working so good as is, i haven't messed with it yet. the symptoms are exactly the same as yours on every barrel i have chambered with that reamer. it's a rch too tight for winchester brass without sb sizing.

i could probably get away with sb sizing every other firing but to stay consistent, i do it every time. some of my brass has 11+ firings on it and no signs of it giving up.